Network Working Group Eric Gray, Editor
Internet Draft Ericsson
Expires: August, 2008
Intended Status: Informational
February 25, 2008
The Architecture of an RBridge Solution to TRILL
draft-ietf-trill-rbridge-arch-05.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that
any applicable patent or other IPR claims of which he or she is
aware have been or will be disclosed, and any of which he or she
becomes aware will be disclosed, in accordance with Section 6 of
BCP 79.
Internet-Drafts are working documents of the Internet
Engineering Task Force (IETF), its areas, and its working
groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as "work
in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on January 25, 2008.
Abstract
RBridges are link layer (L2) devices that use a routing protocol
as a control plane. This combines several of the benefits of the
link layer with those of the network layer. For example RBridges
use existing link state routing, without necessarily requiring
configuration, to improve aggregate throughput, for RBridge to
Gray Expires August, 2008 [Page 1]
Internet-Draft RBridge Architecture February 2007
RBridge traffic. RBridges also may support IP multicast and IP
address resolution optimizations. They are intended to be
applicable to L2 network sizes similar to those of conventional
bridges and are intended to be backward compatible with those
bridges as both ingress/egress and transit. They also support
VLANs (although this generally requires configuration) while
otherwise attempting to retain as much 'plug and play' as is
already available in existing bridges. This document proposes an
architecture for RBridge systems as a solution to the TRILL
problem, defines terminology, and describes basic components and
desired behavior. One (or more) separate documents will specify
protocols and mechanisms that satisfy the architecture presented
herein.
Gray Expires August, 2008 [Page 2]
Internet-Draft RBridge Architecture February 2007
Table of Contents
1. Introduction................................................4
2. Background..................................................7
2.1. Existing Terminology...................................7
2.2. RBridge Terminology...................................10
3. Components.................................................12
3.1. RBridge Device........................................13
3.2. RBridge Data Model....................................14
3.2.1. Unicast TRILL Forwarding Database................14
3.2.2. Multi-destination TRILL Forwarding Database......14
3.2.3. Ingress TRILL Forwarding Database................16
4. Functional Description.....................................17
4.1. TRILL Campus Auto-configuration.......................17
4.2. RBridge Peer Discovery................................18
4.3. Topology Discovery....................................18
4.4. Determination of End Station Points of Attachment.....18
4.5. Learning..............................................19
4.6. Tunneling.............................................19
5. RBridge Operation..........................................20
5.1. RBridge General Operation.............................20
5.2. Ingress/Egress Operations.............................21
5.3. Transit Forwarding Operations.........................24
5.3.1. Unicast..........................................25
5.3.2. Broadcast, Multicast and Flooding................25
5.3.2.1. Broadcast...................................25
5.3.2.2. Multicast...................................27
5.3.2.3. Flooding....................................28
5.4. Routing Protocol Operation............................30
5.5. Other Bridging and Ethernet Protocol Operations.......30
5.5.1. Wiring Closet Problem............................31
6. How RBridges Address the TRILL Problem Space...............32
7. Conclusions................................................32
8. Security Considerations....................................33
9. IANA Considerations........................................34
10. Acknowledgments...........................................34
11. References................................................34
11.1. Normative References.................................34
11.2. Informative References...............................34
12. Author's Addresses........................................35
Intellectual Property Statement...............................35
Disclaimer of Validity........................................36
Copyright Statement...........................................36
Acknowledgment................................................36
Gray Expires August, 2008 [Page 3]
Internet-Draft RBridge Architecture February 2007
1. Introduction
This document describes an architecture that addresses the TRILL
problem and applicability statement [2]. This architecture
describes a solution that is composed of a set of devices called
RBridges. RBridges cooperate together in an Ethernet network to
provide a layer two delivery service that makes efficient use of
available links using a link state routing protocol. The service
provided is analogous to creation of a single, virtual device
composed of an overlay of tunnels, constructed between RBridge
devices, using paths determined by link state routing. RBridges
thus support increased aggregate RBridge to RBridge bandwidth,
and fault tolerance, when compared to conventional Ethernet
bridges (which forward frames via a spanning tree, in a non-VLAN
or single VLAN context, or multiple spanning trees), while still
being compatible with bridges and hubs.
The principal objectives of this architecture is to provide an
overview of the use of these RBridges in meeting the following
goals:
1) Provide a form of optimized layer two delivery service.
2) Use existing technology as much as possible.
3) Allow for configuration free (or minimal configuration)
deployment.
In providing a (optimized) layer two (L2) service, key factors
we want to maintain are: transparency to higher layer (layer 3
and above) delivery services and mechanisms, and use of location
independent addressing. Optimization of the L2 delivery service
consists of: use of an optimized subset of all available paths
and support for optimization of ARP/ND and pruning of multicast
traffic delivery paths.
Not all optimizations are necessarily expected to be supported
in initial specification and some subset of these optimizations
may be specified at a later time. This architecture should
allow some level of optimization support to be provided in
compliant implementations, in as many case as possible.
To accomplish the goal of using existing technologies as much as
possible, we intend to specify minimal extensions to an existing
link-state routing protocol, as well as defining specific sub-
sets of existing bridging technologies that this architecture is
intended to makes use of.
Gray Expires August, 2008 [Page 4]
Internet-Draft RBridge Architecture February 2007
The extent to which routing protocol extensions may be required
depends on the closeness of the "fit" of the chosen routing
protocol (in this case, IS-IS) to RBridge protocol requirements.
The specific of routing protocol use - along with appropriate
extensions and enhancements - will be defined in corresponding
RBridge protocol specifications (see [3] for example).
Specific protocol specifications will also describe the details
of interactions between the RBridge protocol and specific L2
technologies - i.e. - Virtual Local Area Networking (VLAN), L2
Multicast, etc. This document describes the general nature of
the RBridge solution without restricting related specifications.
As an overview, however, the intention is to use a link-state
routing protocol to accomplish the following:
1) Discover RBridge peers.
2) Determine RBridge link topology.
3) Potentially advertise L2 reachability information; note
that - at this time - the default method for acquiring L2
reachability information specified in [3] depends on use of
data-plane learning (see Bridge Learning in the terminology
section below).
4) Establish L2 delivery using shortest path (verses STP, RSTP
or MSTP).
There are additional RBridge protocol requirements - above and
beyond those addressed by any existing routing protocol - that
are identified in this document and need to be addressed in
corresponding RBridge protocol specifications.
To allow for configuration free deployment, specific protocol
specifications should explicitly define the conditions under
which RBridges may - and may not - be deployed as-is (plug and
play), and the mechanisms that are required to allow this. For
example, the first requirement any RBridge protocol must meet is
to derive information required by link-state routing protocol(s)
for protocol start-up and communications between peers - such as
higher-layer addressing and/or identifiers, encapsulation header
information, etc.
At the abstract level, RBridges need to maintain the following
information:
Gray Expires August, 2008 [Page 5]
Internet-Draft RBridge Architecture February 2007
1) Peer information,
2) Topology information,
3) Forwarding information -
a. unicast,
b. flooded, and
c. multicast.
In addition, RBridge specifications may suggest (or require) the
maintenance of other information as needed to support ARP/ND and
multicast optimizations.
Peer information may be acquired via the routing protocol, or
may be discovered as a result of RBridge-specific peer discovery
mechanisms. Details of specific peer information requirements -
as well as how this information will be acquired is specified in
protocol specifications (e.g. - [3]).
Topology information is expected to be acquired via the link-
state routing protocol.
In addition to the requirement that the routing protocol should
be an existing link-state routing protocol, which may provide a
mechanism for (or re-use of an existing) neighbor/peer
discovery, the routing protocol should be able to work with
minimal (or no) configuration - using algorithmically derived
addressing, for example, assuming the use of addresses is
required.
