INTERNET-DRAFT Danny McPherson
Arbor Networks
Naiming Shen
Cisco Systems
Expires: April 2008 October 20, 2007
Intended Status: Proposed Standard
IS-IS Transient Blackhole Avoidance
<draft-ietf-isis-rfc3277bis-00.txt>
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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Abstract
This document describes a simple, interoperable mechanism that can be
employed in IS-IS networks in order to decrease data loss associated
with deterministic blackholing of packets during transient network
conditions. The mechanism proposed here requires no IS-IS protocol
changes and is completely interoperable with the existing IS-IS
specification.
The intention of this document is to provide an update to [RFC 3277].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Specification of Requirements . . . . . . . . . . . . . . . 4
2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Deployment Considerations. . . . . . . . . . . . . . . . . . . 6
4. Manageability Considerations . . . . . . . . . . . . . . . . . 8
5. Security Considerations. . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References. . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References. . . . . . . . . . . . . . . . . . . 10
9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
When an IS-IS router that was previously a transit router becomes
unavailable as a result of some transient condition such as a reboot,
other routers within the routing domain must select an alternative
path to reach destinations which had previously transited the failed
router. Presumably, the newly selected router(s) comprising the path
have been available for some time and, as a result, have complete
forwarding information bases (FIBs) which contain a full set of
reachability information for both internal and external (e.g., BGP)
destination networks.
When the previously failed router becomes available again, in only a
few seconds paths that had previously transited the router are again
selected as the optimal path by the IGP. As a result, forwarding
tables are updated and packets are once again forwarded along the
path. Unfortunately, external destination reachability information
(e.g., learned via BGP) is not yet available to the router, and as a
result, packets bound for destinations not learned via the IGP are
unnecessarily discarded.
A simple interoperable mechanism to alleviate the offshoot associated
with this deterministic behavior is outlined below.
1.1. 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 RFC 2119 [RFC 2119].
2. Discussion
This document describes a simple, interoperable mechanism that can be
employed in IS-IS [ISO 8473] [RFC 1195] networks in order to avoid
transition to a newly available path until other associated routing
protocols such as BGP have had sufficient time to converge.
The benefits of such a mechanism can realized when considering the
scenario depicted in Figure 1.
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D.1
|
+-------+
| RtrD |
+-------+
/ \
/ \
+-------+ +-------+
| RtrB | | RtrC |
+-------+ +-------+
\ /
\ /
+-------+
| RtrA |
+-------+
|
S.1
Figure 1: Example Network Topology
Host S.1 is transmitting data to destination D.1 via a primary path
of RtrA->RtrB->RtrD. Routers A, B and C learn of reachability to
destination D.1 via BGP from RtrD. RtrA's primary path to D.1 is
selected because when calculating the path to BGP NEXT_HOP of RtrD
the sum of the IS-IS link metrics on the RtrA-RtrB-RtrD path is less
than the sum of the metrics of the RtrA-RtrC-RtrD path.
Assume RtrB becomes unavailable and as a result the RtrC path is used
to reach RtrD. Once RtrA's FIB is updated and it begins forwarding
packets to RtrC everything should behave properly as RtrC has
existing forwarding information regarding destination D.1's
availability via BGP NEXT_HOP RtrD.
Assume now that RtrB comes back online. In only a few seconds IS-IS
neighbor state has been established with RtrA and RtrD and database
synchronization has occurred. RtrA now realizes that the best path
to destination D.1 is via RtrB, and subsequently updates it FIB
appropriately. RtrA begins to forward packets destined to D.1 to
RtrB. However, because RtrB has yet to establish and synchronization
it's BGP neighbor relationship and routing information with RtrD,
RtrB has no knowledge regarding reachability of destination D.1, and
therefore discards the packets received from RtrA destined to D.1.
If RtrB were to temporarily set it's LSP Overload bit while
synchronizing BGP tables with it's neighbors, RtrA would continue to
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use the operational RtrA->RtrC->RtrD path, and the IS-IS LSP SHOULD
only be used to obtain reachability to locally connected networks
(rather than for calculating transit paths through the router, as
defined in [ISO 8473]).
However, it should be noted that when RtrB goes away its LSP is still
present in the IS-IS databases of all other routers in the routing
domain. When RtrB comes back it establishes adjacencies. As soon as
its neighbors have an adjacency with RtrB, they will advertise their
new adjacency in their new LSP. The result is that all the other
routers will receive new LSPs from RtrA and RtrD containing the RtrB
adjacency, even though RtrB is still completing its synchronization
and therefore has not yet transmitted it's new LSP.
At this time SPF is computed and everyone will include RtrB in their
tree since they will use the old version of RtrB's LSP (the new one
has not yet arrived). Once RtrB has finished establishing its
adjacencies, it will then regenerate its LSP and flood it. Then all
other routers within the domain will finally compute SPF with the
correct information. Only at that time will the Overload bit be
taken into account.
