Networking Working Group N. Shen
Internet-Draft Cisco Systems
Intended status: Standards Track S. Amante
Expires: September 7, 2017 Apple, Inc.
M. Abrahamsson
T-Systems Nordic
March 6, 2017
IS-IS Routing with Reverse Metric
draft-ietf-isis-reverse-metric-05
Abstract
This document describes the mechanism to allow IS-IS routing to
quickly and accurately shift traffic away from either a point-to-
point or multi-access LAN interface by signaling to an adjacent IS-IS
neighbor with the metric towards itself during network maintenance or
other operational events.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Node and Link Isolation . . . . . . . . . . . . . . . . . 2
1.2. Distributed Forwarding Planes . . . . . . . . . . . . . . 3
1.3. Mobility Cases . . . . . . . . . . . . . . . . . . . . . 3
1.4. Spine-Leaf Applications . . . . . . . . . . . . . . . . . 3
1.5. IS-IS Reverse Metric . . . . . . . . . . . . . . . . . . 3
1.6. Specification of Requirements . . . . . . . . . . . . . . 4
2. IS-IS Reverse Metric TLV . . . . . . . . . . . . . . . . . . 4
3. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 6
3.1. Processing Changes to Default Metric . . . . . . . . . . 6
3.2. Processing Changes to Default Metric for Multi-Topology
IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Multi-Access LAN Procedures . . . . . . . . . . . . . . . 7
3.4. Point-To-Point Link Procedures . . . . . . . . . . . . . 8
3.5. Operational Guidelines . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Node Isolation Challenges . . . . . . . . . . . . . 11
Appendix B. Link Isolation Challenges . . . . . . . . . . . . . 11
Appendix C. Use of Reverse Metric for LDP/IGP Synchronization on
LAN's . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix D. Contributors' Addresses . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The IS-IS [ISO10589] routing protocol has been widely used in
Internet Service Provider IP/MPLS networks. Operational experience
with the protocol, combined with ever increasing requirements for
lossless operations have demonstrated some operational issues. This
document describes the issues and a new mechanism for improving it.
1.1. Node and Link Isolation
IS-IS routing mechanism has the overload-bit, which can be used by
operators to perform disruptive maintenance on the router. But in
many operational maintenance cases, it is not necessary to displace
all the traffic away from this node. It is useful to augment only a
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single link or LAN for the maintenance. More detailed descriptions
of the challenges can be found in Appendix A and Appendix B of this
document.
1.2. Distributed Forwarding Planes
In a distributed forwarding platform, different forwarding line-cards
may have interfaces and IS-IS connections to neighbor routers. If
one of the line-card's software resets, it may take some time for the
forwarding entries to be fully populated on this line-card. The IS-
IS adjacency may be established with a neighbor router long before
the entire BGP prefixes are downloaded to the forwarding table. It
is important to signal to the network not to use this particular IS-
IS adjacency inbound to this router if possible. Temporarily pushing
out the 'Reverse Metric' over this link to discourage the traffic
into this line-card will help to reduce the traffic loss in the
network.
1.3. Mobility Cases
When the IS-IS is run on some mobile devices, either in point-to-
point links or in broadcast networks, it is important to have the
routing metric to influence the traffic in both directions. When a
node is moving farther away, it not only needs to raise the cost for
traffic from this router to the network, but also it is important to
raise the cost for the traffic from the network towards the router.
When a node is moving closer, it can lower the cost on both metrics.
1.4. Spine-Leaf Applications
In the IS-IS Spine-Leaf extension [I-D.shen-isis-spine-leaf-ext], the
leaf nodes will perform equal-cost or unequal-cost load sharing
towards all the spine nodes. In certain operational cases, for
instance, when one of the backbone links on a spine node is
congested, this spine node can push a higher metric towards the
connected leaf nodes to reduce the transit traffic through this spine
node or link.
1.5. IS-IS Reverse Metric
This document proposes that the routing protocol itself be the
transport mechanism to allow one IS-IS router to advertise a "reverse
metric" in an IS-IS Hello (IIH) PDU to an adjacent node on a point-
to-point or multi-access LAN link. This would allow the provisioning
to be performed only on a single node, set a "reverse metric" on a
link and have traffic bidirectionally shift away from that link
gracefully to alternate, viable paths.
