Switch 7700 Configuration Guide, v2

118 CHAPTER 5: ROUTING PROTOCOL OPERATION
abundant route policies to implement flexible filtering and selecting of routes,
which can be extended easily to support new developments of the network.
BGP, as an upper-layer protocol, runs on a special router. 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, keep-alive messages are received and
transmitted to check the correctness of 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 other BGP speakers that exchange information with it
peers and multiple related peers compose a peer group.
BGP runs on a router in any of the following modes:
■ IBGP (Internal BGP)
■ EBGP (External BGP)
The BGP is called IBGP when it runs in an AS and is called EBGP when it runs
among different ASs.
Running of BGP is driven by messages of the following four types:
■ Open message
■ Update message
■ Notification message
■ Keep-alive message
The open message is the first message sent after the creation of a connection,
which is used to create the connection relation between BGP peers. The
notification message is used to notify errors. The keep-alive message is used to
check the validity of a connection. The update message is the most important
information in BGP system, which is used to exchange routing information
between peers. It is composed of up to three parts that are unreachable route,
path attributes and network layer reachability information (NLRI).
Configuring BGP The BGP configuration includes:
■ Enabling BGP
■ Configuring Networks for BGP Distribution
■ Configuring a BGP Peer Group
■ Configuring a BGP Timer
■ Configuring the Local Preference
■ Configuring MED for AS
■ Comparing the MED Routing Metrics from the Peers in Different ASs
■ Configuring a BGP Community