BROCADE IP PRIMER

Chapter 11: Open Shortest Path First (OSPF)Autonomous Systems (AS)In OSPF, an Autonomous System (AS) is a group of routers that operate underthe same routing protocol. An implementation of OSPF, whether it has onearea or a thousand areas, constitutes one Autonomous System. A group ofrouters that are using RIP constitute a different Autonomous System. A groupof routers using BGP constitute yet another Autonomous System. You get theidea. The routing protocol being used defines the Autonomous System.Why is this important? Well, there are not too many network infrastructuresthat run one, and only one, routing protocol. Especially in larger businesses, itis very common to come across connected systems that use OSPF, BGP, RIP,and others. OSPF designates these as individual Autonomous Systems. Theforeign Autonomous Systems (e.g., BGP, RIP, etc.) can share their routes withOSPF, and OSPF can share its routes with the foreign Autonomous Systems.But to do so, we need to define the Autonomous Systems boundary.Autonomous System Boundary Router (ASBR)An Autonomous System Boundary Router (ASBR) is a router that participatesin more than one routing protocol. This is a router that may have one interfaceparticipating in OSPF, and another interface participating in RIP, BGP, or someother protocol. Look back at our area example. Router K (in Area 4) is anASBR. It has one interface in Area 4 participating in OSPF, and it has anotherinterface participating in RIP. Router E in Area 1 is also an ASBR. It has oneinterface participating in OSPF Area 1, and another interface participating inBGP to the Internet.Like an ABR, the ASBR need not be a DR or a BDR. Its role is defined simplybecause it is participating in one or more routing protocols in addition to OSPF.Because of this, it has a responsibility of sharing, or not sharing, routesbetween the routing protocols. This is also referred to as route redistribution.Getting RIP and OSPF To Talk To Each OtherJust because you see a route in your switch's routing table does not mean thatyour routing protocol is going to advertise it. OSPF will advertise connectedroutes (of interfaces that are participating in OSPF) and routes that it learnsfrom other neighbors, but that's it. It will not advertise routes learned from RIP,BGP, etc., nor will it advertise static routes. It can, but you have to tell it explicitlyto do so.BR-Switch#conf tBR-Switch(config)#router ospfBR-Switch(config-ospf-router)#redistribution ripThis command will take all of the routes that the router learned from RIP, andinclude them in the advertisements to OSPF routers.246 Brocade IP Primer