CS419: Computer NetworksLecture 5, Part 4: Feb 23, 2005Internet Routing:CS419Practical aspects of OSPF (Open Shortest Path First)| Link-state protocolz 189 pages long!!!z (versus 31 pages for RIP)| 2-level hierarchyz Virtual links| Designated router on LANs| Hop-by-hop security| External routesNote: The term “Open” was a marketing attack against cisco, whose routing protocol was proprietaryCS419OSPF scalability| OSPF runs in a single Autonomous System (AS)z But an AS can be big| To improve scalability, the AS can be partitioned into areasz Area is composed of subnets and routersz Areas are connected by a single backbone| Two level hierarchyCS419OSPF backbone and areasCS419Backbone is logically (but not necessarily physically) contiguousCS419Designated routers on LAN| A single broadcast LAN with N routers logically looks like N2point-to-point links| Silly to advertise all of these N2links| Rather, the LAN is advertised as a multi-access link| One router is dynamically elected as a designated router to advertise the link and adjacent routersz A backup is also elected| Spanning tree algorithm modified to cope with multi-access linksCS419OSPF security model| Security is hop-by-hop| Each router authenticates its neighborsz But does not authenticate LSUs!| If a single rogue router joins the algorithm, it can bring down the networkz Claim to have an interface with all subnets!CS419External routes| As an AS, OSPF routers at the AS boundary can reach external IP prefixes| These are advertised in OSPF as external routesz Can be “default route”| Two typesz Type 1: Cost is the sum of intra-AS path and external metricz Type 2: Intra-AS cost is ignored when calculating pathCS419External routes exampleCS419Which path should be taken here?CS419The usual answer: it depends| The dilemma of inter-domain routing is that each AS sets its metrics independentlyz No way to impose uniformityz (as least not in the “unregulated” Internet)| Though an AS can autonomously determine how to view external