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Understanding Routing Protocols: Role and Functions of Routers in Autonomous Systems

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  1. Routing Protocols COSC 541 Data Commun. System & Networks Yue Dou

  2. Router Function • Routers responsible for receiving and forwarding packets through the interconnected set of networks • Each router makes routing decision based on knowledge of the topology and conditions of the internet

  3. Routing Protocal • Routers need to make dynamic routing decisions to avoid portions of the network that have failed or congested. • Routers need the routing protocol to exchange routing information about the topology and delays of the internet

  4. Autonomous Systems ( AS ) • An AS consists of a group of routers exchanging information via an interior router protocol • An AS is a set of routers and networks managed by a single organization ( such as an university ) • Except failure, there is a path between any two pair of routers in AS

  5. An Example of Autonomous System

  6. Internet Construction • Internet construct with many autonomous systems • Routing protocol divides into interior and exterior router protocol • Interior router protocol ( IRP ) passes routing information between routers within an autonomous system • Exterior router protocol ( ERP ) passes routing information between routers in different autonomous system

  7. Interior & Exterior router protocol • An interior router protocol needs to build up a rather detailed model of the interconnection of routers within an AS in order to calculate the least-cost path from a give router to any network within the AS. • An exterior router protocol supports the exchange of summary reachability information between separately administered ASs.

  8. Border Gateway Protocol ( BGP ) • Preferred exterior router protocol for the Internet • BGP allow gateway routers in different autonomous systems (ASs ) to cooperate in the exchange of routing information • BGP operates in terms of 4 messages: Open, Update, Keepalive, Notification

  9. BGP 3 Functional Procedures • Neighbor acquisition: one router sends an open message to the other, which may either return a keepalive message or refuse the offer • Neighbor reachability: the two routers periodically issue Keepalive messages to each other • Network reachability: when a router change its routing information, it will broadcast an Update message

  10. BGP Routing Information Exchange • Border router in one AS can issue an Update message to the border router in another AS • The Update message could include: AS_Path: the identity of AS(s) Next_Hop: the IP address of border router NLRI: a list of all of the networks in AS(s)

  11. Open Shortest Path First (OSPF) Protocol • Widely used as the interior router protocol in TCP/IP networks • Each router maintains a routing table that reflects the known topology of the autonomous system of which it is a part • OSPF computes a route through the internet that incurs the least cost based on a user-configurable metric of cost ( like data rate, dollar cost etc.)

  12. An Example of Routing Table Destination Next Hop Distance ------------------------------------------------------ N1 R3 10 N7 R10 12 R7 R10 8 ------------------------------------------------------- Destination: LAN or border router Next Hop: Forwarding router Distance: Sum of cost on the whole route

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