BROCADE IP PRIMER

Open Shortest Path First(OSPF)11We talked about RIP's short-comings in the previous chapter. The routing communityneeded something more robust. In October of 1989, RFC 1131 wasreleased and a new routing protocol was born. It was called Open ShortestPath First (OSPF), and it used Dijkstra's algorithm to calculate the shortestpath between points. Modern implementations (including Brocade's) arebased on OSPFv2, as defined in RFC 2328.Before we go on, I'd like to make one point clear. OSPF is a very robust andcomplex routing protocol. Entire books have been written on OSPF alone. Wewill only be scratching the surface, giving you a sound foundational understandingof the protocol, and how it can serve you.With that, let's compare OSPF to the methods and protocols we've discussedin the previous chapter.Link State vs. Distance Vector (OSPF vs. RIP)Let's dive right into the differences:• Metric. OSPF is a link state protocol. This means that each router participatingin the protocol is responsible for reporting the state of each of itslinks. OSPF uses a link's cost to make its routing decisions. The cost isdetermined by dividing the speed of the link (in Mbps) into 100, by default(this method can be customized; more on this later). Just as in consumerpricing, the lower the cost, the more desirable the route. Distance vector,as you may recall from Chapter 9, means that the protocol bases its routingdecisions on distance. If you have two or more routes to the samedestination, you choose the one with the shortest distance. RIP is themost common example of this type of protocol. For its distance, it useshop count, or the number of routers a packet must traverse to get to itsdestination.• Route Advertisements. RIP advertises its entire routing table. OSPFsends only updates.Brocade IP Primer 239