Objectives After completing this chapter you will be able to: Understand how routers operate Describe Distance Vector and Link State Algorithms Describe hierarchies in IP routing Outline the advantages and disadvantages of routing

Path Taken by an IP Packet in an Internetwork A B C Destination Host (Protocol Address) Router 1 (Physical address) Destination Host (Protocol Address) Router 2 (Physical address) Destination Host (Protocol Address) Destination Host (Physical address)

IP Version 4 Message Header Source Address Version Type of Service Total LengthIHL IdentificationFragment Offset ProtocolTime to LiveHeader Checksum Destination address Padding Options (variable) IP header is normally 20 bytes long Flags DATA (variable)

Default Gateway IP Address Subnet Mask Default Gateway IP Address Subnet Mask Default Gateway IP Address Subnet Mask Default Gateway

Proxy ARP IP Address Subnet Mask Default Gateway IP Address Subnet Mask Default Gateway IP Address Subnet Mask Default Gateway

Routing Tables

Routing Algorithms Distance Vector Algorithms - calculate the best path to each destination separately, usually trying to find a path that minimises a simple metric, such as the number of hop counts to the destination. Example is Routing Information Protocol (RIP). Link State Algorithms - Instead of advertising a list of distances to each known destination, a router running link-state algorithm advertises the states of its local network links. Example is Open Shortest Path First (OSPF).

Commonly used Metrics in IP Routing Path Length / Hop Count Reliability Delay Bandwidth Load Communications Cost

Distance Vector Algorithm A B DE C

IP Routing Protocol Hierarchies Area 1 Router 4 Router 1 Router 5 H2 Router 8 Router 7 Router 11 Router 13 H1 Area 2 Area 3 Router 12 Area 0 (Backbone) Router 2 Router 3 Router 6 Router 9 Router 10

Neighbour Acquisition, Neighbour Reachability and Network Reachability Neighbour Acquisition Neighbour Reachability Network Reachability Acquisition Request / Hello Acquisition Reply / Hello Hello Polling List

Advantages of Routers Flexible - can differentiate between paths using metrics. Can load share over redundant paths. Network Devices understand routers - they understand congestion messages. Easier to administer and control because each segment has a different address. Provide a protective firewall.

Disadvantages of Routers Because routers are protocol-dependent devices, they require software for each protocol that they run. Routers make movement of network devices between network segments more difficult. Some low level protocols, for example, DEC LAT and NetBIOS cannot be routed because they do not contain a network layer address.