Neighbor Discovery

Neighbor Discovery

PIM-DM version 1 packets are encapsulated in Internet Group IGMP packets as shown in Figure 6-5. PIM-DM packets have a common header (see Figure 6-6) which contains a code identifying the PIM-DM message type and the PIM mode, dense, sparse or sparse-dense. The message types are listed in Table 6-1 and neighbor discovery or router query messages (see Figure 6-7) are identified as type 0 (see Table 6-2). Router query messages are used to discover neighbors that are attached to a common network. Discovery may be a misleading term since there is not an explicit neighbor list section comparable to a DVMRP neighbor discovery message.

Figure 6-5: Encapsulation of a PIM-DM version 1 packet in an IGMP datagram

Figure 6-6: PIM-DM version 1 packet header

Table 6-1: PIM-DM version 1 Message Codes

Code

Message Type

0

Router Query

1

Register (Sparse Mode)

2

Register-Stop (Sparse Mode)

3

Join/Prune

4

RP Reachability (Sparse Mode)

5

Assert

6

Graft

7

Graft-ACK

Table 6-2: PIM-DM version 1 Query Message Modes

Code

Mode

0

Dense Mode

1

Sparse

2

Sparse-Dense

Figure 6-7: PIM-DM version 1 Query Message packet format

A better name for a router query message could be a neighbor inform message. When a neighbor receives a query message, the IP address of the neighbor is recorded. No explicit mechanism acknowledges that the query was received. Instead, the receiving router will simply transmit its own query message that has the effect of informing other PIM-DM routers on the network of its existence. When a query message is received from a neighbor, the interface is added to the outgoing interface list. The outgoing interface list is used to determine which interfaces the PIM-DM router should forward multicast traffic. Of course, if there are no other PIM-DM routers on the network, the interface would be added to the outgoing list if there are receivers requesting traffic for a particular multicast group. For a multi-access network, such as an ethernet, the query message is sent to the All-Routers multicast address, 224.0.0.2, and serves as the Designated Router (DR) election mechanism. For dense mode PIM, the designated router only has a function if IGMP version 1 is being used. In this case, the DR becomes the IGMP querier for the network (see Chapter 3). The elected DR is the PIM-DM enabled router with the highest IP address. The query process and DR election is shown in Figure 6-8. For this scenario, router C is elected DR since it has the highest IP address on the multi-access network.

Figure 6-8: PIM-DM router query and DR election

The holdtime parameter in the router query message indicates how much time will elapse before this neighbor is declared dead. Subsequent router queries from a neighbor will reset this time so the query interval must be less than the holdtime interval. The router queries act as a keep-alive mechanism to inform neighboring routers that this router is still alive and well. If PIM-DM is disabled on the interface or the router actually crashes and burns, the holdtime for this router will expire on the neighboring routers. If the holdtime expires for a neighbor that was elected DR for the multi-access network, then a new DR will need to be elected.