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While there are significant
differences in the way BGP and the various IGPs operate, the syntax
of the BGP commands is similar to the syntax for configuring
internal routing protocols.
Use the router
bgp command to activate BGP
and identify the local AS. The AS number will be assigned to you by
the inter-NIC if you are connecting to the Internet, or by some
local policy if you are connecting to an internal network. 
Use the network
command to permit BGP to advertise a network when it is present in
the IP routing table. 
With
an IGP, such as RIP, the network command determines the interfaces
the protocol will run on, and the prefixes advertised are then
derived from the enabled interfaces. BGP
actually determines what it will advertise from the network command.
For instance, if you configure BGP to advertise the 172.18.0.0/16
network, but only have a route to 172.18.10.0/24, BGP will not
advertise the 172.18.0.0/16 network. The route in the routing table
and the network configured must match for BGP to advertise the
prefix.
Use the neighbor
remote-as command to identify
a peer router with which the local router will establish a session.
The value placed in the AS field of the neighbor
command determines whether this peer is an EBGP or an IBGP neighbor.
If the AS field configured in the router
bgp command is identical to
the AS configured in the neighbor
statement, BGP will initiate an internal session. If the field
values are different, BGP will build an external session. 
Use the clear
ip bgp command to remove
entries from the BGP table and reset BGP sessions. Use this command
after making configuration changes to ensure that the change is
activated and that peer routers are informed. 
Note: Use the clear
command with the asterisk option (clear all) whenever you make
changes to a configuration in order for those changes to take
effect.
While it's not technically required
to execute clear ip bgp *
to make a configuration change take effect in the network, rather
than attempt to explain the complicated conditions under which the clear
command is required, it is safer to do it every time a configuration
change is made. This practice ensures that changes will always be
propogated correctly.
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