DVMRP Operation

DVMRP Operation

The basic operation of DVMRP consists of four processes. The first process is neighbor discovery, which is used to find other DVMRP-capable and enabled routers attached to a common network. The second process is that of route exchange. Although the DVMRP route exchange process is similar to RIP, there are important differences that will be demonstrated. The purpose of a multicast routing protocol is the efficient delivery of multicast datagrams to destinations that want to receive them. Therefore, DVMRP must interoperate with IGMP to determine if multicast packets need to be forwarded onto a network (receivers are present) or if the packets need to be prevented from reaching a network (no receivers present). DVMRP can dynamically add or delete networks from the list of networks that desire to receive multicast traffic from a particular group. The final two basic DVRMP processes are used to achieve this dynamic nature. Networks are added to the for warding list using Graft messages, while networks are removed from the forwarding list using Prune messages.

DVMRP messages are sent using the IP packet format (see Figure 5-12) with no options  and with the protocol field set to 2, identifying the packet type as an IGMP message. The type field is set to 19 (0x13) to identify the IGMP packet as a DVMRP message and the code field is used to differentiate between the various DVMRP packets, as shown in Table 5-6.

Figure 5-12: Encapsulation of a DVMRP packet in an IP datagram

Table 5-6: DVMRP Packet Type Identifiers

Packet Type

Code

Probe

1

Route Report

2

Ask-Neighbors2

5

Neighbors2

6

Prune

7

Graft

8

Graft Ack

9