Routing Area Working Group P. Lapukhov
Internet-Draft Facebook
Intended status: Informational A. Premji
Expires: October 22, 2015 Arista Networks
J. Mitchell, Ed.
April 20, 2015
Use of BGP for routing in large-scale data centers
draft-ietf-rtgwg-bgp-routing-large-dc-02
Abstract
Some network operators build and operate data centers that support
over one hundred thousand servers. In this document, such data
centers are referred to as "large-scale" to differentiate them from
smaller infrastructures. Environments of this scale have a unique
set of network requirements with an emphasis on operational
simplicity and network stability. This document summarizes
operational experience in designing and operating large-scale data
centers using BGP as the only routing protocol. The intent is to
report on a proven and stable routing design that could be leveraged
by others in the industry.
Status of This Memo
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Copyright (c) 2015 IETF Trust and the persons identified as the
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Network Design Requirements . . . . . . . . . . . . . . . . . 4
2.1. Bandwidth and Traffic Patterns . . . . . . . . . . . . . 4
2.2. CAPEX Minimization . . . . . . . . . . . . . . . . . . . 4
2.3. OPEX Minimization . . . . . . . . . . . . . . . . . . . . 5
2.4. Traffic Engineering . . . . . . . . . . . . . . . . . . . 5
2.5. Summarized Requirements . . . . . . . . . . . . . . . . . 5
3. Data Center Topologies Overview . . . . . . . . . . . . . . . 6
3.1. Traditional DC Topology . . . . . . . . . . . . . . . . . 6
3.2. Clos Network topology . . . . . . . . . . . . . . . . . . 7
3.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Clos Topology Properties . . . . . . . . . . . . . . 8
3.2.3. Scaling the Clos topology . . . . . . . . . . . . . . 9
3.2.4. Managing the Size of Clos Topology Tiers . . . . . . 10
4. Data Center Routing Overview . . . . . . . . . . . . . . . . 10
4.1. Layer 2 Only Designs . . . . . . . . . . . . . . . . . . 11
4.2. Hybrid L2/L3 Designs . . . . . . . . . . . . . . . . . . 11
4.3. Layer 3 Only Designs . . . . . . . . . . . . . . . . . . 12
5. Routing Protocol Selection and Design . . . . . . . . . . . . 12
5.1. Choosing EBGP as the Routing Protocol . . . . . . . . . . 13
5.2. EBGP Configuration for Clos topology . . . . . . . . . . 14
5.2.1. Example ASN Scheme . . . . . . . . . . . . . . . . . 14
5.2.2. Private Use BGP ASNs . . . . . . . . . . . . . . . . 15
5.2.3. Prefix Advertisement . . . . . . . . . . . . . . . . 16
5.2.4. External Connectivity . . . . . . . . . . . . . . . . 17
5.2.5. Route Summarization at the Edge . . . . . . . . . . . 18
6. ECMP Considerations . . . . . . . . . . . . . . . . . . . . . 19
6.1. Basic ECMP . . . . . . . . . . . . . . . . . . . . . . . 19
6.2. BGP ECMP over Multiple ASNs . . . . . . . . . . . . . . . 20
6.3. Weighted ECMP . . . . . . . . . . . . . . . . . . . . . . 20
6.4. Consistent Hashing . . . . . . . . . . . . . . . . . . . 21
7. Routing Convergence Properties . . . . . . . . . . . . . . . 21
7.1. Fault Detection Timing . . . . . . . . . . . . . . . . . 21
7.2. Event Propagation Timing . . . . . . . . . . . . . . . . 22
7.3. Impact of Clos Topology Fan-outs . . . . . . . . . . . . 22
7.4. Failure Impact Scope . . . . . . . . . . . . . . . . . . 23
7.5. Routing Micro-Loops . . . . . . . . . . . . . . . . . . . 24
8. Additional Options for Design . . . . . . . . . . . . . . . . 25
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8.1. Third-party Route Injection . . . . . . . . . . . . . . . 25
8.2. Route Summarization within Clos Topology . . . . . . . . 25
8.2.1. Collapsing Tier-1 Devices Layer . . . . . . . . . . . 26
8.2.2. Simple Virtual Aggregation . . . . . . . . . . . . . 27
8.3. ICMP Unreachable Message Masquerading . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
12.1. Normative References . . . . . . . . . . . . . . . . . . 29
12.2. Informative References . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
This document describes a practical routing design that can be used
in a large-scale data center ("DC") design. Such data centers, also
known as hyper-scale or warehouse-scale data-centers, have a unique
attribute of supporting over a hundred thousand servers. In order to
accommodate networks of this scale, operators are revisiting
networking designs and platforms to address this need.
The design presented in this document is based on operational
experience with data centers built to support large scale distributed
software infrastructure, such as a Web search engine. The primary
requirements in such an environment are operational simplicity and
network stability so that a small group of people can effectively
support a significantly sized network.
After experimentation and extensive testing, Microsoft chose to use
an end-to-end routed network infrastructure with External BGP (EBGP)
[RFC4271] as the only routing protocol for some of its DC
deployments. This is in contrast with more traditional DC designs,
which may use simple tree topologies and rely on extending Layer 2
domains across multiple network devices. This document elaborates on
the requirements that led to this design choice and presents details
of the EBGP routing design as well as explores ideas for further
enhancements.
This document first presents an overview of network design
requirements and considerations for large-scale data centers. Then
traditional hierarchical data center network topologies are
contrasted with Clos networks that are horizontally scaled out. This
is followed by arguments for selecting EBGP with a Clos topology as
the most appropriate routing protocol to meet the requirements and
the proposed design is described in detail. Finally, the document
reviews some additional considerations and design options.
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2. Network Design Requirements
This section describes and summarizes network design requirements for
large-scale data centers.
2.1. Bandwidth and Traffic Patterns
The primary requirement when building an interconnection network for
large number of servers is to accommodate application bandwidth and
latency requirements. Until recently it was quite common to see the
majority of traffic entering and leaving the data center, commonly
referred to as "north-south" traffic. As a result, traditional
"tree" topologies were sufficient to accommodate such flows, even
with high oversubscription ratios between the layers of the network.
If more bandwidth was required, it was added by "scaling up" the
network elements, e.g. by upgrading the device's line-cards or
fabrics or replacing the device with one with higher port density.
Today many large-scale data centers host applications generating
significant amounts of server-to-server traffic, which does not
egress the DC, commonly referred to as "east-west" traffic. Examples
of such applications could be compute clusters such as Hadoop,
massive data replication between clusters needed by certain
applications, or virtual machine migrations. Scaling traditional
tree topologies to match these bandwidth demands becomes either too
expensive or impossible due to physical limitations, e.g. port
density in a switch.
2.2. CAPEX Minimization
The cost of the network infrastructure alone (CAPEX) constitutes
about 10-15% of total data center expenditure (see [GREENBERG2009]).
However, the absolute cost is significant, and hence there is a need
to constantly drive down the cost of individual network elements.
This can be accomplished in two ways:
o Unifying all network elements, preferably using the same hardware
type or even the same device. This allows for volume pricing on
bulk purchases.
o Driving costs down using competitive pressures, by introducing
multiple network equipment vendors.