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This Reverse Metric mechanism is to be used for both point-to-point
and multi-access LAN links. Unlike the point-to-point link, IS-IS
protocol currently does not have a way to influence the traffic
towards a particular node on LAN links. This proposal enables IS-IS
routing the capability of altering traffic in both directions on
either a point-to-point link or on a multi-access link of a node.
1.6. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. IS-IS Reverse Metric TLV
The Reverse Metric TLV is composed of a 1 octet field of Flags, a 3
octet field containing an IS-IS Metric, and a 1 octet Traffic
Engineering (TE) sub-TLV length field representing the length of a
variable number of Extended Intermediate System (IS) Reachability
sub-TLV's. If the 'S' bit in the Flags field is set to 1, then the
Value field MUST also contain data of 1 or more Extended IS
Reachability sub-TLV's.
The Reverse Metric TLV is optional. The Reverse Metric TLV may be
present in any IS-IS Hello PDU. A sender MUST only transmit a single
Reverse Metric TLV in a IS-IS Hello PDU.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags | Metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metric (Continue) | sub-TLV Len |Optional sub-TLV
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reverse Metric TLV
TYPE: TBD
LENGTH: variable (5 - 255 octets)
VALUE:
Flags (1 octet)
Metric (3 octets)
TE sub-TLV length (1 octet)
TE sub-TLV data (0 - 250 octets)
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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |S|W|
+-+-+-+-+-+-+-+-+
Figure 1: Flags
The Metric field contains a 24-bit unsigned integer of an IS-IS
metric that a neighbor SHOULD add to the existing, configured
"default metric" contained within its IS Neighbors TLV, Extended IS
Reachability TLV's for point-to-point links, or Pseudonode LSP by the
Designated Intermediate System (DIS) for multi-access LAN's, back
toward the router and the link that originated this Reverse Metric
TLV. Refer to "Elements of Procedure", in Section 3 for details on
how an IS-IS router should process the Metric field in a Reverse
Metric TLV.
There is currently only two Flag bits defined.
W bit (0x01): The "Whole LAN" bit is only used in the context of
multi-access LAN's. When a Reverse Metric TLV is transmitted from a
(non-DIS) node to the DIS, if the "Whole LAN" bit is set (1), then a
DIS SHOULD add the received Metric value in the Reverse Metric TLV to
each node's existing "default metric" in the Pseudonode LSP. If the
"Whole LAN" bit is not set (0), then a DIS SHOULD add the received
Metric value in the Reverse Metric TLV to the existing "default
metric" in the Pseudonode LSP for the single node from whom the
Reverse Metric TLV was received. Please refer to "Multi-Access LAN
Procedures", in Section 3.3, for additional details. The W bit MUST
be unset when a Reverse Metric TLV is transmitted in a IIH PDU onto a
point-to-point link to a neighbor, and the W bit MUST be ignored upon
receiving on a point-to-point link.
S bit (0x02): The "TE sub-TLV" bit MUST be set when an IS-IS router
wishes to signal that its neighbor alter parameters contained in the
neighbor's Traffic Engineering "Extended IS Reachability TLV", as
defined in [RFC5305]. This document defines that only the "Traffic
Engineering Default Metric" sub-TLV, sub-TLV Type 18, may be sent
toward neighbors in the Reverse Metric TLV, because that is used in
Constrained Shortest Path First (CSPF) computations. Upon receiving
this TE sub-TLV in a Reverse Metric TLV, a node SHOULD add the
received TE default metric to its existing, configured TE default
metric within its Extended IS Reachability TLV. Use of other sub-
TLV's is outside the scope of this document. The S bit MUST NOT be
set when an IS-IS router does not have TE sub-TLV's that it wishes to
send to its IS-IS neighbor.
