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16.5 DVMRP
16.5.1 Introduction to DVMRP
Distance Vector Multicast Routing Protocol (DVMRP) is a dense mode multicast routing
protocol. It employs a RIP like route exchange mechanism to establish a forwarding
broadcast tree for each source, then a truncated broadcast tree (short path tree to the
source) will be created by dynamic pruning/grafting. Reverse path forwarding (RPF) is
used to decide whether multicast packet should be forwarded to the downstream nodes.
The following are some important DVMRP features:
1. The route exchange process determining RPF information is based on distance
vectors (in the way similar to RIP)
2. Route exchange occurs periodically (every 60 seconds by default)
3. Maximum TTL = 32 hops (rather than the 16 hops in RIP)
4. Mask included in route update packet, CIDR supported.
Comparing to unicast routing, the multicast routing is a reversed route (i.e., you interested
in where the packet comes from instead of where it is going to). That’s why the route table
information in DVMRP is used to determine whether the incoming multicast packet is
arriving at the correct interface. The packet is discarded if the interface is not correct to
prevent multicast loop.
The test to determine whether a packet is arriving at the correct interface is called RPF
check. When a multicast packet arrives at an interface, the DVMRP route table will be
checked to decide the reverse path to the source network. If the interface at which the
packet arrives is the interface to send unicast information to the source, then the RPF
check is success and the packet is forwarded from all down stream interfaces. Otherwise,
there may be something wrong, and the multicast packet is discarded.
Since not all switches support multicast, DVMRP provide support for tunneling multicast
information. Tunneling is a method used between DVMRP switches separated by
non-multicast routing switch(es). The tunnel acts as the virtual network between two
DVMRP switches. The multcast packet is encapsulated in a unicast packet and destined
to a multicast-enabled switch. DVMRP treats tunneling interface the same way as
common physical interfaces.
If two or more switches are connected to a multi-egress network, multiple copies of a
packet may be sent to the subnet. Therefore, a specific forwarder must be specified.
DVMRP fulfills this by routing switch mechanism. When two switches in a multi-egress
network are exchanging routing information, they know the route metric for each other to
get to the source network, and the switch has the smallest metric to the source network
becomes the designated forwarder of that subnet; if the metrics are same, the one with
lower IP address rules.
When DVMRP is enabled on an interface of the switch, probe messages are multicasted
to the other DVMRP switches to discover the neighbors and their capabilities. If no probe
message from a neighbor is received before the neighbor timeout, it is regarded as lost.
In DVMRP, source network route selection information is exchanged in the same basic