In order to allow for good vendor diversity it is important to
minimize the software feature requirements for the network elements.
This strategy provides maximum flexibility of vendor equipment
choices while enforcing interoperability using open standards.
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2.3. OPEX Minimization
Operating large-scale infrastructure could be expensive, provided
that larger amount of elements will statistically fail more often.
Having a simpler design and operating using a limited software
feature-set minimizes software issue related failures.
An important aspect of OPEX minimization is reducing size of failure
domains in the network. Ethernet networks are known to be
susceptible to broadcast or unicast traffic storms that have dramatic
impact on network performance and availability. The use of a fully
routed design significantly reduces the size of the data-plane
failure domains - i.e. limits them to the lowest level in the network
hierarchy. However, such designs introduce the problem of
distributed control-plane failures. This observation calls for
simpler control-plane protocols that are expected to have less
chances of network meltdown. Minimizing software feature
requirements as described in the CAPEX section above also reduces
testing and training requirements.
2.4. Traffic Engineering
In any data center, application load-balancing is a critical function
performed by network devices. Traditionally, load-balancers are
deployed as dedicated devices in the traffic forwarding path. The
problem arises in scaling load-balancers under growing traffic
demand. A preferable solution would be able to scale load-balancing
layer horizontally, by adding more of the uniform nodes and
distributing incoming traffic across these nodes. In situation like
this, an ideal choice would be to use network infrastructure itself
to distribute traffic across a group of load-balancers. The
combination of Anycast prefix advertisement [RFC4786] and Equal Cost
Multipath (ECMP) functionality can be used to accomplish this goal.
To allow for more granular load-distribution, it is beneficial for
the network to support the ability to perform controlled per-hop
traffic engineering. For example, it is beneficial to directly
control the ECMP next-hop set for Anycast prefixes at every level of
network hierarchy.
2.5. Summarized Requirements
This section summarizes the list of requirements outlined in the
previous sections:
o REQ1: Select a topology that can be scaled "horizontally" by
adding more links and network devices of the same type without
requiring upgrades to the network elements themselves.
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o REQ2: Define a narrow set of software features/protocols supported
by a multitude of networking equipment vendors.
o REQ3: Choose a routing protocol that has a simple implementation
in terms of programming code complexity and ease of operational
support.
o REQ4: Minimize the failure domain of equipment or protocol issues
as much as possible.
o REQ5: Allow for traffic engineering, preferably via explicit
control of the routing prefix next-hop using built-in protocol
mechanics.
3. Data Center Topologies Overview
This section provides an overview of two general types of data center
designs - hierarchical (also known as tree based) and Clos based
network designs.
3.1. Traditional DC Topology
In the networking industry, a common design choice for data centers
typically look like a (upside-down) tree with redundant uplinks and
three layers of hierarchy namely core, aggregation/distribution and
access layers (see Figure 1). To accommodate bandwidth demands, each
higher layer, from server towards DC egress or WAN, has higher port
density and bandwidth capacity where the core functions as the
"trunk" of the tree based design. To keep terminology uniform and
for comparison with other designs, in this document these layers will
be referred to as Tier-1, Tier-2 and Tier-3 "tiers" instead of Core,
Aggregation or Access layers.
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+------+ +------+
| | | |
| |--| | Tier-1
| | | |
+------+ +------+
| | | |
+---------+ | | +----------+
| +-------+--+------+--+-------+ |
| | | | | | | |
+----+ +----+ +----+ +----+
| | | | | | | |
| |-----| | | |-----| | Tier-2
| | | | | | | |
+----+ +----+ +----+ +----+
| | | |
| | | |
| +-----+ | | +-----+ |
+-| |-+ +-| |-+ Tier-3
+-----+ +-----+
| | | | | |
<- Servers -> <- Servers ->
Figure 1: Typical DC network topology
3.2. Clos Network topology
This section describes a common design for horizontally scalable
topology in large scale data centers in order to meet REQ1.
3.2.1. Overview
A common choice for a horizontally scalable topology is a folded Clos
topology, sometimes called "fat-tree" (see, for example, [INTERCON]
and [ALFARES2008]). This topology features an odd number of stages
(sometimes known as dimensions) and is commonly made of uniform
elements, e.g. network switches with the same port count. Therefore,
the choice of folded Clos topology satisfies REQ1 and facilitates
REQ2. See Figure 2 below for an example of a folded 3-stage Clos
topology (3 stages counting Tier-2 stage twice, when tracing a packet
flow):
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+-------+
| |----------------------------+
| |------------------+ |
| |--------+ | |
+-------+ | | |
+-------+ | | |
| |--------+---------+-------+ |
| |--------+-------+ | | |
| |------+ | | | | |
+-------+ | | | | | |
+-------+ | | | | | |
| |------+-+-------+-+-----+ | |
| |------+-+-----+ | | | | |
| |----+ | | | | | | | |
+-------+ | | | | | | ---------> M links
Tier-1 | | | | | | | | |
+-------+ +-------+ +-------+
| | | | | |
| | | | | | Tier-2
| | | | | |
+-------+ +-------+ +-------+
| | | | | | | | |
| | | | | | ---------> N Links
| | | | | | | | |
O O O O O O O O O Servers
Figure 2: 3-Stage Folded Clos topology
This topology is often also referred to as a "Leaf and Spine"
network, where "Spine" is the name given to the middle stage of the
Clos topology (Tier-1) and "Leaf" is the name of input/output stage
(Tier-2). For uniformity, this document will refer to these layers
using the "Tier-n" notation.
3.2.2. Clos Topology Properties
The following are some key properties of the Clos topology:
o The topology is fully non-blocking (or more accurately: non-
interfering) if M >= N and oversubscribed by a factor of N/M
otherwise. Here M and N is the uplink and downlink port count
respectively, for a Tier-2 switch as shown in Figure 2.
o Utilizing this topology requires control and data plane supporting
ECMP with the fan-out of M or more.
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o Tier-1 switches have exactly one path to every server in this
topology. This is an important property that makes route
summarization impossible in this topology (see Section 8.2 below).
o Traffic flowing from server to server is load-balanced over all
available paths using ECMP.
3.2.3. Scaling the Clos topology
A Clos topology can be scaled either by increasing network element
port density or adding more stages, e.g. moving to a 5-stage Clos, as
illustrated in Figure 3 below:
Tier-1
+-----+
| |
+--| |--+
| +-----+ |
Tier-2 | | Tier-2
+-----+ | +-----+ | +-----+
+-------------| DEV |--+--| |--+--| |-------------+
| +-----| C |--+ | | +--| |-----+ |
| | +-----+ +-----+ +-----+ | |
| | | |
| | +-----+ +-----+ +-----+ | |
| +-----+-----| DEV |--+ | | +--| |-----+-----+ |
| | | +---| D |--+--| |--+--| |---+ | | |
| | | | +-----+ | +-----+ | +-----+ | | | |
| | | | | | | | | |
+-----+ +-----+ | +-----+ | +-----+ +-----+
| DEV | | DEV | +--| |--+ | | | |
| A | | B | Tier-3 | | Tier-3 | | | |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | |
O O O O O O O O
Servers Servers
Figure 3: 5-Stage Clos topology
The small example topology on Figure 3 is built from devices with a
port count of 4 and provides full bisectional bandwidth to all
connected servers. In this document, one set of directly connected
Tier-2 and Tier-3 devices along with their attached servers will be
referred to as a "cluster". For example, DEV A, B, C, D, and the
servers that connect to DEV A and B, on Figure 3 form a cluster.
