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HAPTER

 33: BGP C

ONFIGURATION

The BGP is called IBGP when it runs in an AS and EBGP when it runs among 
different ASs.

BGP is driven by messages, which include the following types:

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Type 1, OPEN: The first message sent after the creation of a connection to 
create association between BGP peers.

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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).

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Type 3, NOTIFICATION: Used to notify errors.

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Type 4, KEEPALIVE: Used to check connectivity.

■

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).

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 from another 
AS, it will advertise the route, if the route is better than the currently known route 
or is a new route, to all the other BGP speakers in the AS.

BGP speakers among which messages are exchanged are peers to one another. 
Multiple related peers compose a peer group.

In the implementation of Switch 8800 Family series, these policies are used by BGP 
when advertising routes:

■

If there are multiple routes available, a BGP speaker only selects the optimum 
one.

■

A BGP only advertises its own route to its peers.

■

A BPG advertises the routes obtained from EBGP to all its BGP peers (including 
EBGP and IBGP peers).

■

A BGP speaker does not advertise the routes obtained from IBGP to its IBGP 
peers.

■

A BGP speaker advertises the routes obtained from IBGP to its IBGP peers (In 
3Com Switch 8800 Family series, BGP and IGP are asynchronous.)

■

Once the connection is set up, a BGP speaker will advertise all its BGP routes to 
its peers.