ROUTING PROTOCOLS Rizwan Rehman

Static routing  each router manually configured with a list of destinations and the next hop to reach those destinations  ideal for small number of destinations or “stub” networks  stub network - network with only one or two paths to the rest of the network

Dynamic Routing  routers compute routing tables dynamically based on information provided by other routers in the network  routers communicate topology to each other via different protocols  routers then compute one or more next hops for each destination - trying to calculate the most optimal path

Static and Dynamic Routing  Static routing is a simplistic approach  Shortcomings:  Cumbersome to configure  Cannot adapt to link/node failures, addition of new nodes and links  Doesn't scale to large networks  Solution: Dynamic Routing

Desirable Characteristics  Automatically detect and adapt to network topology changes  Optimal routing  Scalability  Robustness  Simplicity  Speed of convergence  Some control of routing choices (e.g. which links we prefer to use)

Convergence -  Convergence is when all the routers have the same routing information  When a network is not converged, there is network downtime  Packets don't get to where they are supposed to be going: routing loops, black holes  Occurs when there is a change in the status of a router or link

7 OSPF Packet Format Destination IP: neighbor’s IP address or (ALLSPFRouters) or (AllDRouters) TTL: set to 1 (in most cases) OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89

8 Discovery of Neighbors  Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.  If two routers share a link, they can become neighbors, and establish an adjacency  After becoming a neighbor, routers exchange their link state databases Scenario: Router restarts

9 Autonomous Systems (AS)

10 BGP  BGP = Border Gateway Protocol  Currently in version 4  Note: In the context of BGP, a gateway is nothing else but an IP router that connects autonomous systems.  Interdomain routing protocol for routing between autonomous systems  Uses TCP to send routing messages  BGP is neither a link state, nor a distance vector protocol. Routing messages in BGP contain complete routes.  Network administrators can specify routing policies

11 BGP