Given the potential choices, IS-IS routing has been chosen at
this time, and is assumed in this architecture. The fact that
this is assumed in this architecture is - in no way - intended
to preclude use of another link-state routing protocol, or any
other routing protocol, in any solution not described in this
architecture.
Forwarding information is derived from the combination of
attached MAC address learning, snooping of multicast-related
protocols (e.g. - IGMP), and routing advertisements and path
computations using the link-state routing protocol.
Other information - such as the mapping of MAC and IP addresses,
or multicast pruning information - may be learned using snooping
Gray Expires August, 2008 [Page 6]
Internet-Draft RBridge Architecture February 2007
of ARP/ND or IGMP (for example) and it is possible that RBridges
may need to participate actively in these protocols.
The remainder of this document outlines the TRILL architecture
of an RBridge-based solution and describes RBridge components,
interactions and functions. Note that this document is not
intended to represent the only solution to the TRILL problem
statement, nor does it specify the protocols that instantiate
this architecture - or that only one such set of protocols is
prescribed. The former may be contained in other architecture
documents and the latter would be contained in separate
specification documents (see - e.g. - [3]).
2. Background
This architecture is based on the RBridge system described in an
Infocom paper [1]. That paper describes the RBridge system as a
specific instance; this document abstracts architectural
features only. The remainder of this section describes the
terminology of this document, which may differ from that of the
original paper.
2.1. Existing Terminology
The following terminology is defined in other documents. A brief
definition is included in this section for convenience and - in
some cases - to remove any ambiguity in how the term may be used
in this document, as well as in derivative documents intended to
specify components, protocol, behavior and encapsulation
relative to the architecture described in this document.
o IEEE 802.1D and IEEE 802.1Q: IEEE documents which include
specification for bridged Ethernet, including Media Access
Control (MAC) bridges and the BPDUs used in spanning tree
protocol (STP) [5], [8].
o ARP: Address Resolution Protocol - a protocol used to find an
address of form X, given a corresponding address of form Y.
In this document, ARP refers to the well-known protocol used
to find L2 (MAC) addresses, using a given L3 (IP) address.
See [7] for further information on IP ARP.
o Bridge: an Ethernet (L2, 802.1D) device with multiple ports
that receives incoming frames on a port and transmits them on
zero or more of the other ports; bridges support both bridge
learning and STP. Transparent bridges do not modify the L2
PDU being forwarded.
Gray Expires August, 2008 [Page 7]
Internet-Draft RBridge Architecture February 2007
o Bridge Learning: process by which a bridge determines on
which (if any) single outgoing port to transmit (forward or
copy) an incoming unicast frame. This process depends on
consistent forwarding as "learning" uses the source MAC
address of frames received on each interface. Layer 2 (L2)
forwarding devices "learn" the location of L2 destinations by
peeking at layer 2 source addresses during frame forwarding,
and store the association of source address and receiving
interface. L2 forwarding devices use this information to
create "filtering database" entries and - gradually -
eliminate the need for flooding.
o Bridge Protocol Data Unit (BPDU): the frame type associated
with bridge control functions (for example: STP/RSTP).
o Bridged LAN: see IEEE 802.1Q-2005, Section 3.3 [8].
o Broadcast Domain: the set of (layer 2) devices that must be
reached (or reachable) by (layer 2) broadcast traffic
injected into the domain.
o Broadcast Traffic: traffic intended for receipt by all
devices in a broadcast domain.
o Ethernet: a common layer 2 networking technology that
includes, and is often equated with, 802.3.
o Filtering Database: database containing association
information of (source layer 2 address, arrival interface).
The interface that is associated with a specific layer 2
source address, is the same interface which is used to
forward frames having that address as a destination. When a
layer 2 forwarding device has no entry for the destination
layer 2 address of any frame it receives, the frame is
"flooded".
o Flooded Traffic: traffic that is subject to flooding - i.e. -
being forwarded on all interfaces, except the one on which it
was received, within a LAN or VLAN.
o Flooding: the process of forwarding traffic to ensure that
frames reach all possible destinations when the destination
location is not known. In "flooding", an 802.1D forwarding
device forwards a frame for any destination not "known" (i.e.
- not in the filtering or forwarding database) on every
active interface except that one on which it was received.
See also VLAN flooding and flooded traffic.
Gray Expires August, 2008 [Page 8]
Internet-Draft RBridge Architecture February 2007
o Frame: in this document, frame refers to an Ethernet (L2)
unit of transmission (PDU), including header, data, and
trailer (or payload and envelope).
o Hub: Ethernet device with multiple ports that transparently
transmits frames arriving on any port to all other ports.
This is a functional definition, as there are devices that
combine this function with certain bridge-like functions that
may - under certain conditions - be referred to as "hubs".
o IS-IS: Intermediate System to Intermediate System routing
protocol. [6] for further information on IS-IS.
o LAN: Local Area Network, is a computer network covering a
small geographic area, like a home, office, or group of
buildings, e.g., as based on IEEE 802.3 technology, see also
IEEE 802.1Q-2005, Section 3.11 [8].
o MAC: Media Access Control - mechanisms and addressing for L2
frame forwarding.
o Multicast Forwarding: forwarding methods that apply to frames
with broadcast or multicast destination MAC addresses.
o Node: a device with an L2 (MAC) address that sources and/or
sinks L2 frames.
o Packet: in this document, packet refers to L3 (or above) data
transmission units (PDU - e.g. - an IP Packet (RFC791 [4]),
including header and data.
o PDU: Protocol Data Unit - unit of data to be transmitted by a
protocol. To distinguish L2 and L3 PDUs, we refer to L2 PDUs
as "frames" and L3 PDUs as "packets" in this (and related)
document(s).
o Router: a device that performs forwarding of IP (L3) packets,
based on L3 addressing and forwarding information. Routers
forward packets from one L2 broadcast domain to another (one,
or more in the IP multicast case) - distinct - L2 broadcast
domain(s). A router terminates an L2 broadcast domain.
o Spanning Tree Protocol (STP): an Ethernet (802.1D) protocol
for establishing and maintaining a single spanning tree among
all the bridges on a local Ethernet segment. Also, Rapid
Spanning Tree Protocol (RSTP). In this document, STP and RSTP
are considered to be the same.
Gray Expires August, 2008 [Page 9]
Internet-Draft RBridge Architecture February 2007
o SPF: Shortest Path First - an algorithm name associated with
routing, used to determine a shortest path graph traversal.
o TRILL: Transparent Interconnect over Lots of Links - the
working group and working name for the problem domain to be
addressed in this document.
o Unicast Forwarding: forwarding methods that apply to frames
with unicast destination MAC addresses.
o Unknown Destination - a destination for which a receiving
device has no filtering database entry. Destination (layer
2) addresses are typically "learned" by (layer 2) forwarding
devices via a process commonly referred to as "bridge
learning" (see definition above).
o VLAN: Virtual Local Area Network, see IEEE 802.1Q-2005 [8].
o VLAN Flooding: flooding as described previously, except that
frames are only forwarded on those interfaces configured for
participation in the applicable VLAN.
2.2. RBridge Terminology
The following terms are defined in this document and intended
for use in derivative documents intended to specify components,
protocol, behavior and encapsulation relative to the
architecture specified in this document.
o Adjacent RBridges: RBridges that communicate directly with
each other without relay through other RBridges.
o Cooperating RBridges: a set of communicating RBridges that
will share a consistent set of forwarding information.
o Designated RBridge (DRB): one approach to resolving several
issues associated with having multiple RBridges as candidate
ingress and egress points for a single LAN (or VLAN) is to
designate an RBridge to act as the ingress and egress in that
case. The RBridge designated to handle ingress and egress
traffic to a specific Ethernet link (or VLAN associated with
that link) having shared access and multiple RBridges is such
a link's "Designated RBridge", or DRB. The Designated RBridge
may (for example) be determined by an election process among
those RBridges having shared access via a single LAN, or may
be selected by some other means.
