Troubleshooting guide

Troubleshooting

Network Flooded with External Traffic

One of the most common uses for BGP is BGP multihoming. Multihoming allows connections to two

different ISPs. An unintended consequence of multihoming is that the ISPs can advertise routes to each

other through the multihomed router. This results in the local router becoming a transit network for

external traffic.This should be prevented by utilizing prefix lists that only allow specific subnets to be

advertised to each provider. This will keep the ISPs from being able to advertise other routes for the

Internet to each other through the local router. (For more information on proper configuration for

multihoming applications, refer to

Example 4 on page 41).

Routes in Incorrect Communities

There are several things that could prevent a remote neighbor from applying the correct policies to routes

that have been defined as members of particular communities.

• The AOS device has not been enabled to send community attributes to this neighbor. Enter the

send-community standard command from the BGP Neighbor Configuration mode context.

• The BGP neighbor defines different policies for the community or the BGP neighbor does not accept

community attributes in customer routes. Consult with the ISP about what communities it supports.

There might also be problems with the local policy that has been configured for communities on the AOS

device. Look at the configured route maps and examine entries that include a match clause for a

community list. Then verify that the set clauses implement the correct policies for communities in this list.

Use the following Enable mode command to view a community list:

#show ip community-list <name>

Use the following command to view the routes that match the community list:

#show ip bgp community-list <name>

Monitor the communities of routes that the router receives by entering this command:

#show ip bgp community [<number> | internet | local-as | no-advertise | no-export]

The CLI displays all routes in the specified community. Enter the command without keywords (#show ip

bgp community) for the community to see all routes known by the router that have a community attribute.