Example Configuring BGP on Service Provider Routers

This example shows the configuration of an ISP edge router.

11.2.3.0/24 AS 65001

Customer Network

Service Provider Network router bgp 387 neighbor 10.0.0.1 remote-as 65001 neighbor 10.0.0.1 default-originate neighbor 10.0.0.1 prefix-list DefaultOnly out neighbor 10.0.0.1 prefix-list CustomerA in neighbor 10.0.0.1 filter-list 15 in neighbor 10.0.0.1 route-map AllCustomersIn in ip as-path access-list 15 permit A65001(_65001)*$ ip prefix-list DefaultOnly permit 0.0.0.0/0 ip prefix-list CustomerA permit 11.2.3.0/24 le 32 ip prefix-list Provider permit 11.2.0.0/16 le 32

route-map AllCustomersIn permit 10 match ip prefix-list Provider set comnunity no-export additive

The customer is assigned the private AS number 65001. The BGP session is opened with the customer IP address, 10.0.0.1.

The ISP sends the default route only to the customer. This route is configured using first the default-originate command and then the prefix-list DefaultOnly.

Received routes from the customer must first pass the prefix-list CustomerA. There is one dedicated prefix-list for each individual customer; the prefix-list permits only the routes that the customer is allowed to announce. If the routes are allowed by the prefix-list, they must also pass the filter-list named "15 in." In this case, the filter allows the private AS of the customer in any number of repetitions, as long as it is the only AS number in the path. This filter-list allows for AS-path prepending configurations on the customer side. If the received route is allowed by both the prefix-list and the filter-list, then the route-map AllCustomersIn is applied.

The route-map is a general route-map that is used for all customers. It checks every route that is received, via the prefix-list Provider, and if the route is within the big block of PA address space that the ISP announces to the rest of the Internet, the customer route is marked with the no-export community. This mark means that the route is used within the ISP AS only and is not sent to the rest of the Internet.

Routes that are received from the customer and are allowed by the prefix-list and filter-list, but do not fall within the PA address space, are allowed by the route-map and are not changed in any way. The ISP propagates these routes to the rest of the Internet.

route-map AllCustomersIn permit 9999

5-56 Configuring BGP on Cisco Routers (BGP) v3.2

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