Configuring DVMRP
DVMRP Overview
OmniSwitch AOS Release 7 Advanced Routing Configuration Guide
March 2011
page 5-5
Neighbor Discovery
DVMRP routers must maintain a database of DVMRP adjacencies with other DVMRP routers. A
DVMRP router must be aware of its DVMRP neighbors on each interface. To gather this information,
DVMRP routers use a neighbor discovery mechanism and periodically multicast DVMRP
Probe messages
to the All-DVMRP-Routers group address (224.0.0.4). Each Probe message includes a Neighbor List of
DVMRP routers known to the transmitting router.
When a DVMRP router (let’s call it “router B”) receives a Probe (let’s say from “router A”), it adds the IP
address of router A to its own internal list of DVMRP neighbors on that interface. It then sends a Probe of
its own with the IP address of router A included in the Probe’s Neighbor List. When a DVMRP router
receives a Probe with its own IP address included in the Neighbor List, the router knows that a two-way
adjacency has been successfully formed between itself and the neighbor that sent the Probe.
Probes effectively serve three main purposes:
•
Probes provide a mechanism for DVMRP routers to locate each other as described above.
•
Probes provide a way for DVMRP routers to determine each others’ capabilities. This is deduced from
the major and minor version numbers in the Probe packet and directly from the capability flags in the
Probe packet.
•
Probes provide a keep-alive function in order to quickly detect neighbor loss.
A DVMRP router sends periodic
Route Report
messages to its DVMRP neighbors. A Route Report
message contains the sender’s current routing table, which contains entries that advertise a source network
(with a mask) and a hop-count that is used as the routing metric.This routing information is used to build
source distribution trees and to perform multicast forwarding. The DVMRP neighbor that advertises the
route with the lowest metric will be used for forwarding. (In case of a tie, the DVMRP neighbor with the
lowest IP address will be used.)
In DVMRPv3, a router will not accept a Route Report from another DVMRP router until it has estab-
lished adjacency with that neighboring router.
Note.
Older versions of DVMRP use Route Report messages to perform neighbor discovery rather than
the Probe messages used in DVMRP Version 3.