Troubleshooting guide

Additional BGP Configuration

(config-as-path-list)#permit \b400\b

After configuring the AS path list, use the match command to reference the list in a route map entry.

(config-route-map)#match as-path <name>

<name> Specifies the name of the AS path list.

The route map is then applied to a BGP neighbor (refer to Applying a Route Map Entry to a BGP

Neighbor on page 30 for more information).

Filtering Routes According to Community

If a network places routes in communities, the routes that the local router advertises can be filtered

according to these communities. The first step to filtering routes according to community is to create a

community list (refer to

Configuring a Community List on page 13) that either permits or denies BGP

routes based on well-known or privately defined BGP communities. Next, unless previously

configured, a route map must be created (refer to

Route Map on page 23). Use the match command to

reference the community list in the route map entry:

(config-route-map)#match community <name> exact-match

<name> Specifies the name of the community list.

exact-match Optional. Specifies that the route map must match the community name exactly. When

the exact-match keyword is used, the entire community string must be defined as advertised for it to

qualify as an exact match. Without this keyword, matches will result if the defined string appears

anywhere in the community field.

Lastly, the route map should be applied to a BGP neighbor as an outbound policy (refer to Applying a

Route Map Entry to a BGP Neighbor on page 30).

It is important to enter any deny commands before the permit commands since the

router processes statements in the AS path list in the order that they are entered.

Use set commands to configure any attributes (prepend AS_PATH,

MULTI_EXIT_DISC, LOCAL_PREF, etc.) to be applied to the routes prior to applying

the route map to the BGP neighbor. Refer to Prepending Private AS Numbers for Load

Balancing on page 28 for detailed explanations of the set command attributes.

This command does not define a community for routes. It selects routes according to

their predefined community or communities. Other BGP neighbors, either internal or

external, should have placed the route in a community.

Use set commands to configure any policies (prepend AS_PATH,

MULTI_EXIT_DISC, LOCAL_PREF, etc.) to be applied to the community. Refer to

Prepending Private AS Numbers for Load Balancing on page 28 for detailed

explanations of the set command attributes.