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

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

McPherson, D.                                           [Page 2]

INTERNET DRAFT                                             February 2001

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

   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 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 for=AD
   warding information regarding destination D.1's availability via BGP

   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 therefore updates it FIB appro=AD
   priately.  RtrA begins to forward packets destined for D.1 to RtrB.
   Though, because RtrB has yet to establish and synchronize 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 synchro=AD=

   nizing BGP tables with it's neighbors, RtrA would continue to use the
   working RtrA->RtrC->RtrD path, and the LSP should only be used to
   obtain reachability to locally connected networks (rather than for
   calculating transit paths through the router, as defined in [1]).

McPherson, D.                                           [Page 3]

INTERNET DRAFT                                             February 2001

   After initial synchronization of BGP tables with neighboring routers,
   RtrB would generate a new LSP, clearing the Overload bit, and RtrA
   could again begin using the optimal path via RtrB.

   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 acces=AD=

   sible to other routers within the routing domain.  It's because of
   this that simply flooding an empty LSP is not sufficient.

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 usefulness obtained
   from that of the IS-IS Overload bit.

   This mechanism has been deployed in several large IS-IS networks for
   several of years.

   Triggers for setting the Overload bit as described are left to the
   implementer.  Some potential triggers could perhaps include "N sec=AD
   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=AD=

   nation network, employing such a mechanism may actually have a nega=AD=

   tive impact on convergence (i.e., the router maintains the only
   available path to reach downstream routers, but the OL bit disallows
   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 have NOT
   implemented the Overload bit correctly, forwarding loops may occur.

McPherson, D.                                           [Page 4]

INTERNET DRAFT                                             February 2001

6. Potential Alternatives

   Alternatively, it may be considered more appealing to employ some=AD
   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 OL bit mechanism
   proposed here model is that transit paths may still be calculated
   through the router.  Another advantage is that a metric-based mecha=AD=

   nism does not require that all nodes in the IS-IS domain correctly
   implement the OL 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=AD
   bility when using link metrics for this purpose.

   Of course, both methods proposed in this document are backwards-com=AD=

   patible and implementation-specific.

7. Security Considerations

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

8. Acknowledgments

   The author of this document makes no claim to the originality of the
   idea.  Others To be supplied...

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", "Work in
        Progress", November 2000.

McPherson, D.                                           [Page 5]

INTERNET DRAFT                                             February 2001

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

10. Authors' Address

   Danny McPherson
   Amber Networks, Inc.
   48664 Milmont Drive
   Fremont, CA  94538
   Phone: 510.687.5200
   Email: danny@ambernetworks.com

McPherson, D.                                           [Page 6]

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