PIM-SM overview
In a network where IP multicast traffic is transmitted for multimedia applications, such traffic is blocked at routed
interface (VLAN) boundaries unless a multicast routing protocol is running. Protocol Independent Multicast (PIM)
is a family of routing protocols that form multicast trees to forward traffic from multicast sources to subnets that
have used a protocol such as IGMP to request the traffic. PIM relies on the unicast routing tables created by any
of several unicast routing protocols to identify the path back to a multicast source (reverse path forwarding, or
RPF.) With this information, PIM sets up the distribution tree for the multicast traffic. The PIM-DM and PIM-SM
protocols on the switches covered in this guide enable and control multicast traffic routing.
IGMP provides the multicast traffic link between a host and a multicast router running PIM-SM. Both PIM-SM and
IGMP must be enabled on VLANs whose member ports have directly connected hosts with a valid need to join
multicast groups.
PIM-DM is used in networks where, at any given time, multicast group members exist in relatively large numbers
and are present in most subnets. However, using PIM-DM in networks where multicast sources and group
members are sparsely distributed over a wide area can result in unnecessary multicast traffic on routers outside
the distribution paths needed for traffic between a given multicast source and the hosts belonging to the multicast
group. In such networks, PIM-SM can be used to reduce the effect of multicast traffic flows in network areas
where they are not needed. And because PIM-SM does not automatically flood traffic, it is a logical choice in
lower bandwidth situations.
PIM-SM features
PIM-SM on the switches covered in this guide include:
Routing protocol support
PIM uses whichever IP unicast routing protocol is running on the router. These can include:
• RIP
• OSPF
• Static routes
• Directly connected interfaces
VLAN interface support:
Up to 127 outbound VLANs (and 1 inbound VLAN) are supported in the multicast routing table (MRT) at any
given time. This means the sum of all outbound VLANs across all current flows on a router may not exceed
127. (A single flow may span one inbound VLAN and up to 127 outbound VLANs, depending on the VLAN
memberships of the hosts actively belonging to the flow.) For 2930F switch, 16 VLANs are supported.
Flow capacity:
Up to 2046 flows are supported in hardware across a maximum of 128 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.)
Multicast group to RP mapping:
PIM-SM uses the BSR protocol to automatically resolve multicast group addresses to C-RP routers. In the
current software release, a router administers BSR operation on a PIM-SM domain basis. (BSR zones and
PIM border router operation are not currently supported by the switches covered in this guide.) Note that BSR
operation does not extend to statically configured RPs.
IGMP compatibility:
PIM-SM is compatible with IGMP version 2, and is fully interoperable with IGMP for determining multicast
flows.
Chapter 4 PIM-SM (Sparse Mode)
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