Gray Expires August, 2008 [Page 10]
Internet-Draft RBridge Architecture February 2007
o Edge RBridge (edge of a TRILL Campus): describes RBridges
that may serve to ingress frames into the TRILL Campus and
egress frames from the TRILL Campus. L2 frames transiting an
TRILL Campus enter, and leave, it via an edge RBridge.
o Egress RBridge: for any specific frame, the RBridge through
which that frame leaves the TRILL Campus. For frames
transiting a TRILL Campus, the egress RBridge is an edge
RBridge where RBridge encapsulation is removed from the
transit frames prior to exiting the TRILL Campus.
o Encapsulation database: in the TRILL context, the database
that the Ingress RBridge uses to map the layer 2 destination
address in the received frame to the egress Rbridge.
o Forwarding Tunnels: in this document, Campus Forwarding
Tunnels (or Forwarding Tunnels) is used to refer to the paths
for forwarding transit frames, encapsulated at an RBridge
ingress and decapsulated at an RBridge egress.
o Ingress RBridge: for any specific frame, the RBridge through
which that frame enters the TRILL Campus. For frames
transiting a TRILL Campus, the ingress RBridge is the edge
RBridge where RBridge encapsulation is added to the transit
traffic entering the TRILL Campus.
o Multi-Destination Frames: Broadcast or Multicast frames, or
Unicast frames destined to a MAC DA that is unknown i.e. -
flooded frames (see flooded traffic). Frames that need to be
delivered to multiple egress RBridges, via the RBridge
Distribution Tree.
o Peer RBridge: The term "Peer RBridge", or (where usage is not
ambiguous) the term "Peer", are used in the RBridge context
to refer to any of the RBridges that make up a TRILL campus.
o RBridge: a logical device as described in this document,
which incorporate both routing and bridging features, thus
allowing for the achievement of TRILL Architecture goals. A
single RBridge device which can cooperate with other RBridge
devices to create a TRILL Campus.
o RBridge Distribution Tree: This term or (where usage is not
ambiguous) the term "distribution tree", refers to a tree
used by RBridges to deliver multi-destination frames. An RDT,
or distribution tree, is computed using a specific RBridge as
the root. May also be referred to as an R-tree.
Gray Expires August, 2008 [Page 11]
Internet-Draft RBridge Architecture February 2007
o TRILL Campus: this term, or the term "Campus" (where usage is
not ambiguous) is used in the RBridge context to refer to the
set of cooperating RBridges and TRILL Links that connect them
to each other.
o TRILL Forwarding Database: this term, or the term "forwarding
database" (where not ambiguous) is used in an RBridge context
to refer to the database that maps the egress TRILL address
to the next hop TRILL link.
o TRILL Header: a 'shim' header that encapsulates the ingress
L2 frame and persists throughout the transit of a TRILL
Campus, which may be further encapsulated within a hop-by-hop
L2 header (and trailer). The hop-by-hop L2 encapsulation in
this case includes the source MAC address of the immediate
upstream RBridge transmitting the frame and destination MAC
address of the receiving RBridge - at least in the unicast
forwarding case.
o TRILL Link: this term, or the term "Link" (where its usage is
not ambiguous) is used in the RBridge context to refer to the
Layer 2 connection that exists either between RBridges, or
between an RBridge and Ethernet end stations.
3. Components
A TRILL Campus is composed of RBridge devices and the forwarding
tunnels that connect them; all other Ethernet devices, such as
bridges, hubs, and nodes, operate conventionally in the presence
of an RBridge.
+----------------------------------------------------------+
| Higher Layer Entities |
+--+--------------+----------------------+--------------+--+
| \ TRILL Layer | RBridge Relay Entity | TRILL Layer / |
+---+-------------+----------------------+-------------+---+
| Data Link Layer | | Data Link Layer |
+-----------------+ +-----------------+
| Physical Layer | | Physical Layer |
+--------+--------+ +--------+--------+
| |
P 1 P 2
Figure 1: Simplified Architecture of an RBridge
Figure 1 shows an RBridge that contains:
Gray Expires August, 2008 [Page 12]
Internet-Draft RBridge Architecture February 2007
o An RBridge Relay Entity connecting two RBridge ports
o At least one physical port (two in this example)
o Higher layer Entities, including at least the IS-IS protocol
o At the TRILL Layer, an RBridge encapsulates incoming Ethernet
frames with a TRILL header to forward them to other RBridges.
3.1. RBridge Device
An RBridge is a device - having some of the characteristics of
both bridges and routers - that forwards frames on an Ethernet
link segment. It has one or more Ethernet ports which may be
wired or wireless; the particular physical layer is not
relevant. An RBridge is defined more by its behavior than its
structure, although it logically contains three tables, which
may be used to describe the externally visible behavior of an
RBridge relative to its peers and may also distinguish RBridges
from conventional bridges.
Conventional bridges contain a learned filtering (or forwarding)
database, and spanning tree port state information. The bridge
learns which nodes are accessible from a particular port by
assuming bi-directional consistency: the source addresses of
incoming frames indicate that the incoming port is to be used as
output for frames destined to that address. Incoming frames are
checked against the learned filtering (forwarding) database and
forwarded to the particular port if a match occurs, otherwise
they are flooded out all active ports (except the incoming
port).
Spanning tree port state information indicates the ports that
are active in the spanning tree. Details of STP operation are
out of scope for this document, however the result of STP is to
disable ports which would otherwise result in more than one path
traversal of the spanning tree.
RBridges, by comparison, have a TRILL forwarding database, used
for forwarding of RBridge encapsulated frames across the TRILL
Campus and by the ingress RBridge to determine the encapsulation
to use for frames received as un-encapsulated from non-RBridge
devices. The TRILL forwarding database is described in the
following sections.
Gray Expires August, 2008 [Page 13]
Internet-Draft RBridge Architecture February 2007
3.2. RBridge Data Model
The following tables represent the logical model of the data
required by RBridges in forwarding unicast and multicast data
across a TRILL Campus.
3.2.1. Unicast TRILL Forwarding Database
The Unicast TRILL Forwarding Database is a forwarding table for
unicast traffic within the TRILL Campus, allowing tunneled
traffic to transit the TRILL Campus from ingress to egress. The
size of a fully populated Unicast TRILL Forwarding Database at
each RBridge is maximally bounded by the product of the number
of Adjacent RBridge peers and VLANs.
RBridges may have separate Unicast TRILL Forwarding Databases
for each VLAN, if this is supported by configuration. Note that
scaling concerns may dictate otherwise, either in specific of
RBridge protocol specification, or in deployment. The Unicast
TRILL Forwarding Database is continually maintained by RBridge
routing protocols and/or MAC learning. (see Section 5.4).
The Unicast TRILL Forwarding Database contains data specific to
RBridge forwarding for unicast traffic. The specific fields
contained in this table are to be defined in RBridge protocol
specifications. In the abstract, however, the table should
contain forwarding direction and encapsulation associated with
an RBridge encapsulated frame received - determined by the TRILL
"shim" header destination and VLAN (if applicable).
3.2.2. Multi-destination TRILL Forwarding Database
The Multi-destination TRILL Forwarding Database consists of a
set of forwarding entries used for support of RBridge
Distribution Trees (RDT). Multi-destination TRILL Forwarding
Database entries are distinct from typical Unicast TRILL
Forwarding Database entries because there may be zero or more of
them that match for any incoming frame.
The Multi-destination TRILL Forwarding Database may overlap the
Unicast TRILL Forwarding Database, or be instantiated as a
separate table, in specific compliant implementations.
