User's Manual

48-33
commands such debug pim event/debug pim packet please, and then copy DEBUG information in 3
minutes and send to Technology Service Center.
48.7 DVMRP
48.7.1 Introduction to DVMRP
DVMRP Protocol, namely, is “Distance Vector Multicast Routing Protocol”. It is a Multicast Routing Protocol in
dense mode, which sets up a Forward Broadcast Tree for each source in a manner similar to RIP, and sets up
a Truncation Broadcast Tree, i.e. the Shortest Path Tree to the source, for each source through dynamic
Prune/Graft.
Some of the important features of DVMRP are:
1. The routing exchange used to determine reverse path checking information is based on distance
vector (in a manner similar to RIP)
2. Routing exchange update occurs periodically (the default is 60 seconds)
3. TTL upper limit = 32 hops (and that RIP is 16)
4. Routing update includes net mask and supports CIDR
In comparison with Unicast routing, Multicast routing is a kind of reverse routing (that is, what you are
interested in is where the packets are from but not where they go), thus the information in DVMRP routing
table is used to determine if an input Multicast packet is received at the correct interface. Otherwise, the
packet will be discarded to prevent Multicast circulation.
The check which determines if the packet gets to the correct interface is called RPF check. When some
Multicast data packets get to some interface, it will determine the reverse path to the source network by
looking up DVMRP router table. If the interface data packets get to is the one which is used to send Unicast
message to the source, then the reverse path check is correct, and the data packets are forwarded out from
all downstream interfaces. If not, then probably there is failure, and the Multicast packet is discarded.
Since not all switches support Multicast, DVMRP supports tunnel multicast communication, tunnel is a
method to send multicast data report among DVMRP switches separated by switches which don’t support
multicast routing. Multicast data packets are encapsulated in unicast data packets and directly sent to the
next switch which supports multicast. DVMRP Protocol treats tunnel interface and general physical interface
equally.
If two or more switches are connected to a multi-entrance network, it is likely to transmit more than one copy
of a data packet to the sub-network. Thus a specified transmitter must be appointed. DVMRP achieves this
goal by making use of routing exchange mechanism; when two switches on the multi-entrance network
exchange routing information, they will be aware of the routing distance from each other to the source network,
thus the switch with the shortest distance to the source network will become the specified transmitter of the
sub-network. If some have the same distance, then the one with the lowest IP prevails.
After some interface of the switch is configured to Function DVMRP Protocol, the switch will multicast Probe
message to other DVMRP switches on this interface, which is used to find neighbors and detect the
capabilities of each other. If no Probe message from the neighbor is received until the neighbor is timed out,
then this neighbor is considered missing.