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 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 (on the 7950 XRS) and the IOM/IMM (on the 7750 SR and 7450 ESS) ingress data paths 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 determines 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 FP is enabled on the 7950 XRS, 7750 SR, or 7450 ESS, one or more multicast paths are enabled depending on the hardware in use. In addition, for all routers, multicast flows managed by IMPM are 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 also assigns multicast flows to a primary path 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 removes 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 or the ingress FP 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.