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+Network Working Group                                       D. McPherson
+Request for Comments: 3277                                           TCB
+Category: Informational                                       April 2002
+
+
+           Intermediate System to Intermediate System (IS-IS)
+                     Transient Blackhole Avoidance
+
+Status of this Memo
+
+   This memo provides information for the Internet community.  It does
+   not specify an Internet standard of any kind.  Distribution of this
+   memo is unlimited.
+
+Copyright Notice
+
+   Copyright (C) The Internet Society (2002).  All Rights Reserved.
+
+Abstract
+
+   This document describes a simple, interoperable mechanism that can be
+   employed in Intermediate System to Intermediate System (IS-IS)
+   networks in order to decrease the 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.
+
+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 have 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, it is only
+   seconds before the 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.
+
+
+
+McPherson                    Informational                      [Page 1]
+
+RFC 3277          IS-IS Transient Blackhole Avoidance         April 2002
+
+
+   A simple interoperable mechanism to alleviate the offshoot associated
+   with this deterministic behavior is discussed below.
+
+2. 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 be realized when considering the
+   following scenario depicted in Figure 1.
+
+                                 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 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 RtrD.
+
+
+
+
+
+
+McPherson                    Informational                      [Page 2]
+
+RFC 3277          IS-IS Transient Blackhole Avoidance         April 2002
+
+
+   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 synchronize its
+   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 its LSP Overload bit while
+   synchronizing BGP tables with its 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]).
+
+   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 its 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 LSP (the new one has
+   not yet arrived).  Once RtrB has finished establishing all 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 could again begin using the
+   optimal path via RtrB.
+
+
+
+
+
+
+McPherson                    Informational                      [Page 3]
+
+RFC 3277          IS-IS Transient Blackhole Avoidance         April 2002
+
+
+   Typically, in service provider networks IBGP connections are done via
+   peerings with 'loopback' addresses.  As such, the newly available
+   router must advertise its 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 is because
+   of this that simply flooding an empty LSP is not sufficient.
+
+3. 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
+   seconds after booting", or "N number of BGP prefixes in the BGP Loc-
+   RIB".
+
+   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 (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).
+
+   Finally, if all systems within an IS-IS routing domain haven't
+   implemented the Overload bit correctly, forwarding loops may occur.
+
+4. Potential Alternatives
+
+   Alternatively, it may be considered more appealing to employ
+   something 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
+   mechanism model 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.
+
+
+
+McPherson                    Informational                      [Page 4]
+
+RFC 3277          IS-IS Transient Blackhole Avoidance         April 2002
+
+
+   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 have
+   not yet been widely deployed.  As such, there's currently little
+   flexibility when using link metrics for this purpose.  Of course,
+   both methods proposed in this document are backwards-compatible.
+
+5. Security Considerations
+
+   The mechanisms specified in this memo introduces no new security
+   issues to IS-IS.
+
+6. Acknowledgements
+
+   The author of this document makes no claim to the originality of the
+   idea.  Thanks to Stefano Previdi for valuable feedback on the
+   mechanism discussed in this document.
+
+7. References
+
+   [1] ISO, "Intermediate system to Intermediate system routing
+       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, A., Nguyen, L., White, R., Zinin, A. and D. McPherson,
+       "OSPF Stub Router Advertisement", RFC 3137, June 2001.
+
+   [4] Li, T. and H. Smit, "IS-IS extensions for Traffic Engineering",
+       Work in Progress.
+
+8. Author's Address
+
+   Danny McPherson
+   TCB
+   Phone: 303.470.9257
+   EMail: danny@tcb.net
+
+
+
+
+
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+McPherson                    Informational                      [Page 5]
+
+RFC 3277          IS-IS Transient Blackhole Avoidance         April 2002
+
+
+9.  Full Copyright Statement
+
+   Copyright (C) The Internet Society (2002).  All Rights Reserved.
+
+   This document and translations of it may be copied and furnished to
+   others, and derivative works that comment on or otherwise explain it
+   or assist in its implementation may be prepared, copied, published
+   and distributed, in whole or in part, without restriction of any
+   kind, provided that the above copyright notice and this paragraph are
+   included on all such copies and derivative works.  However, this
+   document itself may not be modified in any way, such as by removing
+   the copyright notice or references to the Internet Society or other
+   Internet organizations, except as needed for the purpose of
+   developing Internet standards in which case the procedures for
+   copyrights defined in the Internet Standards process must be
+   followed, or as required to translate it into languages other than
+   English.
+
+   The limited permissions granted above are perpetual and will not be
+   revoked by the Internet Society or its successors or assigns.
+
+   This document and the information contained herein is provided on an
+   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
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+McPherson                    Informational                      [Page 6]
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