D-Link xStack Reference Manual page 553

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xStack DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch CLI Manual
Multicast sources do not always join the intended receiver group. The first hop router (DR) can send multicast data
without being the member of a group or having a designated source, which essentially means it has no information about
how to relay this information to the RP distribution tree. This problem is alleviated through Register and Register-Stop
messages. The first multicast packet received by the DR is encapsulated and sent on to the RP which in turn removes
the encapsulation and sends the packet on down the RP distribution tree. When the route has been established, a SPT
can be created to directly connect routers to the source, or the multicast traffic flow can begin, traveling from the DR to
the RP. When the latter occurs, the same packet may be sent twice, one type encapsulated, one not. The RP will detect
this flaw and then return a Register Suppression message to the DR requesting it to discontinue sending encapsulated
packets.
Assert Messages
At times on the PIM-SM enabled network, parallel paths are created from source to receiver, meaning some receivers will
receive the same multicast packets twice. To improve this situation, Assert messages are sent from the receiving device
to both multicast sources to determine which single router will send the receiver the necessary multicast data. The source
with the shortest metric (hop count) will be elected as the primary multicast source. This metric value is included within
the Assert message.
PIM-DM
The Protocol Independent Multicast - Dense Mode (PIM-DM) protocol should be used in networks with a low delay (low
latency) and high bandwidth as PIM-DM is optimized to guarantee delivery of multicast packets, not to reduce overhead.
The PIM-DM multicast routing protocol is assumes that all downstream routers want to receive multicast messages and
relies upon explicit prune messages from downstream routers to remove branches from the multicast delivery tree that do
not contain multicast group members.
PIM-DM has no explicit 'join' messages. It relies upon periodic flooding of multicast messages to all interfaces and then
either waiting for a timer to expire (the Join/Prune Interval) or for the downstream routers to transmit explicit 'prune'
messages indicating that there are no multicast members on their respective branches. PIM-DM then removes these
branches ('prunes' them) from the multicast delivery tree.
Because a member of a pruned branch of a multicast delivery tree may want to join a multicast delivery group (at some
point in the future), the protocol periodically removes the 'prune' information from its database and floods multicast
messages to all interfaces on that branch. The interval for removing 'prune' information is the Join/Prune Interval.
The Protocol Independent Multicast (PIM) commands in the Command Line Interface (CLI) are listed below, along with
their appropriate parameters, in the following table.
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