Cisco Systems 3750E Enabling BGP Routing, Router bgp autonomous-system, Route-map route-map-name

Chapter 38 Configuring IP Unicast Routing

Configuring BGP

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-asrouter 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.

Note To enable BGP, the switch or stack master must be running the IP services feature set.

Beginning in privileged EXEC mode, follow these steps to enable BGP routing, establish a BGP routing process, and specify a neighbor:

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	ip routing	Enable IP routing (required only if IP routing is disabled).
Step 3
	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.
Step 4
	network network-number [mask network-mask]	Configure a network as local to this AS, and enter it in the BGP
	[route-maproute-map-name]	table.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
38-48	OL-9775-02