3Com Switch 8800 Configuration Guide Chapter 21 BGP Configuration21-2BGP runs on a router in any of the following modes:z Internal BGP (IBGP)z External BGP (EBGP)The BGP is called IBGP when it runs in an AS and EBGP when it runs among differentASs.21.1.2 BGP Message TypesBGP is driven by messages, which include the following types:z Type 1, OPEN: The first message sent after the creation of a connection to createassociation between BGP peers.z Type 2, UPDATE: The most important information in BGP system used toexchange routing information between peers. It is composed of up to three parts,that is, unreachable route, path attributes and network layer reachable information(NLRI).z Type 3, NOTIFICATION: Used to notify errors.z Type 4, KEEPALIVE: Used to check connectivity.z Type 5, ROUTE-REFRESH: Used to advertise its own route refreshing capability.The first four types are defined in RFC1771, while the last one is in RFC2918 (RouteRefresh Capability for BGP-4).21.1.3 BGP Routing MechanismOn the first startup of the BGP system, the BGP router exchanges routing informationwith its peers by transmitting the complete BGP routing table, after that only updatemessages are exchanged. In the operating of the system, keepalive messages arereceived and transmitted to check the connections between various neighbors.The router transmitting BGP messages is called a BGP speaker, which receives andgenerates new routing information continuously and advertises the information to theother BGP speakers. When a BGP speaker receives a new route advertisement fromanother AS, it will advertise the route, if the route is better than the current route thathas been learned or is a new route, to all the other BGP speakers in the AS.A BGP speaker calls peers other BGP speakers which exchange information with it andmultiple related peers compose a peer group.I. Route advertisement policyIn the Switch 8800, these policies are used by BGP when advertising routes:z If there are multiple routes available, a BGP speaker only selects the optimumone.z A BGP only advertises its own route to its peers.z A BPG advertises the routes obtained from EBGP to all its BGP peers (includingEBGP and IBGP peers).
