Fujitsu PSWITCH 2048T User Manual page 93

User's Guide
distributed routing algorithm to build per-source-group multicast trees. It is also
called Broadcast and Prune Multicasting protocol. It dynamically generates
per-source-group multicast trees using Reverse Path Multicasting. Trees are
calculated and updated dynamically to track membership of individual groups.
The DVMRP protocol operates as follows:
1. The first message for any (source, group) pair is forwarded to the entire
multicast network, with respect to the packet's time-to-live (TTL).
2. TTL restricts the area to be flooded by the message.
3. Any leaf router that does not have members on directly attached subnetworks
sends back prune messages to the upstream router.
4. The branch that transmitted a prune message is deleted from the delivery tree.
DVMRP transmits two types of messages:
 Protocol messages:
These include Probe messages, Prune messages, Graft messages and Graft
Acknowledgements, and so on, which are sent as multicast packets. Most
protocol messages are addressed to ALL-DVMRP-ROUTERS.
 Routing messages:
These are unicast messages used to exchange routing information. They are
typically sent by a neighboring router when it is restarted, has lost its routing
information, or has come up from a down state. They are also transmitted
when a graft is sent in response to a new member's subscription to an existing
group, or when a new group is subscribed.
When messages arrive, the reverse path to the source of the message is discovered
by examining the routing table. If the message arrived on the interface that would
be used to transmit the message back to the source, the message is transmitted to
downstream routers. Otherwise, the message is not on the optimal delivery tree, so
the packet is dropped. In this way, loops and duplicates are filtered. DVMRP
discovers its neighbors by periodically sending probe messages with a TTL of one
(1). These probe messages contain the list of neighboring DVMRP routers from
which a probe has been received. With this, a two-way neighbor relationship is
established.
When two different routers are connected to the same multi-access network, there
is a possibility of getting duplicate packets. To avoid getting duplicate packets,
nominate a Designated Forwarder (DF) for each network. The router with the least
metric is the designated forwarder. In case of a tie, the router with the lower IP
address is the designated forwarder. Once a designated forwarder is elected, the
multicast trees are built. If the destination group of a packet exists in the local
database and the router on which the packet arrived is the designated forwarder,
then that interface is added to the downstream interface list.
December/2018
FUJITSU PSWITCH
93