Dvmrp; Introduction To Dvmrp - Accton Technology ES4710BD User Manual

correctness must be ensured.
PIM-SM protocol requires the support of RP and BSR. So "show ip pim bsr-router"
command should be run first for BRS information, if no BSR exists, then the unicast route to
BSR should be checked.
Use the "show ip pim rp" command to verify RP information is correct. If no RP information
is displayed, the unicast route should be checked, too.
If PIM-SM problems persist after the above-mentioned procedures, please run debugging
commands such as "debug ip pim" / "debug ip pim bsr", and copy the output debug information in 3
minute and send the information to Edge-Core technical service center.

19.5 DVMRP

19.5.1 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., interest is in where the
packet comes from instead of where it is going to). This is 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). A tunnel acts as the virtual network between two DVMRP switches. The
multicast 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
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