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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
78-17058-01
Chapter32 Configuring IP Unicast Routing Configuring BGP
The switch supports the use of private AS numbers, usually assigned by ser vic e provide rs and given to
systems whose routes are not advertised to external neighbors. The private AS numbers are from 64512
to 65535. You can configure external neighbors to remove private AS numbers from the AS path by using
the neighbor remove-private-as router configuration command. Then when an update is passed to an
external neighbor, if the AS path includes private AS numbers, these numbers are dropped.
If your AS will be passing traffic through it from another AS to a third AS, it is important to be consistent
about the routes it advertises. If BGP advertised a route before all routers in the network had learned
about the route through the IGP, the AS might receive traffic that some routers could not yet route. To
prevent this from happening, BGP must wait until the IGP has propagated information across the AS so
that BGP is synchronized with the IGP. Synchronization is enabled by default. If your AS does not pass
traffic from one AS to another AS, or if all routers in your au tonom ous syste ms are runni n g B GP, you
can disable synchronization, which allows your network to carry fewer rout es in the IGP and allows BGP
to converge more quickly.
Beginning in privileged EXEC mode, follow these steps to enable BGP routing, establish a BGP routing
process, and specify a neighbor:
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 ip routing Enable IP routing (required only if IP routing is disabled).
Step3 router bgp autonomous-system Enable a BGP routing process, assign it an AS number, and
enter router configuration mode. The AS number can be from
1 to 65535, with 64512 to 65535 designated as private
autonomous numbers.
Step4 network network-number [mask network-mask]
[route-map route-map-name]Configure a network as local to this AS, and enter it in the BGP
table.
Step5 neighbor {ip-address | peer-group-name}
remote-as number Add an entry to the BGP neighbor table specifying that the
neighbor identified by the IP address belongs to the specified
AS.
For EBGP, neighbors are usually directly connected, and the IP
address is the address of the interface at t h e othe r en d o f th e
connection.
For IBGP, the IP address can be the add ress of any of the router
interfaces.
Step6 neighbor {ip-address | peer-group-name}
remove-private-as (Optional) Remove private AS numbers from the AS-path in
outbound routing updates.
Step7 no synchronization (Optional) Disable synchronization between BGP and an IGP.
Step8 no auto-summary (Optional) Disable automatic network summarization. By
default, when a subnet is redistributed from an IGP into BGP,
only the network route is inserted into the BGP table.
Step9 bgp fast-external-fallover (Optional) Automatically reset a BGP session when a link
between external neighbors goes down. By default, the session
is not immediately reset.
Step10 end Return to privileged EXEC mode.
Step11 show ip bgp network network-number
show ip bgp neighbor Verify the configuration.
Step12 copy running-config startup-config (Optional) Save your entries in the configuration file.