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3. Elements of Procedure
3.1. Processing Changes to Default Metric
The Metric field, in the Reverse Metric TLV, is a "default metric"
that will either be in the range of 0 - 63 when a "narrow" IS-IS
metric is used (IS Neighbors TLV, Pseudonode LSP) [RFC1195] or in the
range of 0 - (2^24 - 2) when a "wide" Traffic Engineering metric
value is used, (Extended IS Reachability TLV) [RFC5305]. It is
RECOMMENDED that implementations, by default, place the appropriate
maximum default metric value, 63 or (2^24 - 2), in the Metric field
and TE Default Metric sub-TLV of the Reverse Metric TLV, since the
most common use is to indicate the link of the router is overloaded
and to remove the link from the topology, except for use as a last-
resort path.
In order to ensure that an individual TE link is used as a link of
last resort during SPF computation, its metric MUST NOT be greater
than or equal to (2^24 - 1) [RFC5305]. Therefore, a receiver of a
Reverse Metric TLV MUST use the numerically smallest value of either
the sum of its existing default metric and the Metric value in the
Reverse Metric TLV or (2^24 - 2), as the default metric when updating
its Extended IS Reachability TLV and TE default-metric sub-TLV's that
it will then flood throughout the IS-IS domain, using normal IS-IS
procedures. Likewise, originators of a Pseudonode LSP or IS
Neighbors TLV MUST use the numerically smallest value of either the
sum of its existing default metric and the Metric value it receives
in a Reverse Metric TLV or 63 when updating the corresponding
Pseudonode LSP or IS Neighbor TLV before they are flooded. This also
applies when an IS-IS router is only configured or capable of sending
a "narrow" IS-IS default metric, in the range of 0 - 63, but receives
a "wide" Metric value in a Reverse Metric TLV, in the range of 64 -
(2^24 - 2). In this case, the receiving router MUST use the maximum
"narrow" IS-IS default metric, 63, as its IS-IS default metric value
in its updated IS Neighbor TLV or Pseudonode LSP that it floods.
If an IS-IS router is configured to originate a TE Default Metric
sub-TLV for a link, but receives a Reverse Metric TLV from its
neighbor that does not contain a TE Default Metric sub-TLV, then the
IS-IS router MUST add the value in the Metric field of the Reverse
Metric TLV to its own TE Default Metric sub-TLV for that link. The
IS-IS router should then flood the updated Extended IS Reachability
TLV, including its updated TE Default Metric sub-TLV, using normal
IS-IS procedures.
Routers MUST scan the Metric value and TE sub-TLV's in all
subsequently received Reverse Metric TLV's. If changes are observed
by a receiver of the Reverse Metric TLV in the Metric value or TE
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Default Metric sub-TLV value, the receiving router MUST update its
advertised IS-IS default metric or Traffic Engineering parameters in
the appropriate TLV's, recompute its SPF tree and flood new LSP's to
other IS-IS routers.
If the router does not understand the Reverse Metric TLV or is
explicitly configured to ignore received Reverse Metric TLV's, then
it MUST NOT update the default metric in its IS Neighbors TLV,
Extended IS Reachability TLV, TE Default Metric sub-TLV, Multi-
Topology Intermediate Systems TLV, or Pseudonode LSP, nor execute
other procedures that would result from acting on a Reverse Metric
TLV, such as recomputing its SPF tree.
3.2. Processing Changes to Default Metric for Multi-Topology IS-IS
The Reverse Metric TLV is applicable to Multi-Topology IS-IS (M-ISIS)
[RFC5120] capable point-to-point links. If an IS-IS router is
configured for M-ISIS it MUST send only a single Reverse Metric TLV
in IIH PDU's toward its neighbor(s) on the designated link that is
about to undergo maintenance. When an M-ISIS router receives a
Reverse Metric TLV it MUST add the received Metric value to its
default metric in all Extended IS Reachability TLV's for all
topologies. If an M-ISIS router receives a Reverse Metric TLV with a
TE Default Metric sub-TLV, then the M-ISIS router MUST add the
received TE Default Metric value to each of its TE Default Metric
sub-TLV's in all of its MT Intermediate Systems TLV's. If an M-ISIS
router is configured to advertise TE Default Metric sub-TLV's for one
or more topologies, but does not receive a TE Default Metric sub-TLV
in a Reverse Metric TLV, then the M-ISIS router MUST add the value in
Metric field of the Reverse Metric TLV to each of the TE Default
Metric sub-TLV's for all topologies. The M-ISIS should flood its
newly updated MT IS TLV's and recompute its SPF/CSPF accordingly.
