ng in action. The network 10.0.0.0/8 resides in AS 105, which advertises this prefix to AS 3 and AS 2. The path for 10.0.0.0/8 within AS 3 and AS 2 is now 10.0.0.0/8 A AS105. AS 2 in turn advertises the prefix to AS 1, prepending its own ASN to the path. AS 1 now knows the path to 10.0.0.0/8 as 10.0.0.0/8 AS2, AS105. Meanwhile, AS 3 advertises the prefix to AS 100, which then knows the path to 10.0.0.0/8 as 10.0.0.0/8 AS3, AS105. On the other side of the world, AS 102 receives two paths: > 10.0.0.0/8 AS1, AS2, AS105 10.0.0.0/8 AS101, AS100, AS3, AS105 The > on the first line indicates that BGP considers this the preferred path. The path is preferred because it is the shortest path among the known choices. What makes BGP SO confusing to newcomers is the many attributes that can be con- figured. A variety of weights can be attributed to paths, with names like local preference, weight, communities, and multiexit discriminator. To make matters worse, many of these attributes are very similar in function. Specific Routing Protocols 10.0.0.0/8 AS101, AS100, AS3, AS105 10.0.0.0/8 AS100, AS3, AS105 10.0.0.0/8 AS3, AS105 AS 100 AS 101 10.0.0.0/8AS105 AS 102 10.0.0.0/8 AS 105 10.0.0.0/8 AS1, AS2, AS105 Final routes in AS 102: 10.0.0.0/8 AS105 >10.0.0.0/8 AS1, AS2, AS105 10.0.0.0/8 AS101, AS100, AS3, AS105 10.0.0.0/8 AS2, AS105 Figure 10-13. Routing in BGP The protocol also functions differently from other protocols. For example, the net work statement, which is used to enable interfaces in other protocols, is used in BGP to list the specific networks that can be advertised. BGP does not discover neighbors; they must be configured manually. There can only be one autonomous system on any given router, though it may communicate with neighbors in other autonomous systems. BGP is the routing protocol of the Internet. Many of the major service providers allow anonymous Telnet into route servers that act just like Cisco routers. Do an Internet search for the term "looking-glass routers," and you should find plenty of links. These