Configuring BGP on Multihomed Customer Routers

In the example, the customer network is connected to a service provider network using multiple permanent links. BGP is used to exchange routing information between the customer and the provider.

Selecting BGP as the routing protocol between the customer and provider network ensures that a link failure or the failure of a remote router is detected. In this scenario, the customer does not require the use of a public AS number or full Internet routing. Instead, a private AS number is assigned to the customer network, and the ISP sends a default route to the customer through BGP.

The big difference in this case, as compared to a network scenario where static routes and redistribution are used, is that routers within the private AS of the customer now advertise the customer routes via BGP. Thus, the customer is responsible for announcing its own address space. The ISP receives routes from the customer and conditionally propagates them (similar to static routing). If the customer uses provider-assigned (PA) address space and the ISP can summarize the address space, it will not propagate the explicit routes from the customer to the Internet. The private AS number in the AS-path attribute must be removed before the ISP can propagate any of the customer routes.

Because the customer is now creating BGP routes that are received by the ISP, any error made by the customer can influence routing operation within the ISP network and, if propagated, within the Internet as a whole. Announcing a route to a network to which the customer has not been assigned may cause routing problems. There is always a risk that such routing problems can occur in a service provider network. However, the risk is much greater when the customer, whose network administrators usually have less experience with BGP, enters the configuration.

To reduce the risk of erroneous route advertising, the ISP should always filter any BGP information that it has received from the customer network. The ISP should reject routes to networks that are not expected to be in the customer AS. Routes that contain an AS path with unexpected AS numbers should also be rejected.

5-50 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

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