818 CHAPTER 55: IP ROUTING O VERVIEWOperational scope■ Interior gateway protocols (IGPs): Work within an autonomous system, typicallyincludes RIP, OSPF, and IS-IS.■ Exterior gateway protocols (EGPs): Work between autonomous systems. Themost popular one is BGP.n An autonomous system refers to a group of routers that share the same routingpolicy and work under the same administration.Routing algorithm■ Distance-vector protocols: Includes mainly RIP and BGP. BGP is also considereda path-vector protocol.■ Link-state protocols: Includes mainly OSPF and IS-IS.The main differences between the above two types of routing algorithms lie in theway routes are discovered and calculated.Type of the destination address■ Unicast routing protocols: Includes RIP, OSPF, BGP, and IS-IS.■ Multicast routing protocols: Includes PIM-SM and PIM-DM.This chapter focuses on unicast routing protocols. For information on multicastrouting protocols, refer to “Multicast Overview” on page 1085.Version of IP protocolIPv4 routing protocols: RIP, OSPF, BGP and IS-IS.IPv6 routing protocols: RIPng, OSPFv3, BGP4+, IPv6 IS-IS.Routing Protocols andRouting PriorityDifferent routing protocols may find different routes to the same destination.However, not all of those routes are optimal. In fact, at a particular moment, onlyone protocol can uniquely determine the current optimal routing to thedestination. For the purpose of route selection, each routing protocol (includingstatic routes) is assigned a priority. The route found by the routing protocol withthe highest priority is preferred.The following table lists some routing protocols and the default priorities forroutes found by them.Routing approach PriorityDIRECT 0OSPF 10IS-IS 15STATIC 60RIP 100OSPF ASE 150OSPF NSSA 150IBGP 255
