wdiff draft-amante-isis-reverse-metric-00.txt draft-amante-isis-reverse-metric-01.txt

IS-IS Working Group N. Shen
Internet-Draft T. Li
Intended status: Standards Track Cisco Systems, Inc.
Expires: April 16, June 2, 2011 S. Amante
Level 3 Communications
M. Abrahamsson
Tele2
October 13,
November 29, 2010

IS-IS Reverse Metric TLV for Network Maintenance Events
draft-amante-isis-reverse-metric-00
draft-amante-isis-reverse-metric-01

Abstract

This document describes an improved IS-IS neighbor management scheme
which can be used to enhance network performance by allowing
operators to quickly and accurately shift traffic away from a point-
to-point or multi-access LAN interface by allowing one IS-IS router
to signal to a second, adjacent IS-IS neighbor to adjust its IS-IS
metric that should be used to temporarily reach the first IS-IS
router during network maintenance events.

Status of this Memo

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on April 16, June 2, 2011.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Link Isolation Challenges . . . . . . . . . . . . . . . . 3
1.2. IS-IS Reverse Metric . . . . . . . . . . . . . . . . . . . 4
1.3. 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 8
3.4. Order of Operations . . . . . . . . . . . . . . . . . . . 9
3.5. Operational Guidelines . . . . . . . . . . . . . . . . . . 9

4. Reverse Metric TLV Example Use Case . . . . . . . . . . . . . 8 10

5. Operational Considerations . . . . . . . . . . . . . . . . . . 9 11

6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 11

7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 11

8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 11

9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 10 12

Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 12

1. Introduction

The IS-IS [ISO 10589] 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 one issue and a new mechanism for improving it.

1.1. Link Isolation Challenges

During network maintenance events, operators substantially increase
the IS-IS metric simultaneously on both devices attached to the same
link. 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 fault diagnosis and restoration on
a link without disturbing ongoing communications in the network.

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.

1.2. IS-IS Reverse Metric

This document proposes that the routing protocol itself be the
transport mechanism to allow one IS-IS router to advertise to an
adjacent node on a point-to-point or multi-access LAN link a "reverse
metric" in a IS-IS Hello (IIH) PDU. This would allow an operator to
only configure a single router, set a "reverse metric" on a link and
have traffic bidirectionally shift away from that link gracefully to
alternate, viable paths.

1.3. 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 1 octet for the Type, 1 octet
that specifies the number of bytes in the Value field and a variable-
length Value field. The Value field starts with a 1 octet field of
Flags followed by a 3 octet field containing an IS-IS Metric and,
lastly, 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.

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)

Flags

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |S|W|
+-+-+-+-+-+-+-+-+

Figure 1: Flags

The Reverse Metric TLV Type is TBD. Please refer to IANA
Considerations, in Section 7, for more details.

The Metric field contains a 24-bit unsigned integer equal to the of an IS-IS
metric a neighbor SHOULD set in add to the existing, configured "default
metric" contained within its own IS Neighbors TLV or Extended IS
Reachability TLV 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 that originated this Reverse Metric TLV. An IS-IS neighbor MUST overwrite the existing IS-IS metric, in
its corresponding IS Neighbors, Extended IS Reachability TLV or
Pseudonode LSP, with the value it received Refer to
"Elements of Procedure", below in the Metric field Section 3, for details of how an
IS-IS router should process the
Reverse Metric TLV.

The Metric field may be a "default metric", in the range of 0-63, or a "Traffic Engineering Default Metric" [RFC5305], in the range of
0-2^(24-1) depending on the configuration of router's interface that
is originating the Reverse Metric TLV. It is RECOMMENDED that
implementations, by default, place the appropriate maximum default
metric value, 63 or 2^(24-1), in the Metric field of the Reverse
Metric TLV, since the most common use is to remove the link from the
topology, except for use as a last-resort path.

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 MUST replace the IS-IS metric for all nodes in the Pseudonode LSP
with SHOULD add the received Metric value received in the Reverse Metric TLV. TLV to
each node's existing "default metric" in the Pseudonode LSP. If the
"Whole LAN" bit is not set (0), then a DIS MUST replace SHOULD add the IS-IS
metric received
Metric value in the Reverse Metric TLV to the existing "default
metric" in the Pseudonode LSP for just the single node from whom the
Reverse Metric TLV was received. Please refer to the Elements of Procedure, "Multi-Access LAN
Procedures", in Section 3, 3.3, for additional details. In addition, the The W bit MUST
be unset (0) when a Reverse Metric TLV is transmitted in a IIH PDU
onto a point-to-point link to an IS-IS neighbor.

