Cabletron Systems GIGAswitch GSR-16, Руководство пользователя

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DIGITAL GIGAswitch/Router User Reference Manual 111
BGP Configuration Examples
e. Add the peer-host back to the peer-group.
If the
as-count
option is part of the startup configuration, the above steps are
unnecessary.
BGP Configuration Examples
This section presents sample configurations illustrating BGP features. The following
features are demonstrated:
• BGP peering
• Internal BGP (IBGP)
• External BGP (EBGP) multihop
• BGP community attribute
• BGP local preference (local_pref) attribute
• BGP Multi-Exit Discriminator (MED) attribute
• EBGP aggregation
• Route reflection
BGP Peering Session Example
The router process used for a specific BGP peering session is known as a BGP speaker . A
single router can have several BGP speakers. Successful BGP peering depends on the
establishment of a neighbor relationship between BGP speakers. The first step in creating
a BGP neighbor relationship is the establishment of a TCP connection (using TCP port
179) between peers.
A BGP Open message can then be sent between peers across the TCP connection to
establish various BGP variables (BGP Version, AS number (ASN), hold time, BGP
identifier, and optional parameters). Upon successful completion of the BGP Open
negotiations, BGP Update messages containing the BGP routing table can be sent between
peers.
BGP does not require a periodic refresh of the entire BGP routing table between peers.
Only incremental routing changes are exchanged. Therefore, each BGP speaker is required
to retain the entire BGP routing table of their peer for the duration of the peer’s
connection.
BGP “keepalive” messages are sent between peers periodically to ensure that the peers
stay connected. If one of the routers encounters a fatal error condition, a BGP notification
message is sent to its BGP peer, and the TCP connection is closed.