In discussing entries to be included in the Multi-destination
TRILL Forwarding Database, the following entities are
temporarily defined, or further qualified:
Gray Expires August, 2008 [Page 14]
Internet-Draft RBridge Architecture February 2007
o Root RBridge - the RBridge that is the head end of an RDT.
All RBridges within a TRILL Campus are potential Root
RBridges.
o Egress RBridge - an RBridge that is the tail end of a path
corresponding to a specific Multi-destination TRILL
Forwarding Database entry. All RBridges within a TRILL Campus
are potential egress RBridges. Not all RBridges within a
TRILL Campus will be on the shortest path between any ingress
RBridge and any other egress RBridge.
o Local RBridge - the RBridge that forms and maintains the
Multi-destination TRILL Forwarding Database entry (or
entries) under discussion. The local RBridge may be a root
RBridge, or an egress RBridge with respect to any set of
entries in the Multi-destination TRILL Forwarding Database.
o RBridge TRILL Campus Egress Interface - an interface on any
RBridge where a transit RBridge encapsulated frame would be
decapsulated prior to forwarding. With respect to such an
interface, the local RBridge is the egress RBridge.
Each local RBridge will maintain - as a logical representation -
a set of entries for at least the following, corresponding to a
subset of all possible forwarding paths:
o Zero or more entries grouped for each root RBridge - keyed by
some root RBridge identifier - used to determine forwarding
of broadcast, multicast, and flooded frames originally
RBridge encapsulated by that ingress within the TRILL Campus.
o Corresponding to each of these entry groups, one entry for
each of zero or more egress RBridge - where the local RBridge
is on the shortest path toward that egress RBridge.
o Corresponding to each of these entry groups, one entry for
each of zero or more TRILL Campus egress interfaces.
Each entry would contain an indication of which single interface
a broadcast, multicast or flooded frame would be forwarded for
each (root RBridge, egress RBridge) pair. Entries would also
contain any required encapsulation information, etc. required
for forwarding on a given interface, and toward a corresponding
specific egress RBridge.
Gray Expires August, 2008 [Page 15]
Internet-Draft RBridge Architecture February 2007
Note that the above information is one logical representation of
the information required to perform a reverse path forwarding
check (or RPFC) as is discussed in [3].
A local RBridge could maintain a full set of entries from every
RBridge to every other RBridge, however - depending on topology
- only a subset of these entries would ever be used. In
addition, a topology change that changed selection of shortest
paths would also very likely change other elements of the
entries, negating possible benefits from having pre-computed
Multi-destination TRILL Forwarding Database entries.
Multi-destination TRILL Forwarding Database entries should also
include VLAN identification information relative to each set of
Root RBridges, to allow scoping of broadcast, multicast and
flooding forwarding by configured VLANs.
Multi-destination TRILL Forwarding Database entries may also
include Multicast-Group Address specific information relative to
each egress RBridge that is a member of a given well-known
multicast group, to allow scoping of multicast forwarding by
multicast group.
Implicit in this data model is the assumption that the TRILL
"shim" header encapsulation will contain information that
explicitly identifies the TRILL Campus ingress RBridge for any
broadcast, multicast or flooded frame.
Maintenance of this Multi-destination TRILL Forwarding Database
will be defined in appropriate protocol specifications used to
instantiate this architecture. Note that doing this does not
strictly require those specification to adopt this data model.
The protocol specification needs to include mechanisms and
procedures required to establish and maintain the Multi-
destination TRILL Forwarding Database in consideration of
potential SPF recomputations resulting from network topology
changes.
3.2.3. Ingress TRILL Forwarding Database
The Ingress TRILL Forwarding Database determines how arriving
traffic will be encapsulated, for forwarding toward the egress
RBridge, via the TRILL Campus. It becomes configured in much the
same way that bridge learning occurs: by snooping incoming
traffic, and assuming bi-directional consistency.
Gray Expires August, 2008 [Page 16]
Internet-Draft RBridge Architecture February 2007
This learned information at an egress RBridge may be propagated
to all other RBridges in the TRILL Campus via the RBridge
routing protocol, as an alternative to direct MAC learning from
data frames. However, the information propagated in this fashion
may be quite large and filtering to prevent overwhelming edge
RBridges would require extensive per-VLAN state information in
core RBridges. Hence the current model is that the default mode
for learning L2 reachability information is via learning from
the data plane directly in a manner very analogous to bridge
learning.
Using this approach, the ingress TRILL Forwarding Database may
be as large as the number of nodes on the Ethernet LAN, for all
VLANs in which a specific ingress RBridge is a participant.
The Ingress TRILL Forwarding Database essentially determines the
tunnel encapsulation used to transport each specific frame
across the TRILL Campus, for frames entering at this ingress.
4. Functional Description
The RBridge Architecture is largely defined by RBridge behavior;
the logical components are minimal, as outlined in Section 3.
4.1. TRILL Campus Auto-configuration
Cooperating RBridges self-organize to compose a single TRILL
Campus system. The details for how this occurs are given in
protocol specification(s).
At an architectural level, it is sufficient to note that every
end station attached to a TRILL Campus should have a primary
point of attachment to the TRILL Campus, as might be defined
(for example) by a Designated RBridge. Furthermore, if it is
possible that there are 802.1Q (or 802.1D) bridges in a local
LAN, the association of specific RBridges and the end stations
for which they act as primary point of attachment must be
determined in a way that is consistent with forwarding in an
802.1Q (or 802.1D) network. If a DRB election process were
used, each TRILL Link (or VLAN set within a TRILL Link) attached
to a TRILL Campus would have a single Designated RBridge (either
for the link, or for a subset of the VLANs on the link); using
DRBs as an example, the DRB would be where all traffic intended
to transit a TRILL Campus enters and exits.
Other approaches might be used as well. For example, the
primary point of attachment for each end station (or set of end
Gray Expires August, 2008 [Page 17]
Internet-Draft RBridge Architecture February 2007
stations), might be configured, or determined in some other way,
on a per end station (or set of end stations) basis. Any such
approach must either allow for the possibility of LAN bridges or
not be used when such a possibility exists.
This rule applies strictly on a per-VLAN basis.
High-level steps that may be included in auto-configuration are:
RBridge peer discovery, topology discovery, determination of end
station points of attachment (DRB election, for example),
learning and forwarding (tunneling) TRILL encapsulated frames.
4.2. RBridge Peer Discovery
Proper operation of the TRILL solution using RBridges depends on
the existence of a mechanism for discovering peer RBridges.
Failure to discover all peer RBridges leads inevitably to an
incomplete discovery of the RBridge topology.
RBridge peer discovery can be accomplished in a relatively easy
re-use of well-known techniques based on broadcast - such as the
use of IS-IS "hello" messages.
4.3. Topology Discovery
Proper operation of RBridges also depends on the existence of a
mechanism for determining the RBridge topology. An accurate
determination of RBridge topology is required in order to
determine how traffic frames will flow in the topology and thus
avoid the establishment of persistent loops in frame forwarding,
or construction of a partitioned local LAN.
Fortunately, accurate topology determination is a fundamental
requirement of a functioning link-state routing protocol. The
complexity that applies in this architecture directly relates to
the existence of multiple VLANs on a TRILL Link.
For this reason, RBridges (in terms of protocol definition,
implementation and deployment) should avoid unnecessary use of
multiple VLANs - in particular on links that will be, or may be,
used for transit of TRILL encapsulated frames.
4.4. Determination of End Station Points of Attachment
The mechanisms and details of how end stations are associated
with a specific RBridge - when multiple RBridges are available
(connected to a local LAN or VLAN) - for the purpose of
Gray Expires August, 2008 [Page 18]
Internet-Draft RBridge Architecture February 2007
providing ingress to and egress from the Rbridge Campus, will be
provided by protocol specification(s). A potential approach to
be considered is the use of a "Designated RBridge (DRB)
Election", on either a per link, or per VLAN (set) basis.
