3Com 8807 Configuration Manual page 362

362 C 33: BGP C
HAPTER
BGP Message Types
BGP Routing Mechanism
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:
Type 1, OPEN: The first message sent after the creation of a connection to
■
create association between BGP peers.
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).
Type 3, NOTIFICATION: Used to notify errors.
■
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.
Route advertisement policy
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.