Flow capacity
Up to 2046 flows are supported in hardware across a maximum of 128 outbound VLANs. (A flow is composed
of an IP source address and an IP multicast group address, regardless of the number of active hosts
belonging to the multicast group at any given time.)
IGMP compatibility
PIM-DM is compatible with IGMP (V1 to V3) and is fully interoperable with IGMP for determining multicast
flows.
VRRP
PIM-DM is fully interoperable with VRRP to quickly transition multicast routes in the event of a failover.
MIB support
With some exceptions, PIM-DM supports the parts of the multicast routing MIB applicable to PIM-DM
operation.
PIM draft specifications
Compatible with PIM-DM draft specifications (V1 and V2.)
PIM-DM operation
PIM-DM operates at the router level to direct traffic for a particular multicast group along the most efficient path to
the VLANs which have hosts that have joined that group. A unicast source address and a multicast group address
comprise a given source/group (S/G) pair. Multicast traffic moving from a source to a multicast group address
creates a flow to the area(s) of the network requiring the traffic. The flow destination is the multicast group
address and not a specific host or VLAN. A single multicast flow has one source and one multicast group address
(destination), but may reach many hosts in different subnets, depending on which hosts have issued joins for the
same multicast group.
PIM routes the multicast traffic for a particular S/G pair on paths between the source unicast address and the
VLANs where it is requested (by joins from hosts connected to those VLANs.) Physical destinations for a
particular multicast group can be hosts in different VLANs or networks. Individual hosts use IGMP configured per-
VLAN to send joins requesting membership in a particular multicast group. All hosts that have joined a given
multicast group (defined by a multicast address) remain in that group as long as they continue to issue periodic
joins.
PIM-DM interoperates with IGMP and the switch's routing protocols for the switches covered by this guide. The
PIM operates independently of the routing protocol you choose to run on your switches. This means that you can
use PIM-DM with RIP, OSPF, or static routes configured. PIM-DM uses a unicast routing table to find the path to
the originator of the multicast traffic and sets up multicast trees for distributing multicast traffic. This routing
method is known as reverse path forwarding (RPF.)
For the flow of a given multicast group, PIM-DM creates a tree structure between the source and the VLANs
where hosts have joined the group as shown in the following figure. The tree structure consists of:
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