S bit (0x02): The "TE sub-TLV" bit MUST be set (1) when an IS-IS
router wishes to signal that its neighbor alter parameters contained
in the neighbor's IPv4 and/or IPv6 Traffic Engineering "Extended IS Reachability TLV",
as defined in [RFC5305] for IPv4 and
[I-D.ietf-isis-ipv6-te] for IPv6. An IS-IS router MUST overwrite [RFC5305]. This document defines that only the subset
"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 receipt
of its own this TE sub-TLV's with those sub-TLV's received
from a neighbor sub-TLV in the 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 (0) when an IS-IS router does not have TE
sub-TLV's that it wishes to send to its IS-IS neighbor.

3. Elements of Procedure

3.1. Processing Changes to Default Metric

The values used for Metric field, in the "IPv4 Interface Address" and "IPv6 Interface
Address" TE sub-TLV's MUST Reverse Metric TLV, is a "default metric"
that will either be set to all zero in the range of 0 - 63 when sent inside 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. In addition, TLV, since the "IPv4 Neighbor Address" and
"IPv6 Neighbor Address"
most common use is to remove the link from the topology, except for
use as a last-resort path.

In order to ensure that an individual TE sub-TLV's link is used as a link of
last resort during SPF computation, its metric MUST NOT be set greater
than or equal to local node's
interface address(es) that is originating (2^24 - 1) [RFC5305]. Therefore, a receiver of a
Reverse Metric TLV.

3. Elements TLV MUST use the numerically smallest value of either
the sum of Procedure

A router SHOULD first update its own IS-IS existing default metric and/or Traffic
Engineering parameters and the Metric value in the
Reverse Metric TLV or (2^24 - 2), as the default metric when updating
its IS Neighbors TLV, Extended IS Reachability TLV or Pseudonode LSP, and TE default-metric sub-TLV's that
it will then recompute its SPF tree plus
corresponding route metrics and, lastly, flood its updated LSP's, throughout the IS-IS domain, using normal IS-IS mechanisms, as well as start advertising
procedures. Likewise, originators of a Reverse
Metric Pseudonode LSP or IS
Neighbors TLV in IIH's toward a neighbor. A router MUST advertise 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 toward a neighbor only for or 63 when updating the period during which
it wants a neighbor to temporarily update its corresponding
Pseudonode LSP or IS Neighbor TLV before they are flooded. This also
applies when an IS-IS metric router is only configured or TE
parameters.

When capable of sending
a router "narrow" IS-IS default metric, in the range of 0 - 63, but receives
a "wide" Metric value in a Reverse Metric TLV it TLV, in the range of 64 -
(2^24 - 2). In this case, the receiving router MUST immediately
update use the maximum
"narrow" IS-IS default metric, 63, as its own IS Neighbors TLV, Extended IS-IS default metric value
in its updated IS Reachability Neighbor TLV or Pseudonode LSP with the received value(s) in the Metric field or TE
sub-TLV's, then recalculate its SPF tree and associated route metrics
and, finally, flood its updated LSP's to other IS-IS routers. Note that on it floods.

If an IS-IS router is configured to originate a Multi-Access LAN, only the DIS SHOULD act upon information
contained in TE Default Metric
sub-TLV for a link, but receives a received Reverse Metric TLV. All non-DIS nodes MUST
silently ignore TLV from its
neighbor that does not contain a received Reverse TE Default Metric TLV. Please refer to
Section 3.1 for additional details with respect to Multi-Access LAN's
and sub-TLV, then the
IS-IS router MUST add the value in the Reverse Metric TLV.

Routers that receive a field of the Reverse
Metric TLV MAY send a syslog message
or SNMP trap, in order to assist in rapidly identifying the node in
the network its own TE Default Metric sub-TLV for that is asserting an link. The
IS-IS metric or Traffic Engineering
parameters different from that which is configured locally on router should then flood the
device. 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, number
of TE sub-TLV's value or data in the TE sub-TLV's,
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
corresponding metrics to IP prefixes and, finally, flood new LSP's to
other IS-IS routers.

When a routers, according to the recommendations outlined in
Section 3.4, Order of Operations, below.

If the router stops receiving a does not understand the Reverse Metric TLV or is
explicitly configured to ignore received Reverse Metric TLV's, then
it MUST
immediately NOT update the default metric in its own IS Neighbors TLV,
Extended IS Reachability TLV, TE Default Metric sub-TLV, Multi-
Topology Intermediate Systems TLV or Pseudonode LSP with the previously configured IS-IS metric
value and/or Traffic Engineering parameters, recalculate nor execute other
procedures that would result from acting on a Reverse Metric TLV,
such as recomputing its SPF and
associated route metrics and flood updated LSP's within the tree.

