1-4z Exterior gateway protocols (EGPs): Work between autonomous systems. The most popular one isBGP.An autonomous system refers to a group of routers that share the same routing policy and work underthe same administration.Routing algorithmz Distance-vector protocols: RIP and BGP. BGP is also considered a path-vector protocol.z Link-state protocols: OSPF and IS-IS.The main differences between the above two types of routing algorithms lie in the way routes arediscovered and calculated.Type of the destination addressz Unicast routing protocols: RIP, OSPF, BGP, and IS-IS.z Multicast routing protocols: PIM-SM and PIM-DM.This chapter focuses on unicast routing protocols. For information on multicast routing protocols, seethe IP Multicast Configuration Guide.Version of IP protocolIPv4 routing protocols: RIP, OSPFv2, BGP4, and IS-IS.IPv6 routing protocols: RIPng, OSPFv3, BGP4+, and IPv6 IS-IS.Routing Protocols and Routing PriorityDifferent routing protocols may find different routes to the same destination. However, not all of thoseroutes are optimal. In fact, at a particular moment, only one protocol can uniquely determine thecurrent optimal route to the destination. For the purpose of route selection, each routing protocol(including static routes) is assigned a priority. The route found by the routing protocol with the highestpriority is preferred.The following table lists some routing protocols and the default priorities for routes found by them:Routing approach PriorityDIRECT 0OSPF 10IS-IS 15STATIC 60RIP 100OSPF ASE 150OSPF NSSA 150