Architecturally, it is important to note that this determination
must be based on an accurate view of the topology, including
availability of certain links in a given topology for traffic
associated with any given VLAN. Otherwise, it is possible to
partition a TRILL Link-local LAN (assuming that RBridges may be
deployed and configured to replace existing 802.1Q bridges) as a
result of a failure - under circumstances in which such a
partition would not have occurred with previously deployed
802.1Q bridges.
Protocol specification(s) need to define how accurate VLAN
topology is to be determined and applied in determination of end
station primary points of attachment. Protocol specification(s)
also need to state the limitations that any chosen mechanisms
may impose on the solution (in terms of scalability and ease of
deployment, for example). Finally, protocol specification(s)
need to describe how determination is made with respect to which
RBridge(s) are responsible for learning new end station
information, and for flooded and broadcast frames for reaching
known and unknown end stations on any link or VLAN.
4.5. Learning
The protocol specifications need to define how learning of MAC-
layer reachability information is expected to occur - at least
in the default case.
As described previously, a major consideration is the complexity
associated with receiving reachability information for a lot of
end-stations for which an ingress RBridge has no interest. This
is the case, for example, where a large number of VLANs are in
use (see [8]). This issue does not arise if learning is based
on the data plane (similar to bridge learning) - as is currently
described as a default learning mode in [3].
4.6.Tunneling
RBridges pass encapsulated frame traffic to each other
effectively using tunnels. These tunnels use an Ethernet link
layer header, together with a TRILL header.
Gray Expires August, 2008 [Page 19]
Internet-Draft RBridge Architecture February 2007
Specifics of encapsulation are to be defined in appropriate
protocol/encapsulation specifications.
It is the combination of the local MAC desitnation (which is for
a locally attached RBridge) and the TRILL encapsulation that
distinguishes RBridge to RBridge traffic from other traffic.
The link-layer header includes source and destination addresses,
which typically identify the local RBridges (the sending and
receiving RBridges relative to the local TRILL Link).
The TRILL header is required to support loop mitigation for (at
least) unicast traffic within the TRILL Campus; traffic loops in
forwarding between RBridges and non-RBridge devices, as well as
across non-RBridge devices between RBridges, is beyond the scope
of this document.
The TRILL header and encapsulation:
o must clearly identify the traffic as RBridge traffic - the
outer Ethernet header may, for instance, use an Ethertype
number unique to RBridges;
o should also identify a specific (egress) RBridge - the TRILL
header may, for example, include an identifier unique to the
egress RBridge, in the unicast case;
o should include the RBridge transit route, a hopcount, or a
timestamp to prevent indefinite looping of a frame.
5. RBridge Operation
This section is intended primarily to serve as a tutorial for
RBridge operations. As such in any case where this section says
anything in diagrement with specific protocol specifications,
the protocol specification over-rides.
5.1. RBridge General Operation
As described in sections above, operations that apply to all
RBridges include peer and topology discovery (including hello
messaging, negotiation of RBridge identifiers and link-state
routing), determination of RBridge primary points of attachment
(for local end stations - possibly via DRB election), shortest
path (SPF) computation and either learning or advertising reach-
ability for specific L2 (MAC Ethernet destination) addresses
within a broadcast domain.
Gray Expires August, 2008 [Page 20]
Internet-Draft RBridge Architecture February 2007
In addition, all RBridges will compute RBridge Distribution
Trees for delivery of (potentially VLAN scoped) broadcast,
multicast and flooded frames to each peer RBridge. Setting up
these trees early is important as there is otherwise no means
for frame delivery across the TRILL Campus during the learning
phase. Because it is very likely to be impossible (at an early
stage) for RBridges to determine which RBridges are edge
RBridges, it is preferable that each RBridge compute these trees
for all RBridges as early as possible - even if some entries
will not be used.
The specifics of each of these operational steps will be defined
in protocol specifications (such as [3]).
5.2.Ingress/Egress Operations
Operation specific to edge RBridges involves RBridge learning,
advertisement, encapsulation and decapsulation (at ingress and
egress RBridges, respectively).
As described previously, RBridge learning is similar to typical
bridge learning - i.e. - all RBridges listen promiscuously to L2
Frames on each local LAN and acquire end station location
information associated with source MAC addresses in L2 frames
they observe.
If multiple RBridges are available (i.e. - connected to a local
LAN or VLAN), a determination must be made as to which RBridge
is the primary point of attachment for each end station (or end
stations by groups - using, for example, the VLAN associations
of VLAN aware end stations) requiring ingress and egress
services from an RBridge Campus. This is a minimal requirement,
and there is no reason why this same determination might not be
made in all cases, even the degenerate case in which only one
RBridge may be used. This could be the case, for instance, if a
DRB election is done in all cases, including when the DRB can
only be the one and only RBridge to which local end stations
might be attached.
In the degenerate case - where only one RBridge is connected to
a specific Ethernet LAN - obviously that RBridge will be (or
become) the primary point of attachment for local end stations
requiring ingress/egress services from the RBridge Campus.
Minimally, only the RBridge determined as the primary point of
attachment for a given (set of) end station(s) is required to
perform RBridge learning for that (set of) end station(s) while
Gray Expires August, 2008 [Page 21]
Internet-Draft RBridge Architecture February 2007
they remain associated with interface(s) connected to the local
LAN or VLAN.
Note that - in some cases - the determination of primary points
of attachment and learning may be tightly bound operations.
This might be the case if - for example - the determination is
based on literal configuration of end station and RBridge
attachments. In other cases, learning would occur in a more
conventional - and flexible - way, if (for example) an automated
process of selecting a DRB (such as DRB election) is used on a
per-link or per-VLAN (set) basis.
Assuming learning is required by a specific solution described
in any protocol specification(s), as RBridges learn segment-
local MAC source addresses, it creates an entry in its learned
filtering/forwarding database that associates that MAC source
address with the interface on which it was learned.
Similarly - to support ARP/ND optimization - IP-to-MAC mappings
may also be learned by snooping corresponding protocol messages.
Protocol specifications may include either optional or required
behaviors to support ARP/ND, or multicast, learning and
distribution methods.
Periodically, as determined by RBridge protocol specification,
each RBridge may advertise this learned information to its
RBridge peers. These advertisements would propagate to all edge
RBridges (as potentially scoped by associated VLAN information
for each advertisement). Each edge RBridge would incorporate
this information in the form of a Unicast TRILL Forwarding
Database entry.
Note that currently, [3] specifies that this is not the default
mode, and that learning primarily occurs via the data plane at
egress, as well as at ingress.
The trade-off is between the complexity associated with flooding
data verses the complexity associated with flooding reachability
information.
For applications in which it is likely that most edge RBridges
will not want to receive most of the reachability information,
flooding avoidance requires either that the method is not used,
or that intermediate (core, in at least some cases) RBridges
need to keep VLAN specific state information to limit the scope
of advertisement flooding.
Gray Expires August, 2008 [Page 22]
Internet-Draft RBridge Architecture February 2007
If it is desired - in any specific solution - to support
discovery of new end station attachments, RBridges may also
discover that a new end station has become locally attached (for
which they may be, or become, an edge RBridge) as a result of
receiving un-encapsulated frames that require forwarding. Any
such solution must specify how a primary point of attachment is
determined when this occurs. Several possible approaches exist.
If an automated DRB selection process (such as DRB election) is
the approach in use for a specific solution (on a per-link, or
per-VLAN, basis), this determination is automatic for any link
(or VLAN on a local link). If an RBridge is the DRB for a local
link or VLAN,, and has not previously learned that the MAC
destination for a frame is local (this could be the case - for
instance - for the very first frame it observes), then the
RBridge could be required to forward (or flood) the frame via
the TRILL Campus to all other RBridges (potentially within a
VLAN scope).