In practice, the Tier-3 layer of the network, which are typically top
of rack switches (ToRs), is where oversubscription is introduced to
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allow for packaging of more servers in the data center while meeting
the bandwidth requirements for different types of applications. The
main reason to limit oversubscription at a single layer of the
network is to simplify application development that would otherwise
need to account for multiple bandwidth pools: within rack (Tier-3),
between racks (Tier-2), and between clusters (Tier-1). Since
oversubscription does not have a direct relationship to the routing
design it is not discussed further in this document.
3.2.4. Managing the Size of Clos Topology Tiers
If a data-center network size is small, it is possible to reduce the
number of switches in Tier-1 or Tier-2 of Clos topology by a power of
two. To understand how this could be done, take Tier-1 as an
example. Every Tier-2 device connects to a single group of Tier-1
devices. If half of the ports on each of the Tier-1 devices are not
being used then it is possible to reduce the number of Tier-1 devices
by half and simply map two uplinks from a Tier-2 device to the same
Tier-1 device that were previously mapped to different Tier-1
devices. This technique maintains the same bisectional bandwidth
while reducing the number of elements in the Tier-1 layer, thus
saving on CAPEX. The tradeoff, in this example, is the reduction of
maximum DC size in terms of overall server count by half.
In this example, Tier-2 devices will be using two parallel links to
connect to each Tier-1 device. If one of these links fails, the
other will pick up all traffic of the failed link, possible resulting
in heavy congestion and quality of service degradation if the path
determination procedure, does not take bandwidth amount into account.
To avoid this situation, parallel links can be grouped in link
aggregation groups (LAGs, such as [IEEE8023AD]) with widely available
implementation settings that take the whole "bundle" down upon a
single link failure. Equivalent techniques that enforce "fate
sharing" on the parallel links can be used in place of LAGs to
achieve the same effect. As a result of such fate-sharing, traffic
from two or more failed links will be re-balanced over the multitude
of remaining paths that equals the number of Tier-1 devices. This
example is using two links for simplicity it should be noted, that
having more links in a bundle will have less impact on capacity upon
a member-link failure.
4. Data Center Routing Overview
This section provides an overview of three general types of data
center protocol designs - Layer 2 only, Hybrid L2/L3 and Layer 3
only.
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4.1. Layer 2 Only Designs
Originally most data center designs used Spanning-Tree Protocol (STP)
for loop free topology creation, typically utilizing variants of the
traditional DC topology described in Section 3.1. At the time, many
DC switches either did not support Layer 3 routed protocols or
supported it with additional licensing fees, which played a part in
the design choice. Although many enhancements have been made through
the introduction of Rapid Spanning Tree Protocol and Multiple
Spanning Tree Protocol that increase convergence, stability and load
balancing in larger topologies many of the fundamentals of the
protocol limit its applicability in large scale DC's. STP and its
newer variants use an active/standby approach to path selection and
are therefore hard to deploy in horizontally scaled topologies
described in Section 3.2. Further, operators have had many
experiences with large failures due to issues caused by improper
cabling, misconfiguration, or flawed software on a single device.
These failures regularly affected the entire spanning-tree domain and
were very hard to troubleshoot due to the nature of the protocol.
For these reasons, and since almost all DC traffic is now IP,
therefore requiring a Layer 3 routing protocol at the network edge
for external connectivity, designs utilizing STP usually fail all of
the requirements of large scale DC operators. Various enhancements
to link-aggregation protocols such as [IEEE8023AD], generally known
as Multi-Chassis Link-Aggregation (M-LAG) made it possible to use
Layer 2 designs with active-active network paths while relying on STP
as the backup for loop prevention. The major downside of this
approach is proprietary nature of such extensions.
It should be noted that building large, horizontally scalable, Layer
2 only networks without STP is possible recently through the
introduction of TRILL [RFC6325]. TRILL resolves many of the issues
STP has for large scale DC design however currently the maturity of
the protocol, limited number of implementations, and requirement for
new equipment that supports it has limited its applicability and
increased the cost of such designs.
Finally, neither TRILL nor M-LAG approach eliminate the fundamental
problem of the shared broadcast domain, that is so detrimental to the
operations of any Layer 2, Ethernet based solutions.
4.2. Hybrid L2/L3 Designs
Operators have sought to limit the impact of data-plane faults and
build larger scale topologies through implementing routing protocols
in either the Tier-1 or Tier-2 parts of the network and dividing the
Layer-2 domain into numerous, smaller domains. This design has
allowed data centers to scale up, but at the cost of complexity in
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the network managing multiple protocols. For the following reasons,
operators have retained Layer 2 in either the access (Tier-3) or both
access and aggregation (Tier-3 and Tier-2) parts of the network:
o Supporting legacy applications that may require direct Layer 2
adjacency or use non-IP protocols.
o Seamless mobility for virtual machines that require the
preservation of IP addresses when a virtual machine moves to
different Tier-3 switch.
o Simplified IP addressing = less IP subnets is required for the
data center.
o Application load-balancing may require direct Layer 2 reachability
to perform certain functions such as Layer 2 Direct Server Return
(DSR).
o Continued CAPEX differences between Layer-2 and Layer-3 capable
switches.
4.3. Layer 3 Only Designs
Network designs that leverage IP routing down to Tier-3 of the
network have gained popularity as well. The main benefit of these
designs is improved network stability and scalability, as a result of
confining L2 broadcast domains. Commonly an IGP such as OSPF
[RFC2328] is used as the primary routing protocol in such a design.
As data centers grow in scale, and server count exceeds tens of
thousands, such fully routed designs have become more attractive.
Choosing a Layer 3 only design greatly simplifies the network,
facilitating the meeting of REQ1 and REQ2, and has widespread
adoption in networks where large Layer 2 adjacency and larger size
Layer 3 subnets are not as critical compared to network scalability
and stability. Application providers and network operators continue
to also develop new solutions to meet some of the requirements that
previously have driven large Layer 2 domains.
5. Routing Protocol Selection and Design
In this section the motivations for using External BGP (EBGP) as the
single routing protocol for data center networks having a Layer 3
protocol design and Clos topology are reviewed. Then, a practical
approach for designing an EBGP based network is provided.
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5.1. Choosing EBGP as the Routing Protocol
REQ2 would give preference to the selection of a single routing
protocol to reduce complexity and interdependencies. While it is
common to rely on an IGP in this situation, sometimes with either the
addition of EBGP at the device bordering the WAN or Internal BGP
(IBGP) throughout, this document proposes the use of an EBGP only
design.