As such, it is recommended that each time a router establishes an
adjacency, it will update its LSP and flood it immediately, even
before beginning database synchronization. This will allow for the
Overload bit setting to propagate immediately, and remove the
potential for an older version of the reloaded routers LSP to be
used.
After synchronization of BGP tables with neighboring routers (or
expiry of some other timer or trigger), RtrB would generate a new
LSP, clearing the Overload bit, and RtrA (and other routers in the
routing domain) could again begin using the optimal path via RtrB.
Typically, in service provider networks IBGP connections are done via
peering sessions associated with 'loopback' addresses. As such, the
newly available router must advertise it's own loopback (or similar)
IP address, as well as associated adjacencies, in order to make the
loopbacks accessible to other routers within the routing domain.
It's because of this requirement for local destinaion reachability
that simply flooding an empty LSP is not sufficient.
3. Deployment Considerations
Such a mechanism increases overall network availability and allows
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network operators to alleviate the deterministic blackholing behavior
introduced in this scenario. Similar mechanisms [RFC 3137] have been
defined for OSPF, only after realizing the usefulness obtained from
that of the IS-IS Overload bit technique.
This mechanism has been deployed in several large IS-IS networks for
a number of years, and a variety of techniques to configure and
trigger overload bit setting and clearing are available in many
implementations. Such triggers for setting the Overload bit as
described are left to the implementer. Some potential triggers could
perhaps include "N seconds after booting", or "N number of BGP
prefixes in the BGP Loc-RIB".
Unlike similar mechanisms employed in [RFC 3137], if the Overload bit
is set in a router's LSP, NO transit paths are calculated through the
router. As such, if no alternative paths are available to the
destination network, employing such a mechanism may actually have a
negative impact on convergence (i.e., the router maintains the only
available path to reach downstream routers, but the Overload bit
disallows other nodes in the network from calculating paths via the
router, and as such, no feasible path exists to the routers).
It should also be noted that if all systems within an IS-IS routing
domain haven't implemented this Overload bit behavior correctly,
forwarding loops may occur.
Alternatively, it may be considered more appealing to employ
something more akin to [RFC 3137] for this purpose. With this model,
during transient conditions a node advertises excessively high link
metrics to serve as an indication to other nodes in the network that
paths transiting the router are "less desirable" than alternative
paths.
The advantage of a metric-based mechanism over the Overload bit
mechanism proposed here is that transit paths may still be calculated
through the router. Another advantage is that a metric-based
mechanism does not require that all nodes in the IS-IS domain
correctly implement the Overload bit handling procedures.
As traditionally specified, IS-IS provided for only 6 bits of space
for link metric allocation, and 10 bits aggregate path metrics.
Though extensions provided in [RFC 3784] remove this limitation, they
may not yet be fully deployed in many networks. As such, there's
possibly less flexibility when using link metrics for this purpose.
Of course, both methods proposed in this document are backwards-
compatible.
Two other more more recent techniques can help to alleviate these
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transient network conditions further. Graceful restart [rfc 4724]
[RFC 4781] with a control plane only restart, and "BGP free cores".
Furthur discussion of these techniques is beyond the scope of this
document.
4. Manageability Considerations
These extensions which have been designed, developed and deployed for
many years do not have any new impact on management and operation of
the IS-IS protocol via this standardization process.
5. Security Considerations
The mechanisms specified in this memo introduces no new security
issues to IS-IS.
6. Acknowledgments
The original efforts and corresponding acknowledgements provided in
[RFC 3277] have enabled this work.
Others to be provided....
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7. IANA Considerations
This specification introduces no new IANA considerations and
therefore requires no actions on the part of IANA.
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8. References
8.1. Normative References
[ISO 8473] 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 10589:1992.
8.2. Informative References
[RFC 1195] Callon, R., "OSI IS-IS for IP and Dual Environment,"
RFC 1195, December 1990.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 3137] Retana et al., "OSPF Stub Router Advertisement",
RFC 3137, June 2001.
[RFC 3277] McPherson, D., "Intermediate System to Intermediate
System (IS-IS) Transient Blackhole Avoidance", RFC 3277, April
2002.
[RFC 3784] Li, T., Smit, H., "IS-IS extensions for Traffic
Engineering", RFC 3784, June 2004.
[RFC 4724] Sangli, S., Chen, E., Fernando, R., Scudder, J.,
Rekhter, Y., "Graceful Restart Mechanism for BGP", RFC
4724, January 2007.
[RFC 4781] Rekhter, Y., Aggarwal, R., "Graceful Restart
Mechanism for BGP with MPLS", RFC 4781, January 2007.
9. Authors' Addresses
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Danny McPherson
Arbor Networks, Inc.
EMail: danny@arbor.net
Naiming Shen
Cisco Systems, Inc.
EMail: naiming@cisco.com
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