Tracing and Troubleshooting

Tracing and Troubleshooting

DVMRP contains a mechanism for determining the characteristics of a particular router. The first part of this mechanism is to send a unicast request to a DVMRP router requesting this information. The packet is called an Ask-Neighbors2 and it has the format shown in Figure 5-30. The response to an Ask-Neighbors2 packet is the Neighbors2 response packet, whose format is shown in Figure 5-31.

Figure 5-30: DVMRP Ask-Neighbors2 packet

Figure 5-31: DVMRP Neighbors2 packet format

The capabilities field lists the characteristics of the router, and the possible values are listed in Table 5-8 and the flags values in Table 5-9. The Neighbors2 packet contains a section for each interface on the router from which the information was requested. For each router interface, the Neighbors2 packet contains the metric of the interface, the interface flags, the number of neighbors on the network connected to the interface, and the neighbors’ addresses.

Table 5-8: DVMRP Router Capabilities

Bit

Flag

Description

0

Leaf

This is a leaf router

1

Prune

Router understands pruning

2

GenID

Router sends generation IDs

3

Mtrace

Router handles Mtrace requests

4

SNMP

Router supports the DVMRP MIB

Table 5-9: DVMRP Interface Flags

Bit

Flag

Description

0

Tunnel

Neighbor reached via a tunnel

1

Source Route

Tunnel uses IP source routing

2

Reserved

No longer used

3

Reserved

No longer used

4

Down

Operational status down

5

Disabled

Administrative status down

6

Querier

Querier for the interface

7

Leaf

No downstream neighbors on this interface