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Versions: (draft-mcpherson-isis-transient) 00 01 RFC 3277

Network Working Group                                    Danny McPherson
INTERNET DRAFT                                      Amber Networks, Inc.
July 2001

                  IS-IS Transient Blackhole Avoidance

1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

2. 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

McPherson, D.                                           [Page 1]

INTERNET DRAFT                                                 July 2001

3. 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 discussed below.

4. Discussion

   This document describes a simple, interoperable mechanism that can be
   employed in IS-IS [1, 2] 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
   following scenario depicted in Figure 1.

McPherson, D.                                           [Page 2]

INTERNET DRAFT                                                 July 2001

                  | RtrD  |
                  /      \
                 /        \
            +-------+    +-------+
            | RtrB  |    | RtrC  |
            +-------+    +-------+
                 \        /
                  \      /
                  | RtrA  |

     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 destina¡
tion 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 to RtrD is
used.  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

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 syn¡
chronization has occurred.  RtrA now realizes that the best path to des¡
tination D.1 is via RtrB, and therefore updates it FIB appropriately.
RtrA begins to forward packets destined to D.1 to RtrB.  Though, because
RtrB has yet to establish and synchronization it's BGP neighbor rela¡
tionship 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 synchroniz¡
ing BGP tables with it's neighbors, RtrA would continue to use the work¡
ing RtrA->RtrC->RtrD path, and the LSP should only be used to obtain

McPherson, D.                                           [Page 3]

INTERNET DRAFT                                                 July 2001

reachability to locally connected networks (rather than for calculating
transit paths through the router, as defined in [1]).

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 sent it's new LSP yet.

At this time SPF is computed and everyone will include RtrB in their
tree since they will use the old version of RtrB LSP (the new one has
not yet arrived). Once RtrB has finished establishing all its adjacen¡
cies, 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

As such, it is recommended that each time a router establishes an adja¡
cency, 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 could again begin using the optimal path via

Typically, in service provider networks IBGP connections are done via
peerings 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 that sim¡
ply flooding an empty LSP is not sufficient.

McPherson, D.                                           [Page 4]

INTERNET DRAFT                                                 July 2001

5. Deployment Considerations

   Such a mechanism increases overall network availability and allows
   network operators to alleviate the deterministic blackholing behavior
   introduced in this scenario.  Similar mechanisms [3] have been
   defined for OSPF, though 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.

   Triggers for setting the Overload bit as described are left to the
   implementer.  Some potential triggers could perhaps include "N sec¡
   onds after booting", or "N number of BGP prefixes in the BGP Loc-

   Unlike similar mechanisms employed in [3], 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 desti¡
   nation network, employing such a mechanism may actually have a nega¡
   tive impact on convergence (i.e., the router maintains the only
   available path to reach downstream routers, but the Overload bit dis¡
   allows other nodes in the network from calculating paths via the
   router, and as such, no feasible path exists to the routers).

   Finally, if all systems within an IS-IS routing domain haven't imple¡
   mented the Overload bit correctly, forwarding loops may occur.

6. Potential Alternatives

   Alternatively, it may be considered more appealing to employ some¡
   thing more akin to [3] 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 existing paths.

   The advantage of a metric-based mechanism over the Overload bit mech¡
   anism proposed here model is that transit paths may still be calcu¡
   lated through the router.  Another advantage is that a metric- based
   mechanism does not require that all nodes in the IS-IS domain cor¡
   rectly implement the Overload bit.

   However, as currently deployed, IS-IS provides for only 6 bits of
   space for link metric allocation, and 10 bits aggregate path metric.
   Though extensions proposed in [4] remove this limitation, they've not
   yet been widely deployed.  As such, there's currently little flexi¡
   bility when using link metrics for this purpose.  Of course, both

McPherson, D.                                           [Page 5]

INTERNET DRAFT                                                 July 2001

   methods proposed in this document are backwards-compatible.

7. Security Considerations

   The mechanisms specified in this memo introduces no new security
   issues to IS-IS.

8. Acknowledgements

   The author of this document makes no claim to the originality of the
   idea.  Thanks to Stefano Previdi for valuable feedback on the mecha¡
   nism discussed in this document.

9. References

   [1]  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.

   [2]  Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195,
        December 1990.

   [3]  Retana et al., "OSPF Stub Router Advertisement", RFC 3137, June

   [4] Li, T., Smit, H., "IS-IS extensions for Traffic Engineering",
       Work in Progress.

10. Author's Address

   Danny McPherson
   Amber Networks, Inc.
   48664 Milmont Drive
   Fremont, CA  94538
   Phone: 510.687.5226
   Email: danny@ambernetworks.com

McPherson, D.                                           [Page 6]

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