backbone { authtype simple ; interface 172.16.1.5 { priority 10 ; authkey “UTrustME” ; } ; } ; }; # Announce routes learned from OSPF and route # to directly connected network via BGP tp AS 164 export proto bgp as 164 { proto direct ; proto ospf ; }; # Announce routes learned via BGP from # AS number 164 to our OSPF area. export proto ospfase type 2 { proto bgp as 164 { all ; }; }; The definition statements at the beginning of Listing 7.17 define the autonomous system number (ASN) for BGP and the router identifier for OSPF. The autonomoussystem statement says that the ASN of our autonomous system is 249. (As noted earlier, do not use ASN 249 in your configuration.) The first protocol statement in the sample configuration disables RIP. This router does not run RIP. On one side, it connects to the OSPF backbone area; on the other side, it connects to an external routing domain with BGP. The second protocol statement enables OSPF. There is nothing new here; it looks almost identical to the OSPF protocol statement you have already seen in Listing 7.16. The next protocol statement enables BGP. The group clause defines the characteristics of a group of BGP neighbors, which gated calls peers. The IP address of each peer is identified within the group clause by a peer clause. One of the characteristics defined by the group clause is the type of BGP session to establish with the peers. In the example, BGP is used as a classic exterior gateway protocol, thus the type external parameter. BGP can be used for other purposes. As noted in the section on bgpd during the discussion of ASN numbers, it is possible to use BGP to distribute routes within a routing domain instead of between routing domains. When it is used in this way, BGP is referred to as internal BGP (IBGP). Here is the group clause from the sample file in Listing 7.17: group type external peeras 164 This says that BGP will run as an exterior routing protocol and that the ASN of the external autonomous system with which it will communicate is 164. Of course, this should be the real ASN number of your BGP neighbor. On the other hand, assume that you have a large, far-flung enterprise internet. Within that enterprise network are several networks that run OSPF as an interior routing protocol, and have 224

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