PLANET XGS3-24042 - Manual (Page 434)

48-33
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 switch. DVMRP switch makes use of poison reverse to notify the upstream switch for some specific
source: “I am your downstream.” By adding infinity (32) to the routing distance of some specific source it
broadcasts, DVMRP switch responds to the source upstream exchange to fulfill poison reverse. This means
distance correct value is 1 to 2* infinity (32) -1 or 1 to 63, 1 to 63 means it can get to source network, 32
means source network is not arrival, 33 to 63 means the switch which generates the report message will
receive multicast packets from specific source depending on upstream router.