QTECH QSW-8300 Инструкция по настройке - Manual (Page 237)

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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.
In DVMRP, source network routing selection message are exchanged in a basic manner same
to RIP. That is, routing report message is transmitted among DVMRP neighbors periodically
(the default is 60 seconds). The routing information in DVMRP routing selection table is used
to set up source distribution tree, i.e. to determine by which neighbor it passes to get to the
source transmitting multicast packet; the interface to this neighbor is called upstream interface.
The routing report includes source network (use net mask) address and the hop entry for
routing scale.
In order to finish transmission correctly, every DVMRP switch needs to know which
downstream switches need to receive multicast packet from some specific source network
through it. After receiving packets from some specific source, DVMRP switch firstly will
broadcast these multicast packets from all downstream interfaces, i.e. the interfaces on which
there are other DVMRP switches which have dependence on the specific source. After
receiving Prune message from some downstream switch on the interface, it will prune this