When a frame is received, which must be forwarded across the
RBridge Campus, the responsible RBridge would flood the frame
unless it has already created a Unicast TRILL Forwarding
Database entry for the frame's MAC destination address. If it
has a corresponding Unicast TRILL Forwarding Database entry,
then it would use that. The RBridge in this instance would be
an ingress RBridge for the frame being forwarded across the
RBridge Campus.
The encapsulation used by this ingress RBridge would be
determined by the Unicast TRILL Forwarding Database entry - if
one exists - or the Unicast TRILL Forwarding Database-equivalent
entry for the RBridge Distribution Tree.
When the encapsulated frame arrives at egress RBridge(s), it is
decapsulated and forwarded via the egress interface(s) onto the
local link (or VLAN on the local link).
In using the approach of learning from the data plane, the
egress RBridge stores information related to content of the
frame's TRILL encapsulation for use in subsequent reverse
traffic in a manner directly analogous to bridge learning.
Note that an egress RBridge will - in most case - be the RBridge
determined to be the primary point of attachment for a
destination end station on the local link or VLAN accessed via
its egress interface(s). Exceptions to this might exist under
circumstances in which use of distinct RBridges for ingress and
Gray Expires August, 2008 [Page 23]
Internet-Draft RBridge Architecture February 2007
egress for a common (set of) end station(s) does not produce
local forwarding ambiguity. Any protocol specification that
allows this must also unambiguously describe the precise
circumstances under which it is allowed - as well as the
limitations and issues this introduces in the solution
specified.
Also note that specific solution(s) in protocol specification(s)
will need to describe how determination of an end station's
primary point of attachment (RBridge) occurs for the case where
a specific end station has not yet been discovered at any
ingress or egress interface - except under circumstances where
discovery of new end stations is not supported, or explicitly
disallowed. In the example in which a DRB election is used,
this determination is both trivial and automatic. In an
approach where end station and RBridge attachment/association is
configured, this should be relatively obvious - if inflexible.
In the DRB example, if the destination MAC address of a received
frame does not correspond to a learned MAC destination address
at an egress interface, it will forward the frame on all
interfaces for which it is either the designated RBridge. If a
received frame does correspond to a learned MAC destination
address at an egress interface, the RBridge will forward the
frame via that interface only.
Any specific solution's protocol specification(s) should also
allow for the fact that flooded frames may arrive at a single
local LAN (or VLAN) - where local end stations may be attached -
via multiple RBridges. Protocol specification(s) need to account
for how determination of which RBridge is exclusively
responsible for forwarding such frames is to be made. This is
essential to avoid having the same frame arrive multiple times
and potentially from widely disparate directions (i.e. - on
different interfaces of individual bridges).
5.3. Transit Forwarding Operations
There are two models for transit forwarding within a TRILL
Campus: unicast frame forwarding for known destinations, and
everything else. The difference between the two is in how the
encapsulation is determined. Exactly one of these models will be
selected - in any instantiation of this architecture- for each
of the following forwarding modes:
o Unicast frame forwarding
o Forwarding of non-unicast frames
Gray Expires August, 2008 [Page 24]
Internet-Draft RBridge Architecture February 2007
o Broadcast frame forwarding
o Multicast frame forwarding
o Frame flooding
5.3.1. Unicast
In unicast forwarding, the TRILL header is specific to the
egress RBridge and MAC destination in the outer Ethernet
encapsulation is specific to the next hop RBridge.
As the frame is prepared for transmission at each RBridge, the
next hop MAC destination information is determined at that local
RBridge using a corresponding Unicast TRILL Forwarding Database
entry based on the TRILL "shim" header.
5.3.2. Broadcast, Multicast and Flooding
RBridge Distribution Trees are used for forwarding of broadcast,
multicast and unknown destination frames across the TRILL
Campus. In a simple implementation, it is possible to use the
Multi-destination TRILL Forwarding Database entries for all
frames of these types.
However, this approach results in possibly severe inefficiencies
in at least the multicast case.
As a consequence, instantiations of this architecture should
allow for local optimizations on a hop by hop basis.
Examples of such optimizations are included in the sections
below.
5.3.2-1. Broadcast
The path followed in transit forwarding of broadcast frames will
have been established through actions initiated by each RBridge
(as any RBridge is eligible to subsequently become an ingress
RBridge) in the process of computing Multi-destination TRILL
Forwarding Database entries.
The protocol specification will most likely require each RBridge
to assume that it may be a transit as well as an ingress and
egress RBridge and establish forwarding information relative to
itself and each of its peer RBridges, and stored in the Multi-
destination TRILL Forwarding Database. At least one exception
case exists and that is when RBridges are configured to treat a
given link as a point to point link between two RBridges.
Gray Expires August, 2008 [Page 25]
Internet-Draft RBridge Architecture February 2007
Forwarding information should logically exist in two forms:
transit encapsulation information for interfaces over which the
RBridge will forward a multipoint frame to one or more adjacent
RBridges and a decapsulation indication for each interface over
which the RBridge may egress frames from the TRILL Campus. In
each case, the Multi-destination TRILL Forwarding Database
includes some identification of the interface on which a frame
is forwarded toward any specific egress RBridge for frames
received from any specific ingress RBridge.
Note that an interface over which an RBridge may egress frames
is any interface for which the RBridge is the primary point of
end station attachment for one or more end stations, or the
RBridge determined to be responsible for broadcast delivery.
RBridges must not wait to determine that one (or more) Ethernet
end stations are present on an interface before deciding to
forward decapsulated broadcast frames on that interface. Again,
an exception case would exist if RBridges have been configured
to treat a local link as a point to point connection between two
RBridges, or otherwise configured to ignore possible presence of
end stations in this case.
Forwarding information is selected for each broadcast frame
received by any RBridge (based - for example - on identifying
the ingress RBridge, or distribution tree root, for the frame)
for all corresponding Multi-destination TRILL Forwarding
Database entries. Each RBridge may thus be required to replicate
one RBridge encapsulated broadcast frame for each interface that
is determined from Multi-destination TRILL Forwarding Database
entries corresponding to the frame's ingress RBridge (or
distribution tree root). This includes decapsulated broadcast
frames for each interface for which the RBridge is responsible
for providing egress for broadcast frames (as might have been
determined previously by DRB election, for example).
Note that frame replication and forwarding should be scoped by
VLAN, if VLAN support is provided. Also note that an egress
RBridge may be required to transmit a decapsulated frame on the
same interface on which it previously received the corresponding
RBridge encapsulated frame.
This approach for broadcast forwarding might be considered to
add complexity because replication occurs at all RBridges along
the ingress RBridge tree, potentially for both RBridge
encapsulated and decapsulated broadcast frames. However, the
replication process is similar to replication of broadcast
Gray Expires August, 2008 [Page 26]
Internet-Draft RBridge Architecture February 2007
traffic in 802.1D bridges with the exception that additional
replication may be required at each interface for egress from
the TRILL Campus.
Note that the additional replication associated with TRILL
Campus egress may be made to exactly conform to 802.1D bridge
broadcast replication in implementations that model a TRILL
Campus egress as a separate logical interface.
Using this approach results in one and only one copy of the
broadcast frame being delivered to each egress RBridge.
5.3.2-2. Multicast
Multicast forwarding is reducible to broadcast forwarding in the
simplest (default) case. However, protocol specifications may
require, or recommend and implementations may choose - using
mechanisms that are out of scope for this document - to optimize
multicast forwarding. In order for this to work effectively,
however, support for awareness of multicast "interest" is
required for all RBridges.
