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ROUTING PROTOCOLS

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.

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ROUTING PROTOCOLS

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  1. ROUTING PROTOCOLS Rizwan Rehman

  2. 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

  3. 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

  4. 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

  5. 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)

  6. 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 OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89 TTL: set to 1 (in most cases) Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

  8. Discovery of Neighbors • Routers multicasts OSPFHello 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 10.1.10.2 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

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