IS-IS Working Group N. Shen
Internet-Draft T. Li
Intended status: Standards Track Cisco Systems, Inc.
Expires: June 2, 2011 S. Amante
Level 3 Communications
M. Abrahamsson
Tele2
November 29, 2010
IS-IS Reverse Metric TLV for Network Maintenance Events
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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This Internet-Draft will expire on June 2, 2011.
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . 8
3.4. Order of Operations . . . . . . . . . . . . . . . . . . . 9
3.5. Operational Guidelines . . . . . . . . . . . . . . . . . . 9
4. Reverse Metric TLV Example Use Case . . . . . . . . . . . . . 10
5. Operational Considerations . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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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
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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)
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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 of an IS-IS
metric a neighbor SHOULD add to the existing, configured "default
metric" contained within its IS Neighbors TLV or 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 that originated this Reverse Metric TLV. Refer to
"Elements of Procedure", below in Section 3, for details of 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 (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 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 receipt
of 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-
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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 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 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
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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
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, according to the recommendations outlined in
Section 3.4, Order of Operations, below.
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.
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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
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.
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 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 highest source MAC address from IIH PDU's containing Reverse
Metric TLV's it receives as the tie-breaker to determine the sole
Reverse Metric 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 the source MAC address was highest in IIH PDU's containing a
Reverse Metric TLV received from Router B, then the DIS MUST add the
Metric value to the default metric 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 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 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.
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Another scenario is that one node, Router A, may signal the DIS with
the W bit set to 1. The DIS would 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 add the Metric value in the Reverse Metric
TLV from Router B to the default metric for Router B in the
Pseudonode LSP. The DIS MUST honor a neighbor's Reverse Metric TLV
to update its individual default metric in the Pseudonode LSP even if
the DIS receives prior or later requests to assert a Whole LAN metric
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
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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 add the received Metric or TE default
metric sub-TLV value to its own Traffic Engineering Default Metric
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
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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
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.
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,
Jonathan Harrison, Dave Ward and Himanshu Shah for their
contributions.
9. References
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9.1. Normative References
[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
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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
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