3.2. Processing Changes to Default Metric for Multi-Topology IS-IS
domain.

The Reverse Metric TLV is RECOMMENDED that implementations provide a capability applicable to
disable any changes 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 node's
Reverse Metric TLV it MUST add the received Metric value to its
default metric or Traffic Engineering
parameters based upon receipt of properly formatted 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 the an M-ISIS
router is configured to advertise TE Default Metric sub-TLV's for one
or more topologies, but does not understand 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 or is
explicitly configured to ignore received Reverse each of the TE Default
Metric TLV's, then
it will not update nor sub-TLV's for all topologies. The M-ISIS should flood a new IS Neighbors TLV, Extended its
newly updated MT IS
Reachability TLV or Pseudonode LSP TLV's and should not 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
tree or update metrics associated with corresponding routes.

3.1. 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.

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/or 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, i.e.: 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 replace add the Metric value to the default metric and/or TE parameters
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 replace add their
respective own
Metric value to their default metric and/or TE parameters 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 change add the default metric and/or
TE parameters
contained in its Pseudonode LSP for the specific neighbor that sent
the Reverse Metric TLV.

It is possible for one node, Router A, to signal to the DIS with the
W bit set to 1, in which case the DIS would replace the metric and/or
TE parameters for all neighbor adjacencies in the Pseudonode LSP with add the Metric value in
the Reverse Metric TLV to all neighbor adjacencies in the Pseudonode
LSP and transmit a new Pseudonode LSP to all nodes in the IS-IS
domain. Later, a second node on the LAN, Router B, could signal to
the DIS with the W bit also set to 1. In this case, the DIS MUST use
the Reverse Metric TLV
Value field(s) advertised by the router with highest source MAC address of
the two routers from which IIH PDU's containing Reverse
Metric TLV's it received a receives as the tie-breaker to determine the sole
Reverse Metric TLV, Router A
or B. TLV used as the source for the Metric value that will
be added to the default metric for all nodes in the Pseudonode LSP.
If Router B's the source MAC address was highest, highest in IIH PDU's containing a
Reverse Metric TLV received from Router B, then the DIS MUST update add the
Metric value to the default metric and/or Traffic Engineering parameters for of all neighbors in its Pseudonode
LSP and flood the LSP to all nodes in the IS-IS domain. On the other
hand, if the DIS determines that Router A's MAC address was IIH PDU's, containing
Reverse Metric TLV's, have the highest source MAC address, then the
DIS will ignore Router B's Reverse Metric TLV and continue to use the
Metric value found in Router A's Reverse Metric TLV Value field(s) for to add to the
default metric of all neighbors in the Pseudonode LSP. When this
occurs, the DIS MAY send a single syslog message or SNMP trap
indicating that it has received a Reverse Metric TLV from a neighbor,
but is ignoring it due to it being received from a neighbor with a
lower MAC address.

Another scenario is that one node, Router A, may signal the DIS with
the W bit set to 1. The DIS would update add the Metric value to the
default metric for all neighbors in the Pseudonode LSP and flood the
LSP. Later, a second node on the LAN, Router B, could signal the DIS
with the W bit set to 0, which indicates to the DIS that Router B is
requesting the DIS only update add the Metric value in the Reverse Metric
TLV from Router B to the default metric and/or TE parameters for Router B in the
Pseudonode LSP. The DIS MUST honor a neighbor's Reverse Metric TLV
to update its individual IS-IS default metric and/or TE parameters in the Pseudonode LSP even if
the DIS receives prior or later requests to assert a Whole LAN metric or TE parameter(s) change
from other nodes on the same LAN.

In all cases above, the DIS is MUST use 0 as the base default-metric
value for each neighbor contained in the Pseudonode LSP to which the
DIS will add the Metric value in the Reverse Metric TLV(s) it
receives from neighbors on the LAN.

Local configuration on the DIS to adjust the default metric(s)
contained in the Pseudonode LSP, as documented in
[I-D.shen-isis-oper-enhance] MUST take precedence over received
Reverse Metric TLV's.

