Enabling Bgp Routing - Cisco Catalyst 3120 Software Manual

Chapter 38 Configuring IP Unicast Routing
neighboring router during the interval between the primary Route Processor (RP) in a router failing and
the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software
upgrade.
For more information, see the "BGP Nonstop Forwarding (NSF) Awareness" section of the Cisco IOS
IP Routing Protocols Configuration Guide, Release 12.4 at this URL:
http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a00804556
8e.html

Enabling BGP Routing

To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP
must completely recognize the relationships with its neighbors, you must also specify a BGP neighbor.
BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same AS;
external neighbors are in different autonomous systems. External neighbors are usually adjacent to each
other and share a subnet, but internal neighbors can be anywhere in the same AS.
The switch supports the use of private AS numbers, usually assigned by service providers 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 autonomous systems are running BGP, you
can disable synchronization, which allows your network to carry fewer routes in the IGP and allows BGP
to converge more quickly.
To enable BGP, the switch or stack master must be running the IP services feature set.
Note
Beginning in privileged EXEC mode, follow these steps to enable BGP routing, establish a BGP routing
process, and specify a neighbor:
Command
Step 1
configure terminal
Step 2 ip routing
Step 3 router bgp autonomous-system
Step 4 network network-number [mask network-mask]
[route-map route-map-name]
OL-12247-01
Purpose
Enter global configuration mode.
Enable IP routing (required only if IP routing is disabled).
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.
Configure a network as local to this AS, and enter it in the BGP
table.
Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide
Configuring BGP
38-49