Without optimization, multicast frames are injected by the
ingress RBridge onto an RDT by - for instance - encapsulating
the frame with a MAC destination multicast address, and
forwarding it according to its local Multi-destination TRILL
Forwarding Database. Again, without optimization, each RBridge
along the path toward all egress RBridges will similarly forward
the frame according to their local Multi-destination TRILL
Forwarding Database.
Using this approach results in one and only one copy of the
multicast frame being delivered to appropriate egress RBridges.
However, using this approach, multicast delivery is identical to
broadcast delivery - hence very inefficient.
In any optimization approach, RBridge encapsulated multicast
frames will use either a broadcast or a group MAC destination
address. In either case, the recognizably distinct destination
addressing allows a frame forwarding decision to be made at each
RBridge hop. RBridges may thus be able to take advantage of
local knowledge of multicast distribution requirements to
eliminate the forwarding requirement on interfaces for which
there is no recipient interested in receiving frames associated
with any specific group address.
Gray Expires August, 2008 [Page 27]
Internet-Draft RBridge Architecture February 2007
As stated earlier, in order for RBridges to be able to implement
multicast optimization, distribution of learned multicast group
"interest" information must be provided - and propagated - by
all RBridges. Mechanisms for learning and propagating multicast
group participation by RBridges is out of scope in this document
but may be defined in RBridge protocol specification(s).
Note that, because the multicast optimization would - in
principle - further scope and reduce broadcast traffic, two
things may be said:
o It is not necessary that all implementations in a deployment
implement the optimization (though all must support the data
required to implement it in RBridge peers) in order for any
local multicast optimization (consistent with the above
description) to work;
o Introduction of a multicast optimization will not result in
potential forwarding loops where broadcast forwarding would
not do so.
In the simplest case, the ingress RBridge for a given multicast
frame will re-use the MAC destination group address of a
received multicast frame. However this may not be required as
it is possible that the mechanisms specified to support
multicast will require examination of the decapsulated MAC
destination group address at each RBridge that implements the
optimization.
Specifics of multicast forwarding are to be defined in protocol
specifications.
5.3.2-3. Flooding
Flooding is similarly reducible to broadcast forwarding in the
simplest (default) case - with the exception that a frame being
flooded across the TRILL Campus is typically a unicast frame for
which no Unicast TRILL Forwarding Database entry exists at the
ingress RBridge. This is not a minor distinction, however,
because it impacts the way that addressing may be used to
accomplish flooding within the TRILL Campus.
An ingress RBridge that does not have a Unicast TRILL Forwarding
Database entry for a received frame MAC destination address,
will inject the frame onto the ingress RBridge Tree by - for
instance - encapsulating the frame with a MAC destination
broadcast address, and forwarding it according to its local
Multi-destination TRILL Forwarding Database. Without
Gray Expires August, 2008 [Page 28]
Internet-Draft RBridge Architecture February 2007
optimization, each RBridge along the path toward all egress
RBridges will similarly forward the frame according to their
local Multi-destination TRILL Forwarding Database.
Using this approach results in one and only one copy of the
flooded frame being delivered to all egress RBridges.
However implementations may choose to optimize flooding. A
Flooding optimization will only work at any specific RBridge if
that RBridge re-evaluates the original (decapsulated) unicast
frame.
Any flooding optimization would operate similarly to the
multicast optimization described above, except that - instead of
requiring local information about multicast distribution - each
RBridge implementing the optimization will need only to lookup
the MAC destination address of the original (decapsulated) frame
in its local Unicast TRILL Forwarding Database. If an entry is
found, the frame could then be forwarded only if the specific
RBridge is on the shortest path between the originating ingress
RBridge and the appropriate egress RBridge. This could be
implemented - for example - as a specialized Multi-destination
TRILL Forwarding Database entry.
Note that, because a flooding optimization would - in principle
- further scope and reduce flooded traffic, two things may be
said:
o It is not necessary that all implementations in a deployment
support the optimization in order for any local flooding
optimization (consistent with the above description) to work
(hence such an optimization is optional);
o Introduction of the flooding optimization will not result in
potential forwarding loops where flooded forwarding would not
do so.
Because a forwarding decision can be made at each hop, it is
possible to terminate flooding early if a Unicast TRILL
Forwarding Database for the original MAC destination was in the
process of being propagated when flooding for the frame was
started. It is therefore possible to reduce the amount of
flooding to some degree in this case.
Specifics of a flooding optimization - beyond the above proof of
the concept that such a thing could be done safely - is out of
scope for this document and should be out of scope generally in
all protocol specifications for which the above analysis holds.
Gray Expires August, 2008 [Page 29]
Internet-Draft RBridge Architecture February 2007
5.4. Routing Protocol Operation
The details of routing protocol operation are determined by the
choice to use IS-IS routing. These details would be defined in
appropriate protocol specification(s). Protocol specifications
in this case may include both RBridge protocols (such as [3]),
and specifications offering a generalized enhancement to IS-IS.
Protocol specifications should identify the means by which IS-IS
meets the peer and topology discovery, and path computation
needs of the specific protocol - including which IS-IS optional
features and enhancements (if any) are required for support of
specified RBridge operations.
5.5. Other Bridging and Ethernet Protocol Operations
In defining this architecture, several interaction models have
been considered for protocol interaction between RBridges and
other L2 forwarding devices - in particular, 802.1D bridges.
Whatever model we adopt for these interactions must allow for
the possibility of other types of L2 forwarding devices. Hence,
a minimal participation model is most likely to be successful
over the long term, assuming that RBridges are used in a L2
topology that would be functional if RBridges were replaced by
other types of L2 forwarding devices.
Toward this end, RBridges - and the TRILL Campus as a whole -
could (in theory) participate in Ethernet link protocols,
notably the spanning tree protocol (STP) on the ingress/egress
links using exactly one of the following interaction models:
o Transparent Participation (Transparent-STP)
o Active Participation (Participate-STP)
o Blocking Participation (Block-STP)
Only one of these variants would be supported by an instance of
this architecture. All RBridges within a single TRILL Campus
must use the same model for interacting with non-RBridge
protocols. Furthermore, it is the explicit intent that only one
of these models is ultimately supported - at least as a default
mode of compliant implementations.
This architecture assumes RBridges block STP.
Gray Expires August, 2008 [Page 30]
Internet-Draft RBridge Architecture February 2007
5.5.1. Wiring Closet Problem
There is at least one remaining issue with this assumption and
that has been referred to as the "wiring closet problem." The
essential problem is described in this subsection.
Given this configuration of bridges in a wiring closet, and an
RBridge core:
-----> B-1 <----------------> RB-a <-----.
| \
/ > RBridge CORE
| /
-----> B-2 <----------------> RB-b <-----'
The link between (802.1D) bridges B-1 and B-2 is meant to be
disabled by STP. In the RBridge case, however, there is no
indication (from STP) that this link is redundant. Moreover, in
order to avoid breaking bridge learning, either RB-a or RB-b
will be the DR and - as a result, only one of the links (B-
1<=>RB-a, B-2<=>RB-b) will get used.
One solution to this problem is to include - as a configuration
option, for instance - the ability to enable negotiation of (or
use of a pre-defined, or configurable) pseudo-bridge identifier
to be used in any of the variations of STP.
One - (near) zero-configuration - option we've considered would
be to use a well-known bridge identifier that each RBridge would
use as a common pseudo-bridge identifier. Such an ID, used in
combination with other STP configuration parameters, would most
likely have to be guaranteed to win the root bridge election
process in order to be a reasonable and useful default.
However, because this architecture assumes RBridges block STP,
participation in any form of STP is assumed to take place in an
in-line, co-located bridge function. Such a bridge function is
in addition to RBridge architectural functionality described in
this document. Implementations may include such functionality
and will very likely require some minimal configuration to turn
it on, in vendor specific RBridge implementations. An example
of a minimal configuration would be to assign a pseudo-bridge
identifier to (the local in-line co-located bridge associated
with) a specific RBridge port.
