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Network Working Group                                       D. McPhersonRequest for Comments: 3277                                           TCBCategory: Informational                                       April 2002Intermediate System to Intermediate System (IS-IS)Transient Blackhole AvoidanceStatus 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.netMcPherson                    Informational                      [Page 5]

RFC 3277          IS-IS Transient Blackhole Avoidance         April 20029.  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.McPherson                    Informational                      [Page 6]