MPLS and RSVP
7450 ESS MPLS Guide Page 159
Ingress Path Management for P2MP LSP Packets
The SR OS provides the ingress multicast path management (IMPM) capability that allows users
to manage the way IP multicast streams are forwarded over the router’s fabric and to maximize the
use of the fabric multicast path capacity.
IMPM consists of two components, a bandwidth policy and a multicast information policy. The
bandwidth policy configures the parameters of the multicast paths to the fabric. This includes the
rate limit and the multicast queue parameters of each path. The multicast information policy
configures the bandwidth and preference parameters of individual multicast flows corresponding
to a channel, for example, a <*,G> or a <S,G>, or a bundle of channels.
By default both, the IOM-2 and IOM-3/IMM ingress data paths provide two multicast paths
through the fabric referred to as high-priority path and low-priority path respectively. When a
multicast packet is received on an ingress network or access interface or on a VPLS SAP, the
packet’s classification will determine its forwarding class and priority or profile as per the ingress
QoS policy. This then determines which of the SAP or interface multicast queues it must be stored
in. By default SAP and interface expedited forwarding class queues forward over the high-priority
multicast path and the non expedited forwarding class queues forward over the low-priority
multicast path.
When IMPM on the ingress MDA is enabled, one or more multicast paths are enabled depending
on the IOM type. In addition, multicast flows managed by IMPM will be stored in a separate
shared multicast queue for each multicast path. These queues are configured in the bandwidth
policy.
IMPM maps a packet to one of the paths dynamically based on monitoring the bandwidth usage of
each packet flow matching a <*,G> or <S,G> record. The multicast bandwidth manager assigns
multicast flows to a primary path, and ancillary path for IOM-2, based on the flow preference until
the rate limits of each path is reached. At that point in time, a multicast flow is mapped to the
secondary flow. If a path congests, the bandwidth manager will remove and black-hole lower
preference flows to guarantee bandwidth to higher preference flows. The preference of a multicast
flow is configured in the multicast info policy.
A packet received on a P2MP LSP ILM is managed by IMPM when IMPM is enabled on the
ingress MDA and the packet matches a specific multicast record. When IMPM is enabled but the
packet does not match a multicast record, or when IMPM is disabled, a packet received on a P2MP
LSP ILM is mapped to a multicast path differently depending if the ingress IOM is an IOM-2 or
IOM-3.
Ingress P2MP Path Management on IOM-3/IMMs
On an ingress IOM-3/IMM, there are multiple multicast paths available to forward multicast
packets, depending on the hardware being used. Each path has a set of multicast queues and
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