3Com Switch 8800 Configuration Guide Chapter 21 BGP Configuration
21-2
BGP 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 different
ASs.
21.1.2 BGP Message Types BGP is driven by messages, which include the following types:
z Type 1, OPEN: The first message sent after the creation of a connection to create
association between BGP peers.
z Type 2, UPDATE: The most important information in BGP system used to
exchange 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 (Route
Refresh Capability for BGP-4).
21.1.3 BGP Routing Mechanism On the first startup of the BGP system, the BGP router exchanges routing information
with its peers by transmitting the complete BGP routing table, after that only update
messages are exchanged. In the operating of the system, keepalive messages are
received and transmitted to check the connections between various neighbors.
The router transmitting BGP messages is called a BGP speaker, which receives and
generates new routing information continuously and advertises the information to the
other BGP speakers. When a BGP speaker receives a new route advertisement from
another AS, it will advertise the route, if the route is better than the current route that
has 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 and
multiple related peers compose a peer group.
I. Route advertisement policy
In 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 optimum
one.
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 (including
EBGP and IBGP peers).