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I have long maintained that BGP doesn t transmit policies, but sends the information on which policy decisions are made based on information configured into the router by element management Outbound Route Filtering (ORF) has
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1 Cinnamon changes acceptance policies 2 Cinnamon asks other routers for refresh, at a rate cinnamon can tolerate 3 Cinnamon refilters routes
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Figure 912 Route refresh
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forced me to modify that position, giving a final (if qualified) victory to a longrunning debate between myself and Sue Hares, cochair of the IDR Working Group that develops BGP At the next IETF we attend, I owe Sue a drink The idea of ORF is that it can be of mutual benefit to a pair of routers to exchange their acceptance policies, so a router about to send its Adj-RIB-Out can prefilter it, thus requiring only the bandwidth for the updates the receiver will really use (see Figure 913) Several independent proposals have been made for the policies that can be exchanged, and there is an attempt in the Inter-Domain Routing (IDR) committee to come up with a unified proposal that contains the three filtering criteria suggested: 1 Prefix expression for NLRI 2 AS path expression 3 Community These are a subset of the policy options available on commercial routers, but cover the great majority of cases Like soft refresh, ORF is an optional capability that must be advertised and agreed to between peer BGP speakers
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Graceful Restart
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In the real world, things break Operational experience has shown that the control plane of a router may crash, breaking all routing protocol sessions and
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1 Cinnamon changes acceptance policies 2 Cinnamon sends new filtering rules to other routers 3 Other routers prefilter their Adj-RIB-Outs and send only acceptable routes
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Figure 913 Outbound route filtering
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stopping updates to the RIB, but the FIB and forwarding plane may remain operational The BGP graceful restart capability takes an optimistic view of the FIB contents: that it is more useful to continue forwarding while the routing system recovers, accepting the possibility that some FIB entries may have become obsolete, than to stop forwarding altogether until routing works properly again Graceful restart capabilities also are being developed for ISIS and OSPF, and it is arguable that BGP graceful restart makes little sense if the BGP speaker does not also implement IGP graceful restart When the IGP does support graceful restart, it is good practice for BGP to wait until the IGP converges to begin route selection When the routing system on a highly connected router in the Internet s default-free zone (DFZ) fails, strict interpretation of current BGP practice can impact the Internet as a whole Remember that when a BGP speaker loses connectivity to a route, it must withdraw the route and propagate the withdrawal to all peer speakers to whom it previously advertised the route It may advertise a new route with a different next hop, but it must withdraw the failed route The graceful restart capability provides a way to avoid a flood of withdrawals and announcements, on the assumption that the routing control subsystem can be restarted and the router can keep forwarding while its BGP is reinitializing Specifically, advertising this capability tells peer speakers that the router advertising the capability can retain its FIB during a BGP restart It is not intended for use when the speaker explicitly terminates the session with a NOTIFICATION message
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Optimism can be a great character trait in humans, but network engineers have to pay homage to Murphy s Law For the graceful restart capability to be plausibly fault-tolerant, it must have safeguards against its optimism being unjustified More deployment experience is needed to determine the optimal timer values, or possibly the need for other safeguards, to prevent excessive blackholing or route flapping In the capabilities advertisement is a restart time, which is an estimate in seconds of the time it will take the BGP session to reinitialize after a restart If the timer expires and the router that advertised the capability does not return, its peers can declare it down faster than they would if they had to wait for the BGP session to time out One feature of this capability, which could be useful beyond the original intent of the graceful restart capability, is the endof-RIB marker The presence of this marker in an update indicates that all routes have been sent, and, if the interface has been batching any routes, full convergence can begin Indeed, it has been recommended that as long as a BGP speaker can generate an end-of-RIB marker, it can be useful for it to advertise the graceful restart capability even if it cannot retain its FIB during a BGP restart Once the receiving speaker receives the end-of-RIB marker from all peers that have indicated they are restarting, it can begin to run route selection on the received routes Assume the situation in Figure 914, where router cinnamon is restarting Until it receives an end-of-RIB marker from all its peers for a given address family, it has not built its Loc-RIB and thus has no routes to advertise After route selection, the FIB is updated and any previously marked stale data is removed The reason stale data must be removed at this time is to avoid the effects of multiple restarts One of the complexities of graceful restart is properly handling multiple restarts The solution involves the restarting speaker marking its existing routes as stale Stale routes are not treated differently from other routes in the forwarding process A router with this capability optionally may include a stale route retention timer and flush these routes if the timer expires It must support a timer that can be configured as to how long to