Troubleshooting guide

Additional BGP Configuration

(config-bgp-neighbor)#prefix-list <name> [in | out]

<name> Assigns the specified prefix list to this BGP neighbor.

in Specifies that all inbound BGP route updates received from the neighbor are filtered.

out Specifies that all outbound BGP route updates being sent to the neighbor are filtered.

Route Map

Route maps allow configuration of more complex policies than prefix lists. In addition to filtering routes

according to IPv4 network address and prefix length, routes can be filtered according to their AS path,

metric value, or BGP community. A BGP community is a group of routes to which a BGP router applies

the same policies. Refer to

BGP Communities on page 13 for more information on BGP communities.

Route maps can also be configured to apply various attributes to the routes it filters. A route map applied to

outbound data determines how the router advertises routes to a neighbor. The outbound route map can be

configured to perform such tasks as:

• Define the routes that the router can advertise according to specified attributes or prefixes

• Prepend private AS numbers to specific routes to help balance inbound traffic

• Set an MED on specific routes to help balance inbound traffic

• Request that the neighbor advertise the route to certain communities only

When a route map is applied to inbound data, it determines which of the service provider advertised routes

the local router accepts. The inbound route map can be configured to perform such tasks as:

• Filter external routes according to specified attributes or prefixes

• Apply attributes to filtered routes, including:

– Local preference

– Community

–MED value

– Prepended AS path

• Delete communities defined for the routes

The route map itself is created first. Matching criteria and attributes are defined within the route map

configuration menu. Once a route map has been established, it can be assigned to a BGP neighbor.

Use the route-map command to create a route map and enter the Route Map Configuration mode.

(config)#route-map <name> <number>

(config)#route-map <name> [deny | permit] <number>

<name> Specifies a name for the route map.

deny Specifies not to process routes matching the specified route map attributes.

permit Processes routes matching the specified route map attributes.

<number> Specifies a sequence number for this route entry. Range is 1 to 4294967295.

A prefix list can be used to create even more complicated policies when it is applied to a

route map entry rather than a BGP neighbor (as shown above). Refer to Route Map on

page 23 for more information on this option.