For reasons of interoperability, specific protocol proposals to
address the needs of this architecture may specify exactly how a
Gray Expires August, 2008 [Page 31]
Internet-Draft RBridge Architecture February 2007
co-located bridge will operate in this case (if such co-located
bridge functionality is included in an implementation), as well
as whether or not inclusion of such co-location is required.
As a further note, one of the problems that should be addressed
- assuming that this problem is to be resolved - is how to make
certain the solution is robust against configuration error. In
any solution that requires configuration of a pseudo-bridge ID
that is common across a TRILL Campus, for example, it is
possible to guard against configuration errors by using an
election process (based on the root bridge election process) to
determine which configured ID will be used by all RBridges in
common - assuming that multiple pseudo-bridge IDs are
inadvertently configured.
Finally, note that there is a chicken-and-egg problem associated
with RBridge participation in STP where RBridges may themselves
be connected by spanning trees.
6. How RBridges Address the TRILL Problem Space
The RBridge architecture addresses the following aspects of the
requirements identified in reference [2] through the use of a
link-state routing protocol and defined forwarding behaviors:
o Inefficient Paths
o Robustness to Link Interruption
In addition, using a logical model of "separation of functions"
this architecture allows specifications and implementations to
address existing and developing Ethernet extensions and
enhancements, and provides a background against which protocol
specifications may address: concerns about convergence under
dynamic network changes, and optimizations for VLAN, ARP/ND,
Multicast, etc.
7. Conclusions
This document discusses options considered and factors affecting
any protocol specific choices that may be made in instantiating
the TRILL architecture using RBridges.
Specific architectural and protocol instantiations should take
these into consideration. In particular, protocol, encapsulation
and procedure specifications should allow for potential
Gray Expires August, 2008 [Page 32]
Internet-Draft RBridge Architecture February 2007
optimizations described in the architectural document to the
maximum extent possible.
Also, this document addresses considerations relative to
interaction with existing technology and "future-proofing"
solutions. For both simplicity in description, and robust long
term implementation of the technology, this document recommends
the use of clear distinction - at all possible points - of
definitions, protocols, procedures, etc. from related (but not
identical) specifications and interactions.
In particular, this document recommends the use of a
"collocation model" in addressing issues with combining RBridge,
Router and 802.1D bridge behavior.
8. Security Considerations
As one stated requirement of this architecture is the need to be
able to provide an L2 delivery mechanism that is potentially
configuration free, the default operation mode for instances of
this architecture should assume a trust model that does not
require configuration of security information. This is - in fact
- an identical trust model to that used by Ethernet devices in
general.
In consequence, the default mode does not require - but also
does not preclude - the use of established security mechanisms
associated with the existing protocols that may be extended or
enhanced to satisfy this document's architectural definitions.
In general, this architecture suggest the use of a link-state
routing protocol - modified as required to support L2 reach-
ability and link state between RBridges. Any mechanisms defined
to support secure protocol exchanges between link-state routing
peers may be extended to support this architecture as well.
This architecture also suggests use of additional encapsulation
mechanisms and - to the extent that any proposed mechanism may
include (or be extended to include) secure transmission - it may
be desirable to provide such (optional) extensions.
To the extent possible, any extensions of protocol or
encapsulation should allow for at least one mode of operation
that doesn't require configuration - if necessary, for limited
use in a physically secure deployment.
Gray Expires August, 2008 [Page 33]
Internet-Draft RBridge Architecture February 2007
9. IANA Considerations
This document has no direct IANA considerations. It does
suggest, that protocols that instantiate the architecture use a
TRILL header as a wrapper on the payload for RBridge to RBridge
traffic, and this TRILL header may be identified by a new
Ethertype in the tunneled Ethernet link header. This Ethertype,
identified in an Ethernet header, could be allocated by the
IEEE.
10. Acknowledgments
The initial work for this document was largely done by Joe
Touch, based on work he and Radia Perlman completed earlier.
Subsequent changes are not to be blamed on either of them.
In addition, the current text has been helped substantially by
comments and suggestions from the TRILL working group, working
group chairs, and from Ron Bonica, Stewart Bryant, Joel Halpern,
Guillermo Ibanez and Russ White in particular. Also, a great
deal of work was recently done - by Joe Touch, Radia Perlman,
Dinesh Dutt and Silvano Gai - in an effort to align terminology
and concepts used in this document with those also used in the
other TRILL documents.
11. References
11.1. Normative References
None.
11.2. Informative References
[1] Perlman, R., "RBridges: Transparent Routing", Proc.
Infocom 2005, March 2004.
[2] Touch, J., R. Perlman, (ed.) "Transparent Interconnection
of Lots of Links (TRILL): Problem and Applicability
Statement", work in progress, draft-touch-trill-prob-
00.txt, November, 2005.
[3] Perlman, R., S. Gai, D. Dutt, D. Eastlake III, "RBridges:
Base Protocol Specification", work in progress, draft-
ietf-trill-rbridge-protocol-05.txt, July, 2007.
[4] Postel, J., "INTERNET PROTOCOL", RFC 791, September, 1981.
Gray Expires August, 2008 [Page 34]
Internet-Draft RBridge Architecture February 2007
[5] 802.1D-2004 IEEE Standard for Local and Metropolitan Area
Networks: Media Access Control (MAC) Bridges
[6] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
Dual Environments", RFC 1195, December, 1990.
[7] Plummer, D., "An Ethernet Address Resolution Protocol --
or -- Converting Network Protocol Addresses to 48.bit
Ethernet Address for Transmission on Ethernet Hardware",
RFC 826, November, 1982.
[8] 802.1Q-2005 IEEE Standard for Local and Metropolitan Area
Networks: Virtual Bridged Local Area Networks
(Incorporates IEEE Std 802.1Q-1998, IEEE Std 802.1uT-2001,
IEEE Std 802.1vT-2001, and IEEE 802.1sT-2002)
12. Author's Addresses
Editor:
Eric Gray
Ericsson
900 Chelmsford Street
Lowell, MA, 01851
Phone: +1 (978) 275-7470
EMail: Eric.Gray@Ericsson.com
Contributors:
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695, U.S.A.
Phone: +1 (310) 448-9151
EMail: touch@isi.edu
URL: http://www.isi.edu/touch
Radia Perlman
Sun Microsystems
EMail: Radia.Perlman@sun.com
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of
any Intellectual Property Rights or other rights that might be
Gray Expires August, 2008 [Page 35]
Internet-Draft RBridge Architecture February 2007
claimed to pertain to the implementation or use of the
technology described in this document or the extent to which any
license under such rights might or might not be available; nor
does it represent that it has made any independent effort to
identify any such rights. Information on the procedures with
respect to rights in RFC documents can be found in BCP 78 and
BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the
use of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR
repository at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be
required to implement this standard. Please address the
information to the IETF at ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided
on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY,
THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM
ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT
INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY
OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and
restrictions contained in BCP 78, and except as set forth
therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Gray Expires August, 2008 [Page 36]
Filename: draft-ietf-trill-rbridge-arch-05.doc
Directory: E:\Ericsson\IETF\drafts
Template: \\Boreas\homes\touch-xp\ietf\word
template\2-Word-v2.0.template.dot
Title: Network Working Group
Subject:
Author: Eric Gray
Keywords:
Comments:
Creation Date: 2/25/2008 10:06:00 AM
Change Number: 4
Last Saved On: 2/25/2008 11:11:00 AM
Last Saved By: Eric W Gray
Total Editing Time: 27 Minutes
Last Printed On: 2/25/2008 11:12:00 AM
As of Last Complete Printing
Number of Pages: 36
Number of Words: 11,424 (approx.)
Number of Characters: 65,120 (approx.)
