BROCADE IP PRIMER

Chapter 13: Border Gateway Protocol (BGP)(sound like RIP? It should). But the big push to replace EGP had to do with theNSFNet. As I mentioned, the NSFNet acted as a Core through which all trafficwas routed. This generated heavy dependency on the infrastructure, and engineersquickly realized that the Internet would function far better if it wasindependent of a single entity. The Internet was also evolving into a muchmore international network. The dependency on one network, any network,would not suffice.In the late 1980s, a new routing protocol called Border Gateway Protocol (BGP)was written. By 1994, BGP version 4 was the standard routing protocol for theInternet. The protocol allowed the Internet to become an independent entity. Itsupplanted EGP and eliminated the dependency on the NSFNet. The Internethad left the nest, and was flying on its own now.What is BGP?BGP is a robust routing protocol that is capable of preventing routing loops forextraordinarily large networks. This is what makes it so ideal for use with theInternet (the largest network). As of 2007, the Internet routing table housedover 200,000 routes.BGP is described as a path vector protocol. Remember our discussion on RIPin Chapter 9? RIP is a distance vector protocol. It makes its routing decisionsbased on the “distance” from the source to the destination. With RIP, we knowthat “distance” refers to the number of “hops” (or routers) a packet will traverseto get to its destination. BGP makes its routing decisions based on the“path” from the source to the destination. The “path” describes the packet'sjourney through multiple autonomous systems (sound familiar? It should), notthrough individual routers (hops).Another unique feature is that BGP uses TCP (port 179) to communicate to itspeer routers. An initial session is established to a peer router and the entirerouting table is exchanged. After the initial exchange, only updates are sent.In some ways, BGP is kind of weird hybrid of RIP and OSPF. Its routing decisionsare still based on distance (though at a “path” level), like RIP, but it alsoonly shares updates after an initial exchange (like OSPF).IGP vs. EGPSo many acronyms! What next?! Routing protocols are grouped into two categories:Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols(EGPs). Now, I should make it clear that from now on, when I'm referring to“EGP,” I'm referring to a collection of Exterior Gateway Protocols, not to be confusedwith the routing protocol from the 1980s actually named EGP (e.g.,EGPv3). IGP and EGP refer to collections of routing protocols.An IGP is used within a network or series of networks that are under oneentity's control. RIP and OSPF are both examples of IGPs. If I'm designing a networkfor my business, I could use static routes, RIP, or OSPF (or a mixture ofthe three) to allow my routers to communicate to each other throughout my272 Brocade IP Primer