Distance Vector Routing Chapter 12 Remainder of Ch 12 Distance Vector Routing Section: 12.6.7, Pages 726-733 Dr. Clincy

(How the routers’ tables are filled in) Unicast Routing Protocols (RIP, OSPF, BGP) (How the routers’ tables are filled in) Dr. Clincy

Explain how a router uses a routing table when a packet arrives ? Before Starting Explain how a router uses a routing table when a packet arrives ? Dr. Clincy

Routing Protocols At this stage, we understand how a router uses a routing table in making a next hop decision However, what dictates HOW the routing tables are filled in ? Tables are concerned about the next hop only What’s responsible for looking across the entire path or route – what makes the decision of the best route ? Routing Protocols and Algorithms are used Routing protocols allow routers to share info with one another dynamically - as the Internet makes changes, the routing protocols allow routers to inform other routers Routers communicate to their neighboring routers - gossip Routing protocols implement the procedures for combining info received from other routers Routing Algorithms – decision making analysis – the “brains” – using the info provided Dr. Clincy

Autonomous systems Because the Internet is so large, one protocol cannot handle all of the updating of tables – create groups and networks and routers called Autonomous Systems Routing within the autonomous system is called “interior routing” Routing between the autonomous systems is called “exterior routing” NOTE: different interior routing protocols can be used for each autonomous systems HOWEVER, only one exterior routing protocol is used R1, R2, R3 and R4 use an interior and exterior routing protocol – all other routers only use an interior routing protocol Dr. Clincy

How does it work ? Tx Rx A certain “cost” or “metric” is assigned each network In figuring out the best route from Tx to Rx, the set of networks with the smallest sum is chosen More generically, the set of networks best meeting the “metric’s” objective is chosen If #hops was the metric, we would want to traverse the least number of networks in going from Tx to Rx If max throughput was the metric, a fiber optic network would have a better metric than a coaxial network. Dr. Clincy

Explain routing using your street/highway analogy Autonomous systems Default routing Dr. Clincy

Popular Unicast Routing protocols RIP – Routing Information Protocol – treats each network the same (assigns the same cost for each network) OSPF – Open Shortest Path First protocol – assigns a cost for passing through a network based on the type of service required – routes through the network can have different cost – each router would have several tables BGP – Border Gateway Protocol – is an exterior routing protocol that uses a policy that defines what paths should be chosen Dr. Clincy

RIP Algorithm Recall: each router sends message to it’s neighbor Distance Vector Algorithm built from Bellman-Ford Algorithm Recall: each router sends message to it’s neighbor For the router receiving a RIP response 1st – add one to hop count for each destination advertised 2nd – repeat the following steps for each advertised destination 1. If destination is not in table add destination to table 2. Else if destination is in table 1. If next-hop field is the same replace entry in table with advertised one 2. Else next-hop different replace entry if advertised hop count is less Dr. Clincy

Explain RIP in Simple English Dr. Clincy

Example RIP Algorithm Router receives RIP message for some router C The RIP message list destination networks, corresponding hop count and next hop (not listed in diagram) 1st step: increment hop count Net1: no news, don’t change Net2: same next hop, so replace 2 with 5 Net 3: new router, so add Net 6: different next hop, new hop count less, so replace Net 8: different next hop, new hop count the same, don’t change Net 9: different next hop, new hop count larger, do not change Dr. Clincy

Initial routing tables in a small autonomous system Initial tables are created from config file (and hop counts are set to 1) – next hop fields are empty initially because all networks are directly connected Dr. Clincy

Final routing tables for the previous figure For example, suppose packet hitting Router A first had a destination of Net 66 ? Dr. Clincy