Although EBGP is the protocol used for almost all inter-provider
routing on the Internet and has wide support from both vendor and
service provider communities, it is not generally deployed as the
primary routing protocol within the data center for a number of
reasons (some of which are interrelated):
o BGP is perceived as a "WAN only protocol only" and not often
considered for enterprise or data center applications.
o BGP is believed to have a "much slower" routing convergence
compared to IGPs.
o BGP deployment within an Autonomous System typically assumes the
presence of an IGP for next-hop resolution.
o BGP is perceived to require significant configuration overhead and
does not support neighbor auto-discovery.
This document discusses some of these perceptions, especially as
applicable to the proposed design, and highlights some of the
advantages of using the protocol such as:
o BGP has less complexity within its protocol design - internal data
structures and state-machines are simpler when compared to a link-
state IGP such as OSPF. For example, instead of implementing
adjacency formation, adjacency maintenance and/or flow-control,
BGP simply relies on TCP as the underlying transport. This
fulfills REQ2 and REQ3.
o BGP information flooding overhead is less when compared to link-
state IGPs. Since every BGP router calculates and propagates only
the best-path selected, a network failure is masked as soon as the
BGP speaker finds an alternate path, which exists when highly
symmetric topologies, such as Clos, are coupled with EBGP only
design. In contrast, the event propagation scope of a link-state
IGP is an entire area, regardless of the failure type. This meets
REQ3 and REQ4. It is worth mentioning that all widely deployed
link-state IGPs also feature periodic refreshes of routing
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information, while BGP does not expire routing state, even if this
rarely causes significant impact to modern router control planes.
o BGP supports third-party (recursively resolved) next-hops. This
allows for manipulating multi-path to be non-ECMP based or
forwarding based on application-defined forwarding paths, through
establishment of a peering session with an application
"controller" which can inject routing information into the system,
satisfying REQ5. OSPF provides similar functionality using
concepts such as "Forwarding Address", but with more difficulty in
implementation and lack of protocol simplicity.
o Using a well-defined BGP ASN allocation scheme and standard
AS_PATH loop detection, "BGP path hunting" (see [JAKMA2008]) can
be controlled and complex unwanted paths will be ignored. See
Section 5.2 for an example of a working BGP ASN allocation scheme.
In a link-state IGP accomplishing the same goal would require
multi-(instance/topology/processes) support, typically not
available in all DC devices and quite complex to configure and
troubleshoot. Using a traditional single flooding domain, which
most DC designs utilize, under certain failure conditions may pick
up unwanted lengthy paths, e.g. traversing multiple Tier-2
devices.
o EBGP configuration that is implemented with minimal routing policy
is easier to troubleshoot for network reachability issues. In
most implementations, it is straightforward to view contents of
BGP Loc-RIB and compare it to the router's RIB. Also every BGP
neighbor has corresponding Adj-RIB-In and Adj-RIB-Out structures
with incoming and outgoing NRLI information that can be easily
correlated on both sides of a BGP session. Thus, BGP satisfies
REQ3.
5.2. EBGP Configuration for Clos topology
Clos topologies that have more than 5 stages are very uncommon due to
the large numbers of interconnects required by such a design.
Therefore, the examples below are made with reference to the 5-stage
Clos topology (5 stages in unfolded state).
5.2.1. Example ASN Scheme
The diagram below illustrates an example ASN allocation scheme. The
following is a list of guidelines that can be used:
o Only EBGP sessions established over direct point-to-point links
interconnecting the network nodes.
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o 16-bit (two octet) BGP ASNs are used, since these are widely
supported and have better vendor interoperability.
o Private BGP ASNs from the range 64512-65534 are used so as to
avoid ASN conflicts.
o A single BGP ASN is allocated to all of the Clos topology's Tier-1
devices.
o Unique BGP ASN is allocated per each group of Tier-2 devices.
o Unique BGP ASN is allocated to every Tier-3 device (e.g. ToR) in
this topology.
ASN 65534
+---------+
| +-----+ |
| | | |
+-|-| |-|-+
| | +-----+ | |
ASN 646XX | | | | ASN 646XX
+---------+ | | | | +---------+
| +-----+ | | | +-----+ | | | +-----+ |
+-----------|-| |-|-+-|-| |-|-+-|-| |-|-----------+
| +---|-| |-|-+ | | | | +-|-| |-|---+ |
| | | +-----+ | | +-----+ | | +-----+ | | |
| | | | | | | | | |
| | | | | | | | | |
| | | +-----+ | | +-----+ | | +-----+ | | |
| +-----+---|-| |-|-+ | | | | +-|-| |-|---+-----+ |
| | | +-|-| |-|-+-|-| |-|-+-|-| |-|-+ | | |
| | | | | +-----+ | | | +-----+ | | | +-----+ | | | | |
| | | | +---------+ | | | | +---------+ | | | |
| | | | | | | | | | | |
+-----+ +-----+ | | +-----+ | | +-----+ +-----+
| ASN | | | +-|-| |-|-+ | | | |
|65YYY| | ... | | | | | | ... | | ... |
+-----+ +-----+ | +-----+ | +-----+ +-----+
| | | | +---------+ | | | |
O O O O <- Servers -> O O O O
Figure 4: BGP ASN layout for 5-stage Clos
5.2.2. Private Use BGP ASNs
The original range of Private Use BGP ASNs [RFC6996] limited
operators to 1023 unique ASNs. Since it is quite likely that the
number of network devices may exceed this number, a workaround is
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required. One approach is to re-use the ASNs assigned to the Tier-3
devices across different clusters. For example, Private Use BGP ASNs
65001, 65002 ... 65032 could be used within every individual cluster
and assigned to Tier-3 devices.
To avoid route suppression due to the AS_PATH loop detection
mechanism in BGP, upstream EBGP sessions on Tier-3 devices must be
configured with the "AllowAS In" feature that allows accepting a
device's own ASN in received route advertisements. Introducing this
feature does not create an opportunity for routing loops under
misconfiguration since the AS_PATH is always incremented when routes
are propagated between topology tiers. Loop protection is also in
place at the Tier-1 device, which does not accept routes with a path
including its own ASN.
Another solution to this problem would be using four-octet BGP ASNs
([RFC6793]), where there are additional Private Use ASN's available,
see [IANA.AS]. Use of Four-Octet BGP ASNs put additional protocol
complexity in the BGP implementation so should be considered against
the complexity of re-use when considering REQ3 and REQ4. Perhaps
more importantly, they are not yet supported by all BGP
implementations, which may limit vendor selection of DC equipment.
5.2.3. Prefix Advertisement
A Clos topology features a large number of point-to-point links and
associated prefixes. Advertising all of these routes into BGP may
create FIB overload conditions in the network devices. Advertising
these links also puts additional path computation stress on the BGP
control plane for little benefit. There are two possible solutions:
o Do not advertise any of the point-to-point links into BGP. Since
the EBGP based design changes the next-hop address at every
device, distant networks will automatically be reachable via the
advertising EBGP peer and do not require reachability to these
prefixes. However, this may complicate operational
troubleshooting or monitoring systems if the addresses are not
reachable: e.g. using the popular "traceroute" tool will display
IP addresses that are not reachable.
o Advertise point-to-point links, but summarize them on every
device. This requires an address allocation scheme such as
allocating a consecutive block of IP addresses per Tier-1 and
Tier-2 device to be used for point-to-point interface addressing
to the lower layers (Tier-2 uplinks will be numbered out of Tier-1
addressing and so forth).