3.4. Order of Operations

When an IS-IS router starts or stops generating a Reverse Metric TLV,
it will go through a process of updating its own IS-IS metric and
optionally Traffic Engineering parameters in its IS Neighbors TLV,
Extended IS Reachbaility TLV or Pseudonode LSP, flooding updated
LSP's (using normal IS-IS mechanisms), recompute its SPF/CSPF tree
plus corresponding metrics to IP prefixes, update its FIB and begin
advertising the Reverse Metric TLV in IIH PDU's toward its
corresponding neighbor(s) on the appropriate link or LAN. Likewise,
when IS-IS neighbor(s) start or stop receiving a Reverse Metric TLV,
they will go through a similar process. It is critical that devices
which implement the Reverse Metric TLV conduct this process in a
deterministic order that minimizes the possibilities to generate
temporary micro forwarding loops during a metric increase and
decrease.

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.

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 stored (e.g.: hard disk, etc.) configurations, 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
receipt of properly formatted Reverse Metric TLV's.

4. Reverse Metric TLV Example Use Case

The following is a brief example illustrating one use case of the
Reverse Metric TLV. In order to isolate a point-to-point link from
the IS-IS network, an operator would configure one router, Router A,
attached to a point-to-point link with a "Reverse Metric". This
should not affect the configuration of the existing IS-IS default
metric previously configured on the router's interface. Assuming
Router A is using IS-IS Extensions for Traffic Engineering [RFC5305],
this should trigger Router A to update its Traffic Engineering
Default Metric sub-TLV in its own Extended IS Reachability TLV,
recompute its SPF tree and corresponding metrics to IP prefixes in
the IS-IS domain and begin the process of flooding a new LSP
throughout the network. Router A would also begin transmitting a
Reverse Metric TLV, with an appropriate Metric value, in an IIH PDU,
to its adjacent neighbor, Router B. Upon receipt of the Reverse
Metric TLV, Router B would also update its Traffic Engineering
Default Metric sub-TLV with add the received Metric or TE default
metric sub-TLV value in the Reverse to its own Traffic Engineering Default Metric TLV,
sub-TLV, recalculate its SPF tree and associated route topology as
well as start flooding a new LSP containing the updated Extended IS
Reachability TLV throughout the network. As nodes in the network
receive the associated LSP's from Router A and B and recalculate a
new SPF tree, and route topology, traffic should gracefully shift
onto alternate paths away from the A-B link; ultimately, after all
nodes in the network recompute their SPF tree link A-B should only be
used as a link of last-resort. The operator can inspect traffic
counters on the A-B interface to determine if the link was
successfully isolated from the topology and proceed with necessary
fault diagnosis or maintenance of the associated link.

When the maintenance activity is complete, the operator would remove
the reverse metric configuration from Router A, which would cease
advertisement of the Reverse Metric TLV in IIH PDU's to Router B.
Both routers would revert to their originally configured IS-IS
metric, recompute new SPF trees and corresponding metrics to IP
prefixes and originate new LSP's. As the new LSP's are received and
SPF is recalculated by nodes in the IS-IS domain, traffic should
gradually shift back onto link A-B.

5. Operational Considerations

Since the Reverse Metric TLV may not be recognized by adjacent IS-IS
neighbors, operators should inspect input and output traffic
throughput counters on the local router to ensure that traffic has
bidirectionally shifted away from a link before starting any
maintenance activities.

6. Security Considerations

This

The enhancement in this document raises no new security issues makes it possible for IS-IS. 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.

7. 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.

8. Acknowledgements

The authors would like to thank Mike Shand, Dave Katz, Guan Deng,
Ilya Varlashkin, Jay Chen, Les Ginsberg and Peter Ashwood-Smith Ashwood-Smith,
Jonathan Harrison, Dave Ward and Himanshu Shah for their
contributions.

9. References
9.1. Normative References

[I-D.ietf-isis-ipv6-te]
Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
Engineering in IS-IS", draft-ietf-isis-ipv6-te-08 (work in
progress), September 2010.

[ISO 10589]
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, December 1990.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.

9.2. Informative References

[I-D.shen-isis-oper-enhance]
Shen, N., Li, T., Amante, S., and M. Abrahamsson, "IS-IS
Operational Enhancements for Network Maintenance Events",
draft-shen-isis-oper-enhance-00 (work in progress),
October 2010.

Authors' Addresses

Naiming Shen
Cisco Systems, Inc.
225 West Tasman Drive
San Jose, CA 95134
USA

Email: naiming@cisco.com
Tony Li
Cisco Systems, Inc.
225 West Tasman Drive
San Jose, CA 95134
USA

Email: tli@cisco.com

Shane Amante
Level 3 Communications
1025 Eldorado Blvd
Broomfield, CO 80021
USA

Email: shane@level3.net

Mikael Abrahamsson
Tele2

Email: swmike@swm.pp.se