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Chapter 5 - Distance Vector Multicast Routing Protocol

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

Unicast Versus Multicast Routing
An IP unicast routing protocol (RIP, IGRP, EIGRP, OSPF, and BGP) is used to determine a path from a sender (source) to a single receiver (destination). Each router along the path from the source to the destination must contain a routing table that indicates which interface to use to forward the packet in order to reach the final destination. This route can either be learned by a dynamic IP routing protocol, a static route, or a default route. As the packet is routed through the network, routers inspect the destination IP address to determine the next hop to the final destination and the source address is not used in making the routing decision.
Of fundamental importance to this discussion is the fact that the destination IP address is a Class A, B, or C unicast address. In Figure 5-1, we have a simple network with a source (172.16.1.1) that is sending to a destination (172.16.5.1). It is a simple matter for each router to determine the path to the destination. Assume that only default and directly connected routes are being used in routers A, B, and C. When the packet from the source arrives at router A, the destination address in the IP packet is examined and checked against the routing table. Router A has four routes, three are directly attached, and one is a default route that says “send every packet that is not destined for one of my three directly attached networks out the serial link.” Routers B and C have similar routing tables. As the packet travels through the network, each router checks the destination IP address, consults the routing table, and forwards the packet out the proper interface.
Figure 5-1: Routing of a unicast IP packet from source to destination

 


 
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