Cisco 3560G 48TS Software Configuration Guide - Page 655

Chapter 30 Configuring IP Unicast Routing Configuring BGP 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. Note To enable BGP, the switch must be running the EMI. Beginning in privileged EXEC mode, follow these steps to enable BGP routing, establish a BGP routing process, and specify a neighbor: Step 1 Step 2 Step 3 Command configure terminal ip routing router bgp autonomous-system Step 4 Step 5 network network-number [mask network-mask] [route-map route-map-name] neighbor {ip-address | peer-group-name} remote-as number Step 6 Step 7 neighbor {ip-address | peer-group-name} remove-private-as no synchronization 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. 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 the other end of the connection. For IBGP, the IP address can be the address of any of the router interfaces. (Optional) Remove private AS numbers from the AS-path in outbound routing updates. (Optional) Disable synchronization between BGP and an IGP. 78-16156-01 Catalyst 3560 Switch Software Configuration Guide 30-47