Multi-Topology IS-IS [RFC5120] specifies there is no change to
construction of the Pseudonode LSP, regardless of the Multi-Topology
capabilities of a multi-access LAN. If any MT capable node on the
LAN advertises the Reverse Metric TLV to the DIS, the DIS should act
according to the "Multi-Access LAN Procedures" in Section 3.3 to
update, as appropriate, the default metric contained in the
Pseudonode LSP. If the DIS updates the default metric in and floods
a new Pseudonode LSP, those default metric values will be applied to
all topologies during Multi-Topology SPF calculations.
3.3. Multi-Access LAN Procedures
On a Multi-Access LAN, only the DIS SHOULD act upon information
contained in a received Reverse Metric TLV. All non-DIS nodes MUST
silently ignore a received Reverse Metric TLV.
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In the case of multi-access LAN's, the "W" Flags bit is used to
signal from a non-DIS to the DIS whether to change the metric and
optionally Traffic Engineering parameters for all nodes in the
Pseudonode LSP or a single node on the LAN, (the originator of the
Reverse Metric TLV).
A non-DIS node, e.g.: Router B, attached to a multi-access LAN will
send a Reverse Metric TLV with the W bit set to 0 to the DIS, when
Router B wishes the DIS to add the Metric value to the default metric
contained in the Pseudonode LSP specific to just Router B. Other
non-DIS nodes, i.e.: Routers C and D, may simultaneously send a
Reverse Metric TLV with the W bit set to 0 to request the DIS add
their own Metric value to their default metric contained in the
Pseudonode LSP. When the DIS receives a properly formatted Reverse
Metric TLV with the W bit set to 0, the DIS MUST only add the default
metric contained in its Pseudonode LSP for the specific neighbor that
sent the Reverse Metric TLV.
As long as at least one IS-IS node on the LAN sending the signal to
DIS with the W bit set, the DIS would add the metric value in the
Reverse Metric TLV to all neighbor adjacencies in the Pseudonode LSP,
regardless if some of the nodes on the LAN send the Reverse Metric
TLV without the W bit set. The DIS MUST use the metric of the
highest source MAC address of the node sending the TLV with the W bit
set. The DIS MUST use the metric value towards the nodes which
explicitly send the Reverse Metric TLV.
Local provisioning on the DIS to adjust the default metric(s)
contained in the Pseudonode LSP MUST take precedence over received
Reverse Metric TLV's. For instance, local policy of the DIS may be
provisioned to ignore the W bit signaling on a LAN.
3.4. Point-To-Point Link Procedures
On a point-to-point link, there is already a "configured" IS-IS
interface metric to be applied over the link towards the IS-IS
neighbor.
When IS-IS receives the IIH PDU with the "Reverse Metric" on a point-
to-point link and if the local policy allows the supporting of
"Reverse Metric", it MUST add the metric value in the "Metric" field
of the TLV to the locally configured interface metric value to be the
metric for this IS-IS adjacency. The metric MUST NOT exceed the
maximum allowed value used in either "narrow" or "wide" metric mode.
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3.5. Operational Guidelines
A router MUST advertise a Reverse Metric TLV toward a neighbor only
for the period during which it wants a neighbor to temporarily update
its IS-IS metric or TE parameters towards it.
During the period when a Reverse Metric TLV is used, IS-IS routers
that are generating and receiving a Reverse Metric TLV MUST NOT
change their existing IS-IS metric or Traffic Engineering parameters
in their persistent provisioning database, since those parameters are
carefully derived from off-line capacity planning tools and are
difficult to restore to their original values.
Routers that receive a Reverse Metric TLV MAY send a syslog message
or SNMP trap, in order to assist in rapidly identifying the node in
the network that is asserting an IS-IS metric or Traffic Engineering
parameters different from that which is configured locally on the
device.
