3Com Switch 8800 Configuration Guide Chapter 21 BGP Configuration21-3z A BGP speaker does not advertise the routes obtained from IBGP to its IBGPpeers.z A BGP speaker advertises the routes obtained from IBGP to its IBGP peers (In theSwitch 8800, BGP and IGP are asynchronous.)z Once the connection is set up, a BGP speaker will advertise all its BGP routes toits peers.II. Route selection policyIn the Switch 8800, these policies are adopted for BGP to select routes:z First discard the routes unreachable to the next hop.z First select the routes with the highest local preference.z First select the routes rooted from the router itself.z First select the routes with the least AS-paths.z First select the routes with the lowest origin.z First select the routes with the lowest MED value.z First select the routes learned from EBGP.z First select the routes advertised by the router with the lowest ID.21.1.4 MBGPI. MBGP overviewAs described at the beginning of this chapter, BGP, as the practical exterior gatewayprotocol, is widely used in interconnection between autonomous systems. Thetraditional BGP-4 can only manage the routing information of IPv4 and has limitation ininter-AS routing when used in the application of other network layer protocols (such asIPv6 etc).In order to support multiple network layer protocols, IETF extended BGP-4 and formedMBGP (Multiprotocol Extensions for BGP-4, multiple protocols extension of BGP-4).The present MBGP standard is RFC2858.MBGP is backward compatible, that is, a router supporting BGP extension can beinterconnected with a router that does not support it.II. MBGP extension attributesIn the packets BGP-4 uses, three pieces of information related to IPv4 are carried in theupdate packet. They are Network Layer Reachability Information (NLRI), Next_Hop(The next hop address) in path attribute and Aggregator in path attribute (This attributeincludes the BGP speaker address which forms the summary route).When multiple network layer protocols are supported, it is necessary for BGP-4 toreflect the information of the specified network layer protocol to NLRI and the Next_Hop.Two new routing attributes are introduced in MBGP:
