Troubleshooting guide

Basic BGP Configuration Using the CLI

If the routing table does not include a route that has been specified in BGP, a null route must be configured.

For example, a routing table only includes routes to the 24-bit networks, but not to the 20-bit network that

contains them all. A route to network 10.1.0.0 /20 must be manually added so that the BGP interface can

advertise it. The route is added from the Global Configuration mode with the null 0 keyword indicating the

next-hop address:

(config)#ip route 10.1.0.0 255.255.240.0 null 0

Step 3: Configure at Least One BGP Neighbor

BGP is different from many routing protocols because it does not allow a router to automatically search for

peers from which to obtain routes. A separate BGP neighbor must be configured for each router with

which the local router will communicate. Optional policies for each neighbor can be configured to dictate

which routes the BGP interface sends to and accepts from the neighbor (refer to

Additional BGP

Configuration on page 12).

Set the BGP Neighbor ID (IPv4 address)

BGP identifies a peer router by its IPv4 address. A neighbor’s ID is set when a policy is created for it.

Enter the BGP Neighbor Configuration mode from the BGP Configuration mode as follows:

(config-bgp)#neighbor <ipv4 address>

<ipv4 address> Specifies the IPv4 address for the neighbor. IPv4 addresses should be expressed in

dotted decimal notation (for example, 10.10.10.1) or as a prefix length (for example, /24)).

Specify the Remote AS Number

The AS to which this neighbor belongs must also be specified. The remote-as command is issued from

the BGP Neighbor Configuration mode:

(config-bgp-neighbor)#remote-as <value>

<value> Specifies the AS number. Range is 1 to 4294967295.

If a more specific route (for example, from connected interfaces, static routes, routing

protocols, etc.) other than the one to null 0 does not exist in the routing table, then all

traffic for the specified subnet will be dropped. Null 0 routes should only be used when a

more specific route is available to which to direct traffic.

The IPv4 address entered in this command must match the address for the interface

that the remote router is using as its update source.

The local router must be able to reach the IPv4 address configured as the neighbor ID.

View the routing table and verify that it includes a route to this address.