It is RECOMMENDED that implementations provide a capability to
disable any changes to a node's, or individual interfaces of the
node, default metric or Traffic Engineering parameters based upon
receiving properly formatted Reverse Metric TLV's.
4. Security Considerations
The enhancement in this document makes it possible for one IS-IS
router to manipulate the IS-IS default metric or optionally Traffic
Engineering parameters of adjacent IS-IS neighbors. Although IS-IS
routers within a single Autonomous System nearly always reside under
the control of a single administrative authority, it is highly
RECOMMENDED that operators configure authentication of IS-IS PDU's to
mitigate use of the Reverse Metric TLV as a potential attack vector,
particularly on multi-access LAN's.
5. IANA Considerations
This document requests that IANA allocate from the IS-IS TLV
Codepoints Registry a new TLV, referred to as the "Reverse Metric"
TLV, with the following attributes: IIH = y, LSP = n, SNP = n, Purge
= n.
6. Acknowledgments
The authors would like to thank Mike Shand, Dave Katz, Guan Deng,
Ilya Varlashkin, Jay Chen, Les Ginsberg, Peter Ashwood-Smith,
Jonathan Harrison, Dave Ward, Himanshu Shah, Wes George, Danny
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McPherson, Ed Crabbe, Russ White and Robert Razsuk for their
contributions.
This document was produced using Marshall Rose's xml2rfc tool.
7. References
7.1. Normative References
[I-D.shen-isis-spine-leaf-ext]
Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
Routing for Spine-Leaf Topology", draft-shen-isis-spine-
leaf-ext-03 (work in progress), March 2017.
[ISO10589]
ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with
the Protocol for providing the Connectionless-mode Network
Service (ISO 8473)", ISO/IEC 10589:2002.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <http://www.rfc-editor.org/info/rfc1195>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<http://www.rfc-editor.org/info/rfc5120>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <http://www.rfc-editor.org/info/rfc5305>.
7.2. Informative References
[RFC5919] Asati, R., Mohapatra, P., Chen, E., and B. Thomas,
"Signaling LDP Label Advertisement Completion", RFC 5919,
DOI 10.17487/RFC5919, August 2010,
<http://www.rfc-editor.org/info/rfc5919>.
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Appendix A. Node Isolation Challenges
On rare occasions it is necessary for an operator to perform
disruptive network maintenance on an entire IS-IS router node, i.e.:
major software upgrades, power/cooling augments, etc. In these
cases, an operator will set the IS-IS Overload Bit (OL-bit) within
the Link State Protocol Data Units (LSP's) of the IS-IS router about
to undergo maintenance. The IS-IS router immediately floods the
updated LSP's to all IS-IS routers throughout the IS-IS domain. Upon
receipt of the updated LSP's, all IS-IS routers recalculate their
Shortest Path First (SPF) tree excluding IS-IS routers whose LSP's
have the OL-bit set. This effectively removes the IS-IS router about
to undergo maintenance from the topology, thus preventing it from
forwarding any transit traffic during the maintenance period.
After the maintenance activity is completed, the operator resets the
IS-IS Overload Bit within the LSP's of the original IS-IS router
causing it to flood updated IS-IS LSP's throughout the IS-IS domain.
All IS-IS routers recalculate their SPF tree and now include the
original IS-IS router in their topology calculations, allowing it to
be used for transit traffic again.
Isolating an entire IS-IS router from the topology can be especially
disruptive due to the displacement of a large volume of traffic
through an entire IS-IS router to other, sub-optimal paths, (i.e.:
those with significantly larger delay). Thus, in the majority of
network maintenance scenarios, where only a single link or LAN needs
to be augmented to increase its physical capacity or is experiencing
an intermittent failure, it is much more common and desirable to
gracefully remove just the targeted link or LAN from service,
temporarily, so that the least amount of user-data traffic is
affected while intrusive augment, diagnostic and/or replacement
procedures are being executed.
Appendix B. Link Isolation Challenges
Before network maintenance events are performed on individual
physical links or LAN's, operators substantially increase the IS-IS
metric simultaneously on both devices attached to the same link or
LAN. In doing so, the devices generate new Link State Protocol Data
Units (LSP's) that are flooded throughout the network and cause all
routers to gradually shift traffic onto alternate paths with very
little, to no, disruption to in-flight communications by applications
or end-users. When performed successfully, this allows the operator
to confidently perform disruptive augmentation, fault diagnosis or
repairs on a link without disturbing ongoing communications in the
network.
