HP 3500YL - Manual (Page 69)

PIM-DM (Dense Mode)
PIM-DM Operation
Multicast Flow Management
This section provides details on how the routing switch manages forwarding
and pruned flows. This information is useful when planning topologies to
include multicast support and when viewing and interpreting the Show com­
mand output for PIM-DM features.
Initial Flood and Prune. As mentioned earlier, when a router running PIM­
DM receives a new multicast flow, it initially floods the traffic to all down­
stream multicast routers. PIM-DM then prunes the traffic on paths to VLANs
that have no host joins for that multicast address. (Note that PIM-DM does not
re-forward traffic back to its source VLAN.)
Maintaining the Prune State. For a multicast group “X” on a given VLAN,
when the last host belonging to group “X” leaves the group, PIM places that
VLAN in a prune state, meaning the group “X” multicast traffic is blocked to
that VLAN. The prune state remains until a host on the same VLAN issues a
join for group “X”, in which case the router cancels the prune state and
changes the flow to the forwarding state.
State Refresh Packets and Bandwidth Conservation. A multicast
switch, if directly connected to a multicast source such as a video conferenc­
ing application, periodically transmits state refresh packets to downstream
multicast routers. On routers that have pruned the multicast flow, the state
refresh packets keep the pruned state alive. On routers that have been added
to the network after the initial flooding and pruning of a multicast group, the
state refresh packets inform the newly added router of the current state of
that branch. This means that if all multicast routers in a network support the
state refresh packet, then the multicast router directly connected to the
multicast source performs only one flood-prune cycle to the edge of the
network when a new flow (multicast group) is introduced, and preserves
bandwidth for other uses. Note, however, that some vendors’ multicast routers
do not offer the state refresh feature. In this case, PIM-DM must periodically
advertise an active multicast group to these devices by repeating the flood/
prune cycle on the paths to such routers. For better traffic management in
multicast-intensive networks where some multicast routers do not offer the
state refresh feature, you may want to group such routers where the increased
bandwidth usage will have the least effect on overall network performance.
3-7