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Versions: 00 draft-ietf-isis-transient

Network Working Group                                    Danny McPherson
INTERNET DRAFT                                      Amber Networks, Inc.
December 2000

                  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                                            December 2000

3. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].

4. 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 mechanism to alleviate the offshoot associated with this
   deterministic behavior is discussed below.

5. Discussion

   This document describes a simple, interoperable mechanism that can be
   employed in IS-IS [1] and [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                                            December 2000

                | 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
   forwarding information regarding destination D.1's availability via

   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
   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 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
   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

McPherson, D.                                           [Page 3]

INTERNET DRAFT                                            December 2000

   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, 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 simply
   flooding an empty LSP is not sufficient.

6. 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 similar 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
   implementor.  Some potential triggers could perhaps include "N
   seconds 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
   destination network, employing such a mechanism may actually have a
   negative impact on convergence.

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

McPherson, D.                                           [Page 4]

INTERNET DRAFT                                            December 2000

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.  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.

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 5]

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