Subscriber interface routes (IPv4 and IPv6)

Optimized routing and elimination of downstream shunt traffic during normal operation can be achieved by statically favoring the routes on the network side that are advertised with an increased metric by active nodes (SRRP master state).

The downside of this static approach is that during the port or card failure and consequently a SRRP switchover, the node with the failed port or card continues to advertise routes with the same high metric if the subscriber interface is in the ‛UP’ state (or a single SAP under it). That is, the network side is not aware of the switchover. It continues to forward traffic to the standby node, and as a result, heavy shunt traffic ensues. To effectively deal with this, the network side must be aware of the routing change that occurred in the access layer.

When failure is detected, the metric for the route is changed automatically based on the following configuration:

configure
    service <type> <id>
        subscriber-interface <ip-int-name>
            address <ip-address> gw-ip-address <gw-address> track-srrp <srrp-inst> holdup-time <msec>
            ipv6
            subscriber-prefixes
                prefix <ipv6-prefix> pd track-srrp <srrp-id> holdup-time <msec>
                prefix <ipv6-prefix> wan-host track-srrp <srrp-id> holdup-time <msec>


    policy-options
        begin
           policy-statement <name>
            entry 1 
                from 
                    protocol direct
                    state ‛srrp-master’
                    exit
               action accept
                metric set 100
                exit
            exit
            entry 2 
                from 
                    protocol direct
                    state ‛srrp-non-master’
                    exit
                action accept
                metric subtract 10
                exit
            exit
            entry 3
                from 
                    protocol direct
                    exit
                action accept
                exit
            exit

This configuration ensures that the route metric is changed for the subscriber interface routes based on the SRRP state while the other, non-subscriber directly attached routes are unaffected by SRRP.

The route advertisement based on SRRP State requirement is applicable to BGP and IGP.

The routing policy also provides the flexibility to prevent route advertisement (action reject) instead of changing the route metric.

Although this feature is designed to minimize or eliminate the use of the redundant interface, it is important to note that the redundant interfaces are still used in the case of transient conditions. An example of such condition would be:

1. Messaging SAP Fails

2. SRRP switches over

3. Stale routing in the core is still in the effect while the metric is being propagated (or the route is being advertised or withdrawn). During this time, traffic is flowing over the redundant interface.

4. Network convergence is complete

5. Traffic in the network core is redirected to the new active node (SRRP master state)