See the Advanced Configuration Guide for more information about IMPM as well as detailed configuration examples.
IMPM allows the system to manage Layer 2 and Layer 3 IP multicast flows by sorting them into the available multicast paths through the switch fabric. The ingress multicast manager tracks the amount of available multicast bandwidth per path and the amount of bandwidth used per IP multicast stream. The following traffic is managed by IMPM when enabled:
IPv4 and IPv6 routed multicast traffic
VPLS IGMP snooping traffic
VPLS PIM snooping for IPv4 traffic
VPLS PIM snooping for IPv6 traffic (only when sg-based forwarding is configured)
Two policies define how each path should be managed, the bandwidth policy, and how multicast channels compete for the available bandwidth, the multicast information policy.
Chassis multicast planes should not be confused with IOM/IMM multicast paths. The IOM/IMM uses multicast paths to reach multicast planes on the switch fabric. An IOM/IMM may have less or more multicast paths than the number of multicast planes available in the chassis.
Each IOM/IMM multicast path is either a primary or secondary path type. The path type indicates the multicast scheduling priority within the switch fabric. Multicast flows sent on primary paths are scheduled at multicast high priority while secondary paths are associated with multicast low priority.
The system determines the number of primary and secondary paths from each IOM/IMM forwarding plane and distributes them as equally as possible between the available switch fabric multicast planes. Each multicast plane may terminate multiple paths of both the primary and secondary types.
The system ingress multicast management module evaluates the ingress multicast flows from each ingress forwarding plane and determines the best multicast path for the flow. A specific path can be used until the terminating multicast plane is ‟maxed” out (based on the rate limit defined in the per-mcast-plane-capacity commands) at which time either flows are moved to other paths or potentially blackholed (flows with the lowest preference are dropped first). In this way, the system makes the best use of the available multicast capacity without congesting individual multicast planes.
The switch fabric is simultaneously handling both unicast and multicast flows. The switch fabric uses a weighted scheduling scheme between multicast high, unicast high, multicast low and unicast low when deciding which cell to forward to the egress forwarding plane next. The weighted mechanism allows some amount of unicast and lower priority multicast (secondary) to drain on the egress switch fabric links used by each multicast plane. The amount is variable based on the number of switch fabric planes available on the amount of traffic attempting to use the fabric planes. The per-mcast-plane-capacity commands allows the amount of managed multicast traffic to be tuned to compensate for the expected available egress multicast bandwidth per multicast plane. In conditions where it is highly desirable to prevent multicast plane congestion, the per-mcast-plane-capacity commands should be used to compensate for the non-multicast or secondary multicast switch fabric traffic.