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Server subnets on Tier-3 devices must be announced into BGP without
using route summarization on Tier-2 and Tier-1 devices. Summarizing
subnets in a Clos topology results in route black-holing under a
single link failure (e.g. between Tier-2 and Tier-3 devices) and
hence must be avoided. The use of peer links within the same tier to
resolve the black-holing problem by providing "bypass paths" is
undesirable due to O(N^2) complexity of the peering mesh and waste of
ports on the devices. An alternative to the full-mesh of peer-links
would be using a simpler bypass topology, e.g. a "ring" as described
in [FB4POST], but such a topology adds extra hops and has very
limited bisection bandwidth, in addition requiring special tweaks to
make BGP routing work - such as possibly splitting every device into
an ASN on its own. In Section 8.2 another, less intrusive, method
for performing a limited form route summarization in Clos networks
and the associated trade-offs are described.
5.2.4. External Connectivity
A dedicated cluster (or clusters) in the Clos topology could be used
for the purpose of connecting to the Wide Area Network (WAN) edge
devices, or WAN Routers. Tier-3 devices in such cluster would be
replaced with WAN routers, and EBGP peering would be used again,
though WAN routers are likely to belong to a public ASN if Internet
connectivity is required in the design. The Tier-2 devices in such a
dedicated cluster will be referred to as "Border Routers" in this
document. These devices have to perform a few special functions:
o Hide network topology information when advertising paths to WAN
routers, i.e. remove Private BGP ASNs from the AS_PATH attribute.
This is typically done to avoid ASN number collisions between
different data centers and also to provide a uniform AS_PATH
length to the WAN for purposes of WAN ECMP to Anycast prefixes
originated in the topology. An implementation specific BGP
feature typically called "Remove Private AS" is commonly used to
accomplish this. Depending on implementation, the feature should
strip a contiguous sequence of private ASNs found in AS_PATH
attribute prior to advertising the path to a neighbor. This
assumes that all BGP ASN's used for intra data center numbering
are from the private ASN range. The process for stripping the
private ASNs is not currently standardized, but most
implementations commonly follow the logic described in
[REMOVE-PRIVATE-AS] vendor's document.
o Originate a default route to the data center devices. This is the
only place where default route can be originated, as route
summarization is risky for the "scale-out" topology.
Alternatively, Border Routers may simply relay the default route
learned from WAN routers. Advertising the default route from
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Border Routers requires that all Border Routers to be fully
connected to the WAN Routers upstream, to provide resistance to a
single-link failure causing the black-holing of traffic. To
prevent chance of operator or implementation error that may impact
EBGP sessions to the WAN routers simultaneously (although these
scenarios are not planned for by many operators since they
represents a multiple failure) it is more desirable to take this
approach rather than introducing complicated conditional default
origination schemes provided by some implementations.
5.2.5. Route Summarization at the Edge
It is often desirable to summarize network reachability information
prior to advertising it to the WAN network due to high amount of IP
prefixes originated from within the data center in a fully routed
network design. For example, a network with 2000 Tier-3 devices will
have at least 2000 servers subnets advertised into BGP, along with
the infrastructure or other prefixes. However, as discussed before,
the proposed network design does not allow for route summarization
due to the lack of peer links inside every tier.
However, it is possible to lift this restriction for the Border
Routers, by devising a different connectivity model for these
devices. There are two options possible:
o Interconnect the Border Routers using a full-mesh of physical
links or using any other "peer-mesh" topology, such as ring or
hub-and-spoke. Configure BGP accordingly on all Border Leafs to
exchange network reachability information - e.g. by adding a mesh
of iBGP sessions. The interconnecting peer links need to be
appropriately sized for traffic that will be present in the case
of a device or link failure underneath the Border Routers.
o Tier-1 devices may have additional physical links provisioned
toward the Border Routers (which are Tier-2 devices from the
perspective of Tier-1). Specifically, if protection from a single
link or node failure is desired, each Tier-1 devices would have to
connect to at least two Border Routers. This puts additional
requirements on the port count for Tier-1 devices and Border
Routers, potentially making it a non-uniform, larger port count,
device with the other devices in the Clos. This also reduces the
number of ports available to "regular" Tier-2 switches and hence
the number of clusters that could be interconnected via Tier-1
layer.
If any of the above options are implemented, it is possible to
perform route summarization at the Border Routers toward the WAN
network core without risking a routing black-hole condition under a
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single link failure. Both of the options would result in non-uniform
topology as additional links have to be provisioned on some network
devices.
6. ECMP Considerations
This section covers the Equal Cost Multipath (ECMP) functionality for
Clos topology and discusses a few special requirements.
6.1. Basic ECMP
ECMP is the fundamental load-sharing mechanism used by a Clos
topology. Effectively, every lower-tier device will use all of its
directly attached upper-tier devices to load-share traffic destined
to the same IP prefix. Number of ECMP paths between any two Tier-3
devices in Clos topology equals to the number of the devices in the
middle stage (Tier-1). For example, Figure 5 illustrates the
topology where Tier-3 device A has four paths to reach servers X and
Y, via Tier-2 devices B and C and then Tier-1 devices 1, 2, 3, and 4
respectively.
Tier-1
+-----+
| DEV |
+->| 1 |--+
| +-----+ |
Tier-2 | | Tier-2
+-----+ | +-----+ | +-----+
+------------>| DEV |--+->| DEV |--+--| |-------------+
| +-----| B |--+ | 2 | +--| |-----+ |
| | +-----+ +-----+ +-----+ | |
| | | |
| | +-----+ +-----+ +-----+ | |
| +-----+---->| DEV |--+ | DEV | +--| |-----+-----+ |
| | | +---| C |--+->| 3 |--+--| |---+ | | |
| | | | +-----+ | +-----+ | +-----+ | | | |
| | | | | | | | | |
+-----+ +-----+ | +-----+ | +-----+ +-----+
| DEV | | | Tier-3 +->| DEV |--+ Tier-3 | | | |
| A | | | | 4 | | | | |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | |
O O O O <- Servers -> X Y O O
Figure 5: ECMP fan-out tree from A to X and Y
The ECMP requirement implies that the BGP implementation must support
multi-path fan-out for up to the maximum number of devices directly
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attached at any point in the topology in upstream or downstream
direction. Normally, this number does not exceed half of the ports
found on a device in the topology. For example, an ECMP fan-out of
32 would be required when building a Clos network using 64-port
devices. The Border Routers may need to have wider fan-out to be
able to connect to multitude of Tier-1 devices if route summarization
at Border Router level is implemented as described in Section 5.2.5.
If a device's hardware does not support wider ECMP, logical link-
grouping (link-aggregation at layer 2) could be used to provide
"hierarchical" ECMP (Layer 3 ECMP followed by Layer 2 ECMP) to
compensate for fan-out limitations. Such approach, however,
increases the risk of flow polarization, as less entropy will be
available to the second stage of ECMP.
