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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, the XCM ingress data path provides 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.
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. 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 XMA 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.
Ingress P2MP Path Management on XCM
On an ingress XCM, there are 16 multicast paths available to forward multicast packets. Each path
has a set of multicast queues and associated with it. Paths 0 and 15 are enabled by default and
represent the high-priority and low-priority paths respectively. Each VPLS SAP, access interface,
and network interface will have a set of per forwarding class multicast and/or broadcast queues
which are defined in the ingress QoS policy associated with them. The expedited queues will be
attached to Path 0 while the non-expedited queues will be attached to Path 15.
7950 SR OS MPLS Configuration Guide
MPLS and RSVP
Page 105
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