Configuring NetFlow for BGP

     

    The Border Gateway Protocol (BGP), defined in RFC 1771, provides loop-free interdomain routing between autonomous systems. (An autonomous system [AS] is a set of routers that operate under the same administration.) BGP is often run among the networks of Internet service providers (ISPs).

     

    Enabling BGP Routing

    Enter the global configuration mode and issue the following commands to enable BGP routing and establish a BGP routing process:

    Command Purpose
    router bgp as-number Enables the BGP routing process, which places the router in router configuration mode
    network network-number [mask network-mask] [route-map route-map-name] Flags a network as local to this autonomous system and enters it to the BGP table

     

    Configuring BGP Neighbors

    BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same autonomous system; external neighbors are in different autonomous systems. Normally, external neighbors are adjacent to each other and share a subnet, while internal neighbors may be anywhere in the same autonomous system.

    To configure BGP neighbors, issue the following command in router configuration mode:

    Command Purpose
    neighbor {ip-address|peer-group-name} remote-as as-number Specifies a BGP neighbor

     

    BGP Neighbor Configuration Examples

    The following example shows how BGP neighbors on an autonomous system are configured to share information.

    router bgp 109 
    network 131.108.0.0
    network 192.31.7.0
    neighbor 131.108.200.1 remote-as 167
    neighbor 131.108.234.2 remote-as 109
    neighbor 150.136.64.19 remote-as 99

    In the example, a BGP router is assigned to autonomous system 109, and two networks are listed as originating in the autonomous system. Then the addresses of three remote routers (and their autonomous systems) are listed. The router being configured will share information about networks 131.108.0.0 and 192.31.7.0 with the neighboring routers. The first router listed is in a different autonomous system; the second neighbor's remote-as router configuration command specifies an internal neighbor (with the same autonomous system number) at address 131.108.234.2 and the third neighbor's remote-as router configuration command specifies a neighbor on a different autonomous system.

     

    Including AS Info in Netflow Exports

    If you have configured BGP on your network, and want Netflow to report on autonomous systems (AS info), issue the following command on the router in global configuration mode:

    Command Purpose
    ip flow-export destination{hostname|ip_address}9996 Exports the Netflow cache entries to the specified IP address. Use the IP address of the NetFlow Analyzer server and the configured Netflow listener port. The default port is 9996.
    ip flow-export {version}[peer-as | origin-as] Exports NetFlow cache entries in the specified version format (5 or 7). If your router uses BGP, you can specify that either the origin or peer ASs are included in exports � it is not possible to include both.