ecksum 192.192.192.4 192.192.192.5 0x8000002A 0x003415 Router Link States (Area 1) Link ID ADV Router Checksum Link count 192.192.192.5 192.192.192.5 0x8000002A 0x00B046 1 Summary Net Link States (Area 1) Link ID ADV Router Checksum 10.10.10.0 192.192.192.5 0x8000002A 0x0002A2 Chapter 10: Routing Protocols OSPF Router with ID (192.168.1.116) (Process ID 1) If all of this seems needlessly complicated to you, you're not alone. The complexity of OSPF is one of the reasons many people choose EIGRP instead. If you're working in a multivendor environment, however, EIGRP is not an option. BGP is a very different protocol from the others described here. The most obvious difference is that BGP is an external gateway protocol, while all the previously discussed protocols were internal gateway protocols. BGP can be hard to understand for those who have only ever dealt with internal protocols like EIGRP and OSPF, because the very nature of the protocol is different. As BGP is not often seen in the corporate en- vironment, I'll cover it only briefly here. BGP does not deal with hops or links, but rather with autonomous systems. A network in BGP is referred to as a prefix. A prefix is advertised from an autonomous system. BGP then propagates that information through the connected autonomous systems until all the autonomous systems know about the prefix. Routes in BGP are considered most desirable when they traverse the least possible number of autonomous systems. When a prefix is advertised, the autonomous system number is prefixed onto the autonomous system path. This path is the equivalent of a route in an internal gateway protocol. When an autonomous system learns of a prefix, it learns of the path associated with it. When the autonomous system advertises that prefix to another autonomous system, it prepends its own ASN to the path. As the prefix is advertised to more and more autonomous systems, the path gets longer and longer. The shorter the path, the more desirable it is. Figure 10-13 shows a simple example of BGP routi