Conditional Advertising in Multihomed Customer Networks Cont

The address space is further subnetted at the customer site. One of the subnets (subnet 13.5.1.0/24) is identified as being a central part of the customer core network.

The customer edge routers participate in the IGP routing of the customer. This participation means that these routers have information about which of the subnets within the address space 13.5.0.0/16 are currently reachable. If these subnets are available, there is an explicit route to each of them. If any of the subnets go down, or if the path toward them goes down, the route to that subnet is removed from the routing table.

The BGP advertisement in each of the customer edge routers is configured to advertise the full address space that is used by the customer. When this route is advertised by the customer edge routers, the ISP network, and thus the rest of the Internet, see the complete address space of the customer as one single route, 13.5.0.0/16.

Advertisement of the customer address space by BGP is conditioned by the appearance of the static route, IP route 13.5.0.0 255.255.0.0 13.5.1.1. If the static route is valid, then the BGP route 13.5.0.0/16 is advertised. The static route is a recursive route, which means that the router takes another look in the routing table for the address 13.5.1.1 before determining what to do with the static route. The idea is that 13.5.1.1 is reachable via the IGP. The subnet 13.5.1.0/24 is announced by the IGP. If this subnet is reachable by the edge router, then the static route to 13.5.0.0/16 is valid. If there is no route to 13.5.1.1, then the static route is invalid.

Note The condition, whether or not to advertise the entire customer address space 13.5.0.0/16, is controlled by the IGP reachability of a single subnet, 13.5.1.0/24.

The IGP configuration also includes origination of the default route by both edge routers.

Configuring BGP on Service Provider Routers

This topic describes how to configure BGP on a service provider network to establish routing between a multihomed customer and a single service provider.

Configuring BGP on Service Provider Routers

The service provider must:

• Advertise a default route to the customer through BGP

The service provider must:

• Advertise a default route to the customer through BGP

• Filter incoming BGP updates with a prefix-list to verify that the customer announces only the assigned address space

• Filter incoming BGP updates with an AS-path filter-list to verify that the customer uses only its own AS number

Optionally, the no-export community should be set on customer routes.

In the ISP network, the two edge routers must have BGP sessions configured for the customer. There is no point in feeding the full Internet routing table to the customer, because the table contains the same set of routes for both links and the customer always uses the ISP for all traffic toward the Internet. Injection of a default route in the customer network would accomplish the same task.

The customer is responsible for its own advertisements. Because customers are much less likely to be experienced in BGP configuration than the ISP, they are more likely to make errors. Therefore, the ISP must protect itself and the rest of the Internet from those errors.

The service provider should use a prefix-list that allows only customer-assigned routes and denies any other route to ensure that private address space or any other illegal networks that are erroneously announced by the customer never reach the ISP BGP table. Filtering based on the AS path also provides some protection from customer configuration errors. Only routes that originated within the customer AS are allowed. A filter-list performs this check.

If the customer address space is PA address space and it represents only a small part of a larger block that is announced by the ISP, the explicit BGP routes that are received from the customer need not be advertised to the rest of the Internet. The ISP can announce the big block, attracting any traffic toward any subnet within the block. After the traffic enters the ISP network, the more explicit routes to the customer network are available and used. In this case, the provider edge router can tag the BGP routes that are received from the customer with the no-export well-known community, restricting them from being sent by the ISP to any other AS.

5-54 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

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