Most BGP implementations declare paths to be equal from ECMP
perspective if they match up to and including step (e)
Section 9.1.2.2 of [RFC4271]. In the proposed network design there
is no underlying IGP, so all IGP costs are assumed to be zero or
otherwise the same value across all paths and policies may be applied
as necessary to equalize BGP attributes that vary in vendor defaults,
as has been seen occasionally with MED and origin code. Routing
loops are unlikely due to the BGP best-path selection process which
prefers shorter AS_PATH length, and longer paths through the Tier-1
devices which don't allow their own AS in the path and have the same
ASN are also not possible.
6.2. BGP ECMP over Multiple ASNs
For application load-balancing purposes it is desirable to have the
same prefix advertised from multiple Tier-3 devices. From the
perspective of other devices, such a prefix would have BGP paths with
different AS_PATH attribute values, while having the same AS_PATH
attribute lengths. Therefore, BGP implementations must support load-
sharing over above-mentioned paths. This feature is sometimes known
as "multipath relax" and effectively allows for ECMP to be done
across different neighboring ASNs if all other attributes are equal
as described in the previous section.
6.3. Weighted ECMP
It may be desirable for the network devices to implement weighted
ECMP, to be able to send more traffic over some paths in ECMP fan-
out. This could be helpful to compensate for failures in the network
and send more traffic over paths that have more capacity. The
prefixes that require weighted ECMP would have to be injected using
remote BGP speaker (central agent) over a multihop session as
described further in Section 8.1. If support in implementations is
available, weight-distribution for multiple BGP paths could be
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signaled using the technique described in
[I-D.ietf-idr-link-bandwidth].
6.4. Consistent Hashing
It is often desirable to have the hashing function used to ECMP to be
consistent (see [CONS-HASH]), to minimizing the impact on flow to
next-hop affinity changes when a next-hop is added or removed to ECMP
group. This could be used if the network device is used as a load-
balancer, mapping flows toward multiple destinations - in this case,
losing or adding a destination will not have detrimental effect of
currently established flows. One particular recommendation on
implementing consistent hashing is provided in [RFC2992], though
other implementations are possible. This functionality could be
naturally combined with weighted ECMP, with the impact of the next-
hop changes being proportional to the weight of the given next-hop.
Notice that the usual downside of consistent hashing is increased
load on hardware resource utilization, as typically more space is
required to implement a consistent-hashing region.
7. Routing Convergence Properties
This section reviews routing convergence properties in the proposed
design. A case is made that sub-second convergence is achievable if
the implementation supports fast EBGP peering session deactivation
and timely RIB and FIB update upon failure of the associated link.
7.1. Fault Detection Timing
BGP typically relies on an IGP to route around link/node failures
inside an AS, and implements either a polling based or an event-
driven mechanism to obtain updates on IGP state changes. The
proposed routing design does not use an IGP, so the only mechanisms
that could be used for fault detection are BGP keep-alive process (or
any other type of keep-alive mechanism) and link-failure triggers.
Relying solely on BGP keep-alive packets may result in high
convergence delays, in the order of multiple seconds (on many BGP
implementations the minimum configurable BGP hold timer value is
three seconds). However, many BGP implementations can shut down
local EBGP peering sessions in response to the "link down" event for
the outgoing interface used for BGP peering. This feature is
sometimes called as "fast fallover". Since links in modern data
centers are often point-to-point fiber connections, a physical
interface failure is often detected in milliseconds and subsequently
triggers a BGP re-convergence.
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Ethernet technologies may support failure signaling or detection
standards such as [IEEE8021AG] and [IEEE8023AH], which may make
failure detection more robust. Alternatively, some platforms may
support Bidirectional Forwarding Detection (BFD) [RFC5880] to allow
for sub-second failure detection and fault signaling to the BGP
process. However, use of either of these presents additional
requirements to vendor software and possibly hardware, and may
contradict REQ1. Until recently with [RFC7130], BFD also did not
allow detection of a single member link failure on a LAG, which would
limit's it's usefulness in some designs.
7.2. Event Propagation Timing
In this design the impact of BGP Minimum Route Advertisement Interval
(MRAI) timer (See section 9.2.1.1 of [RFC4271]) should be considered.
Per the standard it is required for BGP implementations to space out
consecutive BGP UPDATE messages by at least MRAI seconds, which is
often a configurable value. The initial BGP UPDATE messages after an
event carrying withdrawn routes are commonly not affected by this
timer. The MRAI timer may present significant convergence delays
when a BGP speaker "waits" for the new path to be learned from its
peers and has no local backup path information.
In a Clos topology each EBGP speaker has either one path or N paths
for the same prefix, where N is a significantly large number, e.g.
N=32 (the ECMP fan-out). Therefore, if a path fails there is either
no backup path at all, or the backup is readily available in BGP Loc-
RIB. In the former case, the BGP withdrawal announcement will
propagate un-delayed and trigger re-convergence on affected devices.
In the latter case, the best-path will be re-evaluated and the local
ECMP group corresponding to the new next-hop set changed. If the BGP
path was the best-path selected previously, an "implicit withdraw"
will be sent via a BGP UPDATE message as described as option b in
Section 3.1 of [RFC4271] due to the BGP AS_PATH attribute changing.
7.3. Impact of Clos Topology Fan-outs
Clos topology has large fan-outs, which may impact the "Up->Down"
convergence in some cases, as described in this section. In a
situation when a link between Tier-3 and Tier-2 device fails, the
Tier-2 device will send BGP WITHDRAW message to all upstream Tier-1
devices, and Tier-1 devices will relay this message to all downstream
Tier-2 devices (except for the originator). Tier-2 devices other
than the one originating the WITHDRAW should then wait for ALL
adjacent Tier-1 devices to send a WITHDRAW message before it removes
the affected prefixes and sends corresponding WITHDRAW downstream to
connected Tier-3 devices. If the original Tier-2 device or the
relaying Tier-1 devices introduce some delay into their
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announcements, the result could be WITHDRAW message "dispersion",
that could be as long as multiple seconds. In order to avoid such
behavior, BGP implementations must support "update groups". The
"update group" is defined as a collection of neighbors sharing the
same outbound policy - the local speaker will send BGP updates to the
members of the group synchronously.
The impact of such "dispersion" grows with the size of topology fan-
out and could also grow under network convergence churn.
7.4. Failure Impact Scope
A network is declared to converge in response to a failure once all
devices within the failure impact scope are notified of the event and
have re-calculated their RIB's and consequently updated their FIB's.
Larger failure impact scope typically means slower convergence since
more devices have to be notified, and additionally results in a less
stable network. In this section we describe BGP's advantages over
link-state routing protocols in reducing failure impact scope for a
Clos topology.
