Networking Working Group N. Shen
Internet-Draft Cisco Systems
Intended status: Standards Track S. Amante
Expires: July 12, 2018 Apple, Inc.
M. Abrahamsson
T-Systems Nordic
January 8, 2018
IS-IS Routing with Reverse Metric
draft-ietf-isis-reverse-metric-08
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 12, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Shen, et al. Expires July 12, 2018 [Page 1]
Internet-Draft IS-IS Reverse Metric January 2018
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 LANs . . . . . . . . . . . . . 8
3.6. Operational Guidelines . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
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 . . . . . . . . . . . . . 11
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
single link or LAN for the maintenance. More detailed descriptions
Shen, et al. Expires July 12, 2018 [Page 2]
Internet-Draft IS-IS Reverse Metric January 2018
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-
to-point or multi-access LAN link. This would allow the provisioning
Shen, et al. Expires July 12, 2018 [Page 3]
Internet-Draft IS-IS Reverse Metric January 2018
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.
The metric value in the "reverse metric" TLV and the TE metric in the
sub-TLV being advertised is an offset or relative metric to be added
on top of the existing local link and TE metric value of the
receiver.
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-TLVs. If the "sub-TLV len" is non-zero, then the Value field
MUST also contain data of 1 or more Extended IS Reachability sub-
TLVs.
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. If a received IS-IS Hello
PDU contains more than one Reverse Metric TLV, an implementation
SHOULD ignore all the Reverse Metric TLVs in this error condition.
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
Shen, et al. Expires July 12, 2018 [Page 4]
Internet-Draft IS-IS Reverse Metric January 2018
TYPE: TBD (be replaced by the value that IANA allocates)
LENGTH: variable (5 - 255 octets)
VALUE:
Flags (1 octet)
Metric Offset (3 octets)
sub-TLV length (1 octet)
sub-TLV data (0 - 250 octets)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |U|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" of the IS-IS link. 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 LANs. 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.
U bit (0x02): The "Unreachable" bit is used by the IS-IS node to
request the neighbor for setting the accumulated metric value to be
limited to (2^24-1). This "U" bit applies to both the default metric
of Extended IS Reachability TLV and the TE default-metric sub-TLV of
the link. This is only relevant to the IS-IS "wide" metric mode.
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
Shen, et al. Expires July 12, 2018 [Page 5]
Internet-Draft IS-IS Reverse Metric January 2018
[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-
TLVs is outside the scope of this document. The "sub-TLV Len" value
MUST be set to zero when an IS-IS router does not have TE sub-TLVs
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]
[RFC5817]. It is important to use the same IS-IS metric mode in both
ends of the link. On the receiving side of the 'reverse-metric' TLV,
the accumulated value of configured metric and the reverse-metric
needs to be limited to 63 in "narrow" metric mode and to (2^24 - 2)
in "wide" metric mode. This applies to both the default metric of
Extended IS Reachability TLV and the TE default-metric sub-TLV in LSP
or Pseudonode LSP with the "wide" metric mode case. If the "U" bit
is present in the flag, the accumulated metric value is to be limited
to (2^24 - 1) instead, and this applies to both the normal link
metric and TE metric in IS-IS "wide" metric mode.
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 NOT change the value of its TE Default Metric sub-
TLV for that link.
Routers MUST scan the Metric Offset value and TE sub-TLVs in all
subsequently received Reverse Metric TLVs. 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 TLVs, recompute its SPF tree and flood new LSPs to
other IS-IS routers.
Shen, et al. Expires July 12, 2018 [Page 6]
Internet-Draft IS-IS Reverse Metric January 2018
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 PDUs 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 TLVs 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-TLVs in all of its MT Intermediate Systems TLVs. If an M-ISIS
router is configured to advertise TE Default Metric sub-TLVs 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-TLVs for all topologies. The M-ISIS should flood
its newly updated MT IS TLVs 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.
The Reverse Metric TE sub-TLV also applies to the DIS. If a DIS is
configured to apply TE over the link and it receives TE metric sub-
TLV in Reverse Metric TLV, it should update TE Default Metric sub-TLV
value of corresponding Extended IS Reachability TLV or insert new one
if it was not present there.
In the case of multi-access LANs, the "W" Flags bit is used to signal
from a non-DIS to the DIS whether to change the metric and optionally
Shen, et al. Expires July 12, 2018 [Page 7]
Internet-Draft IS-IS Reverse Metric January 2018
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 TLVs. 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.
3.5. LDP/IGP Synchronization on LANs
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.
Shen, et al. Expires July 12, 2018 [Page 8]
Internet-Draft IS-IS Reverse Metric January 2018
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. 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
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 TLVs.
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 PDUs to
mitigate use of the Reverse Metric TLV as a potential attack vector,
particularly on multi-access LANs.
Shen, et al. Expires July 12, 2018 [Page 9]
Internet-Draft IS-IS Reverse Metric January 2018
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, possibly from the "Unassigned" range of 244-250, 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, Uma
Chunduri, Alexander Okonnikov, Jonathan Harrison, Dave Ward, Himanshu
Shah, Wes George, Danny McPherson, Ed Crabbe, Russ White, Robert
Razsuk and Tom Petch for their comments and 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, <https://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, <https://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, <https://www.rfc-
editor.org/info/rfc5120>.
Shen, et al. Expires July 12, 2018 [Page 10]
Internet-Draft IS-IS Reverse Metric January 2018
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://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, <https://www.rfc-editor.org/info/rfc5443>.
[RFC5817] Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton,
"Graceful Shutdown in MPLS and Generalized MPLS Traffic
Engineering Networks", RFC 5817, DOI 10.17487/RFC5817,
April 2010, <https://www.rfc-editor.org/info/rfc5817>.
[RFC5919] Asati, R., Mohapatra, P., Chen, E., and B. Thomas,
"Signaling LDP Label Advertisement Completion", RFC 5919,
DOI 10.17487/RFC5919, August 2010, <https://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, <https://www.rfc-editor.org/info/rfc6138>.
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 (LSPs) of the IS-IS router about
to undergo maintenance. The IS-IS router immediately floods the
updated LSPs to all IS-IS routers throughout the IS-IS domain. Upon
receipt of the updated LSPs, all IS-IS routers recalculate their
Shortest Path First (SPF) tree excluding IS-IS routers whose LSPs
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 LSPs of the original IS-IS router
causing it to flood updated IS-IS LSPs 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
Shen, et al. Expires July 12, 2018 [Page 11]
Internet-Draft IS-IS Reverse Metric January 2018
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 LANs, 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 (LSPs) 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.
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 LANs, 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
Shen, et al. Expires July 12, 2018 [Page 12]
Internet-Draft IS-IS Reverse Metric January 2018
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 (SLAs) for all network services.
Appendix C. 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
Mikael Abrahamsson
T-Systems Nordic
Kistagangen 26
Stockholm
SE
Email: Mikael.Abrahamsson@t-systems.se
Shen, et al. Expires July 12, 2018 [Page 13]