The subscriber can receive multicast content through the subscriber SAP, the redirected interface, or a combination of both.
Multicast redundancy is only supported on a MC-LAG topology. Multicast traffic can be delivered over the subscriber SAP, the redirected interface, or a combination of both.
Subscriber IGMP states can be synchronized across multiple routers to ensure minimal interruption of (video delivery) service during network outages. The IGMP/MLD state of a subscriber-host in the system is tied to the state of the underlying MC-LAG protection mechanism. For example, IGMP states is activated only for subscribers that are anchored under the group-interfaces with master SRRP state or under an active MC-LAG port.
For multicast redirection on a MC-LAG topology, it must be ensured that the redirected interface (the interface to which multicast forwarding is redirected) is under the same MC-LAG as the subscriber. Otherwise, IGMP states on the redirected interface is derived independently of the IGMP states for the subscriber from which IGMP/MLD messages are redirected.
The IGMP/MLD synchronization process in conjunction with underlying access protection mechanisms works as follows:
IGMP/MLD states for the subscriber updates only if IGMP/MLD messages (Joins/Leaves/Reports, and so on) are received directly from the downstream node on an active MC-LAG link. This is valid irrespective of the IGMP querier status for the subscriber.
In all other cases, assuming that some protection mechanism in the access is present, the IGMP/MLD messages are discarded and consequently no IGMP/MLD state is updated. Similar logic applies to regular Layer 3 interfaces, where SRRP is replaced with VRRP.
After the subscriber IGMP/MLD state is updated as a result of a directly-received IGMP/MLD message on an active subscriber (SRRP master state or active MC-LAG), the sync IGMP/MLD message is sent to the standby subscriber over the Multi-Chassis Synchronization (MCS) protocol. The synchronized IGMP state is populated in the MCS database in all pairing routers.
If an IGMP/MLD sync (MCS) message is received from the peering node, the IGMP state for the standby subscriber is updated in the MCS database but it is not downloaded into the forwarding plane unless there is a switchover. If the IGMP/MLD sync message is received for the active subscriber, the message is discarded.
IGMP/MLD queries are sent out only by IGMP/MLD querier. In an MC-LAG environment, this is the node with the active MC-LAG link.
IGMP/MLD states from the MCS database are:
activated on non-querier subscriber in case that neither SRRP nor MC-LAG is deployed. It is assumed that the querier subscriber has received the original IGMP/MLD message and consequently sent the IGMP/MLD MCS Sync to the non-querier (standby). Non-querier interface accepts the MCS sync message and also it propagate these IGMP/MLD states to PIM.
The querier subscriber does not accept the IGMP/MLD update from the MCS database.
aware of the state of MC-LAG. As soon as the standby MC-LAG becomes active, the IGMP/MLD states is activated and is propagated to PIM. Traffic is forwarded as soon as multicast streams are delivered to the node and the IGMP states under the subscriber are activated. On a standby MC-LAG, IGMP states are not propagated from the MCS database to PIM and consequently subscribers.
aware of the SRRP state. Because the subscriber with SRRP master state is considered active, the states are propagated to PIM as well. On standby SRRP, IGMP states are be propagated from MCS database to PIM and consequently to subscribers.
For MC-lag setups, after the switchover is triggered by MC-LAG or SRRP, the IGMP/MLD states from MCS database on the newly active MC-LAG node or subscriber under the new SRRP instance in the master state is sent to PIM and consequently to the forwarding plane effectively turning on multicast forwarding.
An active and standby subscriber refers to the state of underlying protection mechanism (active MC-LAG).
The subscribers themselves are always instantiated (or active) on both nodes. However, traffic forwarding over those subscribers is driven by the state of the underlying protection mechanism (MC-LAG). Hence the terms active and standby subscriber.
In subscriber environment, SRRP should be always activated in dual-homing scenario. SRRP in subscriber environment ensures that downstream traffic is forwarded by the same node that is forwarding upstream traffic. In this fashion, accounting and QoS for the subscriber are consolidated within a single node.
To summarize, in multi-chassis environment with subscribers, IGMP synchronization enabled and an access layer protection mechanism in place (MC-LAG), the behavior for is the following:
IGMP/MLD states are synchronized between the chassis.
On a MC-LAG setup, only the SRRP instance in master state or active MC-LAG forwards downstream multicast traffic.
Length of outage during the switchover is determined by the detection and recovery of the underlying protection mechanism (MC-LAG or MCS) in addition to local propagation of IGMP/MLD states from MCS database to PIM and consequently to forwarding plane.