BGP is behaves like a distance-vector protocol in the sense that only
the best path from the point of view of the local router is sent to
neighbors. As such, some failures are masked if the local node can
immediately find a backup path and does not have to send any updates
further. Notice that in the worst case ALL devices in a data center
topology have to either withdraw a prefix completely or update the
ECMP groups in the FIB. However, many failures will not result in
such a wide impact. There are two main failure types where impact
scope is reduced:
o Failure of a link between Tier-2 and Tier-1 devices: In this case,
a Tier-2 device will update the affected ECMP groups, removing the
failed link. There is no need to send new information to
downstream Tier-3 devices, unless the path was selected as best by
the BGP process, in which case only an "implicit withdraw" needs
to be sent, which should not affect forwarding. The affected
Tier-1 device will lose the only path available to reach a
particular cluster and will have to withdraw the associated
prefixes. Such prefix withdrawal process will only affect Tier-2
devices directly connected to the affected Tier-1 device. The
Tier-2 devices receiving the BGP UPDATE messages withdrawing
prefixes will simply have to update their ECMP groups. The Tier-3
devices are not involved in the re-convergence process.
o Failure of a Tier-1 device: In this case, all Tier-2 devices
directly attached to the failed node will have to update their
ECMP groups for all IP prefixes from non-local cluster. The
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Tier-3 devices are once again not involved in the re-convergence
process, but may receive "implicit withdraws" as described above.
Even though in case of such failures multiple IP prefixes will have
to be reprogrammed in the FIB, it is worth noting that ALL of these
prefixes share a single ECMP group on Tier-2 device. Therefore, in
the case of implementations with a hierarchical FIB, only a single
change has to be made to the FIB. Hierarchical FIB here means FIB
structure where the next-hop forwarding information is stored
separately from the prefix lookup table, and the latter only store
pointers to the respective forwarding information.
Even though BGP offers some failure scope reduction, reduction of the
fault domain using summarization is not always possible with the
proposed design, since using this technique may create routing black-
holes as mentioned previously. Therefore, the worst control-plane
failure impact scope is the network as a whole, for instance in a
case of a link failure between Tier-2 and Tier-3 devices. The amount
of impacted prefixes in this case would be much less than in the case
of a failure in the upper layers of a Clos network topology. The
property of having such large failure scope is not a result of
choosing EBGP in the design but rather a result of using the "scale-
out" Clos topology.
7.5. Routing Micro-Loops
When a downstream device, e.g. Tier-2 device, loses all paths for a
prefix, it normally has the default route pointing toward the
upstream device, in this case the Tier-1 device. As a result, it is
possible to get in the situation when Tier-2 switch loses a prefix,
but Tier-1 switch still has the path pointing to the Tier-2 device,
which results in transient micro-loop, since Tier-1 switch will keep
passing packets to the affected prefix back to Tier-2 device, and
Tier-2 will bounce it back again using the default route. This
micro-loop will last for the duration of time it takes the upstream
device to fully update its forwarding tables.
To minimize impact of the micro-loops, Tier-2 and Tier-1 switches can
be configured with static "discard" or "null" routes that will be
more specific than the default route for specific prefixes missing
during network convergence. For Tier-2 switches, the discard route
should be a summary route, covering all server subnets of the
underlying Tier-3 devices. For Tier-1 devices, the discard route
should be a summary covering the server IP address subnet allocated
for the whole data-center. Those discard routes will only take
precedence for the duration of network convergence, until the device
learns a more specific prefix via a new path.
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8. Additional Options for Design
8.1. Third-party Route Injection
BGP allows for a "third-party", i.e. directly attached, BGP speaker
to inject routes anywhere in the network topology, meeting REQ5.
This can be achieved by peering via a multihop BGP session with some
or even all devices in the topology. Furthermore, BGP diverse path
distribution [RFC6774] could be used to inject multiple BGP next hops
for the same prefix to facilitate load-balancing, or using the BGP
ADD-PATH capability [I-D.ietf-idr-add-paths] if supported by the
implementation. Unfortunately, in many implementations ADD-PATH has
been found to only support IBGP properly due to the use cases it was
originally optimized for, which limits the "third-party" peering to
iBGP only, if the feature is used.
To implement route injection in the proposed design a third-party BGP
speaker may peer with Tier-3 and Tier-1 switches, injecting the same
prefix, but using a special set of BGP next-hops for Tier-1 devices.
Those next-hops are assumed to resolve recursively via BGP, and could
be, for example, IP addresses on Tier-3 devices. The resulting
forwarding table programming could provide desired traffic proportion
distribution among different clusters.
8.2. Route Summarization within Clos Topology
As mentioned previously, route summarization is not possible within
the proposed Clos topology since it makes the network susceptible to
route black-holing under single link failures. The main problem is
the limited number of parallel paths between network elements, e.g.
there is only a single path between any pair of Tier-1 and Tier-3
devices. However, some operators may find route aggregation
desirable to improve control plane stability.
If planning on using any technique to summarize within the topology
modeling of the routing behavior and potential for black-holing
should be done not only for single or multiple link failures, but
also fiber pathway failures or optical domain failures if the
topology extends beyond a physical location. Simple modeling can be
done by checking the reachability on devices doing summarization
under the condition of a link or pathway failure between a set of
devices in every Tier as well as to the WAN routers if external
connectivity is present.
Route summarization would be possible with a small modification to
the network topology, though the trade-off would be reduction of the
total size of the network as well as network congestion under
specific failures. This approach is very similar to the technique
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described above, which allows Border Routers to summarize the entire
data-center address space.
8.2.1. Collapsing Tier-1 Devices Layer
In order to add more paths between Tier-1 and Tier-3 devices, group
Tier-2 devices into pairs, and then connect the pairs to the same
group of Tier-1 devices. This is logically equivalent to
"collapsing" Tier-1 devices into a group of half the size, merging
the links on the "collapsed" devices. The result is illustrated in
Figure 6. For example, in this topology DEV C and DEV D connect to
the same set of Tier-1 devices (DEV 1 and DEV 2), whereas before they
were connecting to different groups of Tier-1 devices.
Tier-2 Tier-1 Tier-2
+-----+ +-----+ +-----+
+-------------| DEV |------| DEV |------| |-------------+
| +-----| C |--++--| 1 |--++--| |-----+ |
| | +-----+ || +-----+ || +-----+ | |
| | || || | |
| | +-----+ || +-----+ || +-----+ | |
| +-----+-----| DEV |--++--| DEV |--++--| |-----+-----+ |
| | | +---| D |------| 2 |------| |---+ | | |
| | | | +-----+ +-----+ +-----+ | | | |
| | | | | | | |
+-----+ +-----+ +-----+ +-----+
| DEV | | DEV | | | | |
| A | | B | Tier-3 Tier-3 | | | |
+-----+ +-----+ +-----+ +-----+
| | | | | | | |
O O O O <- Servers -> O O O O
Figure 6: 5-Stage Clos topology
Having this design in place, Tier-2 devices may be configured to
advertise only a default route down to Tier-3 devices. If a link
between Tier-2 and Tier-3 fails, the traffic will be re-routed via
the second available path known to a Tier-2 switch. It is not
possible to advertise a summary route covering prefixes for a single
cluster from Tier-2 devices since each of them has only a single path
down to this prefix. It would require dual-homed servers to
accomplish that. Also note that this design is only resilient to
single link failure. It is possible for a double link failure to
isolate a Tier-2 device from all paths toward a specific Tier-3
device, thus causing a routing black-hole.
A result of the proposed topology modification would be reduction of
Tier-1 devices port capacity. This limits the maximum number of
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attached Tier-2 devices and therefore will limit the maximum DC
network size. A larger network would require different Tier-1
devices that have higher port density to implement this change.
