Accton Technology ES4626 - Manual (Page 646)

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16.4 DVMRP
16.4.1 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 treat 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