Specifications

304 ExtremeWare XOS 11.0 Concepts Guide
IP Multicast Routing
PIM Overview
The switch supports both dense mode and sparse mode operation. You can configure dense mode or
sparse mode on a per-interface basis. After they are enabled, some interfaces can run dense mode, while
others run sparse mode.
PIM Dense Mode
Protocol-Independent Multicast - Dense Mode (PIM-DM) is a multicast routing protocol. PIM-DM
routers perform reverse path multicasting (RPM). However, instead of exchanging its own unicast route
tables for the RPM algorithm, PIM-DM uses the existing unicast routing table for the reverse path. As a
result, PIM-DM requires less system memory.
PIM-DM is a broadcast and prune protocol, which allows you to prune and graft multicast routes.
PIM Sparse Mode
Unlike PIM-DM, Protocol-Independent Multicast - Sparse Mode (PIM-SM) is an explicit join and prune
protocol, and it supports shared trees as well as shortest path trees (SPTs). The routers must explicitly
join the group(s) in which they are interested in becoming a member, which is beneficial for large
networks that have group members that are sparsely distributed.
Using PIM-SM, the router sends a join message to the rendezvous point (RP). The RP is a central
multicast router that is responsible for receiving and distributing multicast packets. You can configure a
dynamic or static RP.
When a router has a multicast packet to distribute, it encapsulates the packet in a unicast message and
sends it to the RP. The RP decapsulates the multicast packet and distributes it among all member
routers.
When a router determines that the multicast rate has exceeded a configured threshold, that router can
send an explicit join to the originating router. When this occurs, the receiving router gets the multicast
directly from the sending router and bypasses the RP.
NOTE
You can run either PIM-DM or PIM-SM per virtual LAN (VLAN_.
PIM Mode Interoperation
An Extreme Networks switch can function as a PIM multicast border router (PMBR). A PMBR
integrates PIM-SM and PIM-DM traffic.
When forwarding PIM-DM traffic into a PIM-SM network, the PMBR notifies the RP that the PIM-DM
network exists. The PMBR forwards PIM-DM multicast packets to the RP, which, in turn, forwards the
packets to those routers that have joined the multicast group.
The PMBR also forwards PIM-SM traffic to a PIM-DM network, based on the (*.*.RP) entry. The PMBR
sends a join message to the RP, and the PMBR forwards traffic from the RP into the PIM-DM network.
No commands are required to enable PIM mode interoperation. PIM mode interoperation is
automatically enabled when a dense mode interface and a sparse mode interface are enabled on the
same switch.