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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. '1' ** Obsolete normative reference: RFC 3137 (ref. '3') (Obsoleted by RFC 6987) -- Possible downref: Non-RFC (?) normative reference: ref. '4' Summary: 7 errors (**), 0 flaws (~~), 2 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Danny McPherson 2 INTERNET DRAFT Amber Networks, Inc. 3 July 2001 5 IS-IS Transient Blackhole Avoidance 6 8 1. Status of this Memo 10 This document is an Internet-Draft and is in full conformance with 11 all provisions of Section 10 of RFC 2026. 13 Internet-Drafts are working documents of the Internet Engineering 14 Task Force (IETF), its areas, and its working groups. Note that 15 other groups may also distribute working documents as Internet- 16 Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six months 19 and may be updated, replaced, or obsoleted by other documents at any 20 time. It is inappropriate to use Internet- Drafts as reference 21 material or to cite them other than as "work in progress." 23 The list of current Internet-Drafts can be accessed at 24 http://www.ietf.org/ietf/1id-abstracts.txt 26 The list of Internet-Draft Shadow Directories can be accessed at 27 http://www.ietf.org/shadow.html. 29 2. Abstract 31 This document describes a simple, interoperable mechanism that can be 32 employed in IS-IS networks in order to decrease data loss associated 33 with deterministic blackholing of packets during transient network 34 conditions. The mechanism proposed here requires no IS-IS protocol 35 changes and is completely interoperable with the existing IS-IS 36 specification. 38 3. Introduction 40 When an IS-IS router that was previously a transit router becomes 41 unavailable as a result of some transient condition such as a reboot, 42 other routers within the routing domain must select an alternative 43 path to reach destinations which had previously transited the failed 44 router. Presumably, the newly selected router(s) comprising the path 45 have been available for some time and, as a result, have complete 46 forwarding information bases (FIBs) which contain a full set of 47 reachability information for both internal and external (e.g., BGP) 48 destination networks. 50 When the previously failed router becomes available again, in only a 51 few seconds paths that had previously transited the router are again 52 selected as the optimal path by the IGP. As a result, forwarding 53 tables are updated and packets are once again forwarded along the 54 path. Unfortunately, external destination reachability information 55 (e.g., learned via BGP) is not yet available to the router, and as a 56 result, packets bound for destinations not learned via the IGP are 57 unnecessarily discarded. 59 A simple interoperable mechanism to alleviate the offshoot associated 60 with this deterministic behavior is discussed below. 62 4. Discussion 64 This document describes a simple, interoperable mechanism that can be 65 employed in IS-IS [1, 2] networks in order to avoid transition to a 66 newly available path until other associated routing protocols such as 67 BGP have had sufficient time to converge. 69 The benefits of such a mechanism can realized when considering the 70 following scenario depicted in Figure 1. 72 D.1 73 | 74 +-------+ 75 | RtrD | 76 +-------+ 77 / \ 78 / \ 79 +-------+ +-------+ 80 | RtrB | | RtrC | 81 +-------+ +-------+ 82 \ / 83 \ / 84 +-------+ 85 | RtrA | 86 +-------+ 87 | 88 S.1 90 Figure 1: Example Network Topology 92 Host S.1 is transmitting data to destination D.1 via a primary path of 93 RtrA->RtrB->RtrD. Routers A, B and C learn of reachability to destina� 94 tion D.1 via BGP from RtrD. RtrA's primary path to D.1 is selected 95 because when calculating the path to BGP NEXT_HOP of RtrD the sum of the 96 IS-IS link metrics on the RtrA-RtrB-RtrD path is less than the sum of 97 the metrics of the RtrA-RtrC-RtrD path. 99 Assume RtrB becomes unavailable and as a result the RtrC path to RtrD is 100 used. Once RtrA's FIB is updated and it begins forwarding packets to 101 RtrC everything should behave properly as RtrC has existing forwarding 102 information regarding destination D.1's availability via BGP NEXT_HOP 103 RtrD. 105 Assume now that RtrB comes back online. In only a few seconds IS-IS 106 neighbor state has been established with RtrA and RtrD and database syn� 107 chronization has occurred. RtrA now realizes that the best path to des� 108 tination D.1 is via RtrB, and therefore updates it FIB appropriately. 109 RtrA begins to forward packets destined to D.1 to RtrB. Though, because 110 RtrB has yet to establish and synchronization it's BGP neighbor rela� 111 tionship and routing information with RtrD, RtrB has no knowledge 112 regarding reachability of destination D.1, and therefore discards the 113 packets received from RtrA destined to D.1. 115 If RtrB were to temporarily set it's LSP Overload bit while synchroniz� 116 ing BGP tables with it's neighbors, RtrA would continue to use the work� 117 ing RtrA->RtrC->RtrD path, and the LSP should only be used to obtain 118 reachability to locally connected networks (rather than for calculating 119 transit paths through the router, as defined in [1]). 