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The challenge with the above solution are as follows. First, it is
quite common to have routers with several hundred interfaces onboard
and individual interfaces that are transferring several hundred
Gigabits/second to Terabits/second of traffic. Thus, it is
imperative that operators accurately identify the same point-to-point
link on two, separate devices in order to increase (and, afterward,
decrease) the IS-IS metric appropriately. Second, the aforementioned
solution is very time consuming and even more error-prone to perform
when its necessary to temporarily remove a multi-access LAN from the
network topology. Specifically, the operator needs to configure ALL
devices's that have interfaces attached to the multi-access LAN with
an appropriately high IS-IS metric, (and then decrease the IS-IS
metric to its original value afterward). Finally, with respect to
multi-access LAN's, there is currently no method to bidirectionally
isolate only a single node's interface on the LAN when performed more
fine-grained diagnosis and repairs to the multi-access LAN.
In theory, use of a Network Management System (NMS) could improve the
accuracy of identifying the appropriate subset of routers attached to
either a point-to-point link or a multi-access LAN as well as
signaling from the NMS to those devices, using a network management
protocol, to adjust the IS-IS metrics on the pertinent set of
interfaces. The reality is that NMS are, to a very large extent, not
used within Service Provider's networks for a variety of reasons. In
particular, NMS do not interoperate very well across different
vendors or even separate platform families within the same vendor.
The risks of misidentifying one side of a point-to-point link or one
or more interfaces attached to a multi-access LAN and subsequently
increasing its IS-IS metric are potentially increased latency, jitter
or packet loss. This is unacceptable given the necessary performance
requirements for a variety of applications, the customer perception
for near lossless operations and the associated, demanding Service
Level Agreement's (SLA's) for all network services.
Appendix C. Use of Reverse Metric for LDP/IGP Synchronization on LAN's
This document primarily outlines the use of IS-IS Reverse Metric TLV
for networks that use IP forwarding. However, it is also critical to
consider application of the IS-IS Reverse Metric TLV to networks that
use MPLS forwarding, specifically networks that use IS-IS as the IGP
and LDP for signaling MPLS labels used for forwarding. In these
networks, it is often the case that IS-IS will become operational and
determine the shortest path through a link or LAN prior to LDP
becoming operational (forming an adjacency with a LDP neighbor and
exchanging LDP labels), which results in temporary blackholing for
data traffic reliant on MPLS forwarding.
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This scenario should be avoided in MPLS networks where IS-IS is the
IGP and LDP signaling is used to exchange tunnel labels over a LAN.
In these cases, it is recommended that the IS-IS Reverse Metric TLV
be utilized when IS-IS and LDP adjacencies are in the process of
becoming established among one, or several, routers attached to a
common multi-access LAN.
Specifically, when an IS-IS adjacency is being established from a
non-DIS node, the non-DIS should transmit a IS-IS Reverse Metric TLV
toward the DIS with the W-bit not set (0), as per
"Elements of Procedure" in Section 3 of this document, until the non-
DIS router either: a) completes transmission of a LDP End-of-LIB
marker [RFC5919] toward the DIS; or, b) expiration of a local (pre-
configured) timer that indicates that LDP adjacency should be fully
operational to the DIS. At this point, the non-DIS router should
cease advertisement of the IS-IS Reverse Metric TLV, which should
cause the (re-)advertisement of normal default metric(s) to itself in
the Pseudonode LSP.
Appendix D. Contributors' Addresses
Tony Li
Email: tony.li@tony.li
Authors' Addresses
Naiming Shen
Cisco Systems
560 McCarthy Blvd.
Milpitas, CA 95035
USA
Email: naiming@cisco.com
Shane Amante
Apple, Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
Email: samante@apple.com
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Mikael Abrahamsson
T-Systems Nordic
Kistagangen 26
Stockholm
SE
Email: Mikael.Abrahamsson@t-systems.se
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