Another problem is traffic re-balancing under link failures. Since
three are two paths from Tier-1 to Tier-3, a failure of the link
between Tier-1 and Tier-2 switch would result in all traffic that was
taking the failed link to switch to the remaining path. This will
result in doubling of link utilization on the remaining link.
8.2.2. Simple Virtual Aggregation
A completely different approach to route summarization is possible,
provided that the main goal is to reduce the FIB pressure, while
allowing the control plane to disseminate full routing information.
Firstly, it could be easily noted that in many cases multiple
prefixes, some of which are less specific, share the same set of the
next-hops (same ECMP group). For example, looking from the
perspective of a Tier-3 devices, all routes learned from upstream
Tier-2's, including the default route, will share the same set of BGP
next-hops, provided that there is no failures in the network. This
makes it possible to use the technique similar to described in
[RFC6769] and only install the least specific route in the FIB,
ignoring more specific routes if they share the same next-hop set.
For example, under normal network conditions, only the default route
need to be programmed into FIB.
Furthermore, if the Tier-2 devices are configured with summary
prefixes covering all of their attached Tier-3 device's prefixes the
same logic could be applied in Tier-1 devices as well, and, by
induction to Tier-2/Tier-3 switches in different clusters. These
summary routes should still allow for more specific prefixes to leak
to Tier-1 devices, to enable for detection of mismatches in the next-
hop sets if a particular link fails, changing the next-hop set for a
specific prefix.
Re-stating once again, this technique does not reduce the amount of
control plane state (i.e. BGP UPDATEs/BGP LocRIB sizing), but only
allows for more efficient FIB utilization, by spotting more specific
prefixes that share their next-hops with less specifics.
8.3. ICMP Unreachable Message Masquerading
This section discusses some operational aspects of not advertising
point-to-point link subnets into BGP, as previously outlined as an
option in Section 5.2.3. The operational impact of this decision
could be seen when using the well-known "traceroute" tool.
Specifically, IP addresses displayed by the tool will be the link's
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point-to-point addresses, and hence will be unreachable for
management connectivity. This makes some troubleshooting more
complicated.
One way to overcome this limitation is by using the DNS subsystem to
create the "reverse" entries for the IP addresses of the same device
pointing to the same name. The connectivity then can be made by
resolving this name to the "primary" IP address of the devices, e.g.
its Loopback interface, which is always advertised into BGP.
However, this create dependency on DNS subsystem, which may happen to
be unavailable during an outage.
Another option is to make the network device perform IP address
masquerading, that is rewriting the source IP addresses of the
appropriate ICMP messages sent off of the device with the "primary"
IP address of the device. Specifically, the ICMP Destination
Unreachable Message (type 3) codes 3 (port unreachable) and ICMP Time
Exceeded (type 11) code 0, which are involved in proper working of
the "traceroute" tool. With this modification, the "traceroute"
probes sent to the devices will always be sent back with the
"primary" IP address as the source, allowing the operator to discover
the "reachable" IP address of the box.
9. Security Considerations
The design does not introduce any additional security concerns.
General BGP security considerations are discussed in [RFC4271] and
[RFC4272]. Furthermore, the Generalized TTL Security Mechanism
[RFC5082] could be used to reduce the risk of BGP session spoofing.
10. IANA Considerations
This document includes no request to IANA.
11. Acknowledgements
This publication summarizes work of many people who participated in
developing, testing and deploying the proposed network design, some
of whom were George Chen, Parantap Lahiri, Dave Maltz, Edet Nkposong,
Robert Toomey, and Lihua Yuan. Authors would also like to thank
Linda Dunbar, Susan Hares, Russ White and Robert Raszuk for reviewing
the document and providing valuable feedback and Mary Mitchell for
grammar and style suggestions.
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12. References
12.1. Normative References
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC6996] Mitchell, J., "Autonomous System (AS) Reservation for
Private Use", BCP 6, RFC 6996, July 2013.
12.2. Informative References
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC
4272, January 2006.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, December 2006.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, October 2007.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC6325] Perlman, R., Eastlake, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC6774] Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K.
Kumaki, "Distribution of Diverse BGP Paths", RFC 6774,
November 2012.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793, December
2012.
[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992, November 2000.
[RFC6769] Raszuk, R., Heitz, J., Lo, A., Zhang, L., and X. Xu,
"Simple Virtual Aggregation (S-VA)", RFC 6769, October
2012.
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[RFC7130] Bhatia, M., Chen, M., Boutros, S., Binderberger, M., and
J. Haas, "Bidirectional Forwarding Detection (BFD) on Link
Aggregation Group (LAG) Interfaces", RFC 7130, February
2014.
[I-D.ietf-idr-add-paths]
Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", draft-ietf-idr-
add-paths-10 (work in progress), October 2014.
[I-D.ietf-idr-link-bandwidth]
Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
Extended Community", draft-ietf-idr-link-bandwidth-06
(work in progress), January 2013.
[GREENBERG2009]
Greenberg, A., Hamilton, J., and D. Maltz, "The Cost of a
Cloud: Research Problems in Data Center Networks", January
2009.
[IEEE8021AG]
IEEE 802.1Q, , "IEEE Standard for Local and metropolitan
area networks - Media Access Control (MAC) Bridges and
Virtual Bridged Local Area Networks", October 2012.
[IEEE8023AH]
IEEE 802.3, , "IEEE Standard for Information technology -
Local and metropolitan area networks - Carrier sense
multiple access with collision detection (CSMA/CD) access
method and physical layer specifications", December 2008.
[INTERCON]
Dally, W. and B. Towles, "Principles and Practices of
Interconnection Networks", ISBN 978-0122007514, January
2004.
[ALFARES2008]
Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable,
Commodity Data Center Network Architecture", August 2008.
[IANA.AS] IANA, , "Autonomous System (AS) Numbers", April 2015,
<http://www.iana.org/assignments/as-numbers/>.
[IEEE8023AD]
IEEE 802.3ad, , "IEEE Standard for Link aggregation for
parallel links", October 2000.
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[REMOVE-PRIVATE-AS]
Cisco Systems, , "Removing Private Autonomous System
Numbers in BGP", August 2005,
<http://www.cisco.com/en/US/tech/tk365/
technologies_tech_note09186a0080093f27.shtml>.
[FB4POST] Farrington, N. and A. Andreyev, "Facebook's Data Center
Network Architecture", May 2013,
<http://nathanfarrington.com/papers/facebook-oic13.pdf>.
[JAKMA2008]
Jakma, P., "BGP Path Hunting", 2008,
<https://blogs.oracle.com/paulj/entry/bgp_path_hunting>.
[CONS-HASH]
Wikipedia, , "Consistent Hashing",
<http://en.wikipedia.org/wiki/Consistent_hashing>.
Authors' Addresses
Petr Lapukhov
Facebook
1 Hacker Way
Menlo Park, CA 94025
US
Email: petr@fb.com
Ariff Premji
Arista Networks
5453 Great America Parkway
Santa Clara, CA 95054
US
Email: ariff@arista.com
URI: http://arista.com/
Jon Mitchell (editor)
Email: jrmitche@puck.nether.net
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