121 However, it should be noted that when RtrB goes away its LSP is still 122 present in the IS-IS databases of all other routers in the routing 123 domain. When RtrB comes back it establishes adjacencies. As soon as its 124 neighbors have an adjacency with RtrB, they will advertise their new 125 adjacency in their new LSP. The result is that all the other routers 126 will receive new LSPs from RtrA and RtrD containing the RtrB adjacency, 127 even though RtrB is still completing its synchronization and therefore 128 has not yet sent it's new LSP yet. 130 At this time SPF is computed and everyone will include RtrB in their 131 tree since they will use the old version of RtrB LSP (the new one has 132 not yet arrived). Once RtrB has finished establishing all its adjacen� 133 cies, it will then regenerate its LSP and flood it. Then all other 134 routers within the domain will finally compute SPF with the correct 135 information. Only at that time will the Overload bit be taken into 136 account. 138 As such, it is recommended that each time a router establishes an adja� 139 cency, it will update its LSP and flood it immediately, even before 140 beginning database synchronization. This will allow for the Overload bit 141 setting to propagate immediately, and remove the potential for an older 142 version of the reloaded routers LSP to be used. 144 After synchronization of BGP tables with neighboring routers (or expiry 145 of some other timer or trigger), RtrB would generate a new LSP, clearing 146 the Overload bit, and RtrA could again begin using the optimal path via 147 RtrB. 149 Typically, in service provider networks IBGP connections are done via 150 peerings with 'loopback' addresses. As such, the newly available router 151 must advertise it's own loopback (or similar) IP address, as well as 152 associated adjacencies, in order to make the loopbacks accessible to 153 other routers within the routing domain. It's because of this that sim� 154 ply flooding an empty LSP is not sufficient. 156 5. Deployment Considerations 158 Such a mechanism increases overall network availability and allows 159 network operators to alleviate the deterministic blackholing behavior 160 introduced in this scenario. Similar mechanisms [3] have been 161 defined for OSPF, though only after realizing the usefulness obtained 162 from that of the IS-IS Overload bit technique. 164 This mechanism has been deployed in several large IS-IS networks for 165 a number of years. 167 Triggers for setting the Overload bit as described are left to the 168 implementer. Some potential triggers could perhaps include "N sec� 169 onds after booting", or "N number of BGP prefixes in the BGP Loc- 170 RIB". 172 Unlike similar mechanisms employed in [3], if the Overload bit is set 173 in a router's LSP, NO transit paths are calculated through the 174 router. As such, if no alternative paths are available to the desti� 175 nation network, employing such a mechanism may actually have a nega� 176 tive impact on convergence (i.e., the router maintains the only 177 available path to reach downstream routers, but the Overload bit dis� 178 allows other nodes in the network from calculating paths via the 179 router, and as such, no feasible path exists to the routers). 181 Finally, if all systems within an IS-IS routing domain haven't imple� 182 mented the Overload bit correctly, forwarding loops may occur. 184 6. Potential Alternatives 186 Alternatively, it may be considered more appealing to employ some� 187 thing more akin to [3] for this purpose. With this model, during 188 transient conditions a node advertises excessively high link metrics 189 to serve as an indication to other nodes in the network that paths 190 transiting the router are "less desirable" than existing paths. 192 The advantage of a metric-based mechanism over the Overload bit mech� 193 anism proposed here model is that transit paths may still be calcu� 194 lated through the router. Another advantage is that a metric- based 195 mechanism does not require that all nodes in the IS-IS domain cor� 196 rectly implement the Overload bit. 198 However, as currently deployed, IS-IS provides for only 6 bits of 199 space for link metric allocation, and 10 bits aggregate path metric. 200 Though extensions proposed in [4] remove this limitation, they've not 201 yet been widely deployed. As such, there's currently little flexi� 202 bility when using link metrics for this purpose. Of course, both 203 methods proposed in this document are backwards-compatible. 205 7. Security Considerations 207 The mechanisms specified in this memo introduces no new security 208 issues to IS-IS. 210 8. Acknowledgements 212 The author of this document makes no claim to the originality of the 213 idea. Thanks to Stefano Previdi for valuable feedback on the mecha� 214 nism discussed in this document. 216 9. References 218 [1] ISO, "Intermediate system to Intermediate system routeing 219 information exchange protocol for use in conjunction with the 220 Protocol for providing the Connectionless-mode Network Service 221 (ISO 8473)," ISO/IEC 10589:1992. 223 [2] Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195, 224 December 1990. 226 [3] Retana et al., "OSPF Stub Router Advertisement", RFC 3137, June 227 2001. 229 [4] Li, T., Smit, H., "IS-IS extensions for Traffic Engineering", 230 Work in Progress. 232 10. Author's Address 234 Danny McPherson 235 Amber Networks, Inc. 236 48664 Milmont Drive 237 Fremont, CA 94538 238 Phone: 510.687.5226 239 Email: danny@ambernetworks.com