Networking Working Group N. Shen
Internet-Draft Cisco Systems
Intended status: Standards Track S. Amante
Expires: November 6, 2017 Apple, Inc.
M. Abrahamsson
T-Systems Nordic
May 5, 2017
IS-IS Routing with Reverse Metric
draft-ietf-isis-reverse-metric-06
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. Spine-Leaf Applications . . . . . . . . . . . . . . . . . 3
1.4. LDP IGP Synchronization . . . . . . . . . . . . . . . . . 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. LDP/IGP Synchronization on LAN's . . . . . . . . . . . . 9
3.6. Link Overload Attribute Bit . . . . . . . . . . . . . . . 9
3.7. Operational Guidelines . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Node Isolation Challenges . . . . . . . . . . . . . 12
Appendix B. Link Isolation Challenges . . . . . . . . . . . . . 12
Appendix C. 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, in
particular if the router is a PE (Provider Edge) router in ISP's MPLS
VPN. The IS-IS adjacency may be established with a neighbor router
long before the entire BGP VPN 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. At the meantime, the remote
PE routers will select a different set of PE routers for the BGP best
path calculation or use a different link towards the same PE router
on which another line-card is recovering.
1.3. 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.
1.4. LDP IGP Synchronization
In the [RFC5443], a mechanism is described to achieve LDP IGP
synchronization by using the maximum link metric value on the
interface. But in the case of a new IS-IS node joining the broadcast
network (LAN), it is not optimal to change all the nodes on the LAN
to the maximum link metric value, as described in [RFC6138]. This
Reverse Metric can be used in this case to discourage both outbound
and inbound traffic without affecting the traffic of other existing
IS-IS nodes on the LAN.
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-
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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.
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 Offset
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metric Offset (Continue) | sub-TLV Len |Optional sub-TLV
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reverse Metric TLV
TYPE: TBD
LENGTH: variable (5 - 255 octets)
VALUE:
Flags (1 octet)
Metric Offset (3 octets)
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sub-TLV length (1 octet)
sub-TLV data (0 - 250 octets)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |W|
+-+-+-+-+-+-+-+-+
Figure 1: Flags
The Metric Offset 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 Offset 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 Offset 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 Offset 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.
The "sub-TLV Len" value is non-zero 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 "sub-TLV Len" value
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MUST be set to zero 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 Offset 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 Offset 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 Offset 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 Offset 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 Offset 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.
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Routers MUST scan the Metric Offset 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 Offset 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.
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 Offset 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 Offset 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. The decision process
of the routers on this LAN MUST follow the procedure in section
7.2.8.2 of [ISO10589], and use the "Two-way connectivity check"
during the topology and route calculation.
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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 Offset 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 Offset 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" (63) or "wide" (2^24 -
2) metric mode.
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3.5. LDP/IGP Synchronization on LAN's
As described in [RFC6138] when a new IS-IS node joins a broadcast
network, it is unnecessary and sometimes even harmful to put IS-IS
maximum link metric on all the nodes. [RFC6138] proposes a solution
to have the new node not advertising the adjacency towards the
pseudo-node when it is not in a "cut-edge" position.
With the introduction of Reverse Metric in this document, a simpler
alternative solution to the above mentioned problem can be used. The
Reverse Metric allows the new node on the LAN to have the inbound
metric value to be the maximum and this puts the link of this new
node in the last resort position without impacting the other IS-IS
nodes on the same LAN.
Specifically, when IS-IS adjacencies are being established by the new
node on the LAN, besides setting the maximum link metric value (2^24
- 2) on the interface of the LAN for the LDP IGP synchronization as
described in [RFC5443], it SHOULD advertise the maximum metric offset
value in the Reverse Metric TLV in its IIH PDU to the LAN. It SHOULD
continue this advertisement until it completes all the LDP label
binding exchanges with all the neighbors over this LAN, either by
receiving the LDP End-of-LIB [RFC5919] for all the sessions or by
exceeding the provisioned timeout value on the node.
3.6. Link Overload Attribute Bit
Not every TE tunnel is setup using IS-IS link metric or IS-IS link TE
metric across the domain. Although the larger than normal link
metric or TE metric can be one way to indicate to the PCE controller
that the node on the other side of the link is trying to reduce the
inbound traffic, but a more explicit way is to have the router set a
bit in the "link-attribute" sub-TLV [RFC5029] to express this link is
currently overloaded. How the controller or the source of the TE
tunnel use the "link overload" information in altering the TE tunnel
path is outside the scope of this document.
3.7. 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.
The use of Reverse Metric does not alter IS-IS metric parameters
stored in a router's persistent provisioning database.
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
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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.
This document also request that IANA allocate from the link-attribute
bit value for sub-TLV 19 of TLV 22. This new bit is referred to as
the "Link Overload" bit.
6. Acknowledgments
The authors would like to thank Mike Shand, Dave Katz, Guan Deng,
Ilya Varlashkin, Jay Chen, Les Ginsberg, Peter Ashwood-Smith, Uma
Chunduri, Alexander Okonnikov, Jonathan Harrison, Dave Ward, Himanshu
Shah, Wes George, Danny 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
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[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>.
[RFC5029] Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
September 2007, <http://www.rfc-editor.org/info/rfc5029>.
[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
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
2009, <http://www.rfc-editor.org/info/rfc5443>.
[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>.
[RFC6138] Kini, S., Ed. and W. Lu, Ed., "LDP IGP Synchronization for
Broadcast Networks", RFC 6138, DOI 10.17487/RFC6138,
February 2011, <http://www.rfc-editor.org/info/rfc6138>.
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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. Contributors' Addresses
Tony Li
Email: tony.li@tony.li
Authors' Addresses
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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
Mikael Abrahamsson
T-Systems Nordic
Kistagangen 26
Stockholm
SE
Email: Mikael.Abrahamsson@t-systems.se
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