In case that failure is repaired on the original active node (non-central standby node) while SRRP preemption (preempt) is configured, the corresponding active subscribers on the protecting node is withdrawn from the data plane and the activity (SRRP master state) is switched to the original node.
This behavior ensures that the resources in the central backup are freed upon failure restoration and are available for protection of other entities in the network (other links/nodes).
In the preemption case, the upstream traffic is steered toward the newly active BNG via gratuitous ARP (GARP). In other words, the virtual MAC is advertised from the newly active node, and consequently the access and aggregation nodes update their Layer 2 forwarding entries. This action should cause no interruption in the upstream traffic.
In the downstream direction, the service interruption is equivalent to the time it takes to withdraw the routes from the network side on the standby node. In this case, there are two scenarios:
A route per group interface (SRRP) is advertised in the network from the central standby node. In this case, downstream traffic interruption is a function of the convergence time of the routing protocol deployed on the network side. After the routing is converged, all downstream subscriber traffic is attracted to the newly active node. In the meantime, the redundant interface can be used to shunt traffic from the central standby node to the newly active node, but only for the subscriber hosts that have not yet been withdrawn from the data plane on the protecting node. This withdrawal process may take some time and therefore downstream traffic for some subscriber hosts is restored before the others during the routing convergence period.
/32 subscriber-host routes are advertised from the protecting node. The total recovery time for downstream traffic depends on the routing convergence. The routing convergence may be slower than in the previous case because more routes (/32s) need to be withdrawn from the network. The redundant interface can be used in the meantime for the subscriber hosts that have not yet been withdrawn from the data plane in protecting node.