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It is needed only because there is no mechanism in

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It is needed only because there is no mechanism instandard BGP to request retransmission of BGP routes.BGP route refresh is an optional BGP capability that allows aBGP router to request retransmission of BGP routes from aneighbor.Thesoft-reconfiguration inboundfeature consumes large volumes of memory in the Internetenvironment. The number of routes that can be received from a peer router on the Internet is solarge that it is not feasible to store an extra copy.The only reason for making the extra copy is to be able to replay the data through the newrouting policy without tearing down the session and re-establishing it.What is needed is a mechanism to ask the neighbor router to do aclear soft outbound.If thiswere possible, the extra copy would not be needed. The neighboring router, of course, has itsown copy in its BGP table, which it could resend to the local router whenever it is signaled todo so.There is no such mechanism in standard BGP, but there is an optional BGP capability thatallows one router to request a refresh from its neighbor: route refresh.
© 2005, Cisco Systems, Inc.Route Selection Using Policy Controls3-115The table compares the various methods of BGP session reset, stating the advantages anddisadvantages of each.Type of ResetAdvantagesDisadvantagesHard resetNo memory overhead.The prefixes in the BGP, IP, and FIBtables that are provided by theneighbor are lost.Not recommended.Outbound soft resetNo configuration, no storing ofrouting table updates.Does not reset inbound routing tableupdates.Dynamic inbound softresetDoes not clear the BGP sessionor cache.Does not require storing of routingtable updates, and has nomemory overhead.Both BGP routers must support theroute refresh capability (Cisco IOSSoftware Release 12.1 and laterreleases).Configured inbound softreset (uses theneighborsoft-reconfigurationcommand)Can be used when both BGProuters do not support theautomatic route refresh capability.Requires preconfiguration.Stores all received (inbound) routingpolicy updates without modification,and is thus memory-intensive.Recommended only when absolutelynecessary.
3-116Configuring BGP on Cisco Routers (BGP) v3.2© 2005, Cisco Systems, Inc.Example: Route RefreshThe example shows the steps in a route refresh.© 2005 Cisco Systems, Inc. All rights reserved.BGP v3.2 3-14Route Refresh (Cont.)Step 1 - Route refresh is negotiated when the BGP session is established.Step 2 - Inbound routing policy is changed on RTR-B.Step 4 - RTR-B sends route refresh message to RTR-A.Step 5 - RTR-A resends all BGP routes to RTR-B.Step 3 - Operator requests inbound route refresh.The ability to use the route refresh feature must be negotiated by the router when the BGPsession is first established. The local router keeps a record that the capability is available withthe neighbor. There is no need to keep a copy of the routing information that is received fromthe neighbor if it has the ability to refresh.

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