To preserve QoS and Accounting, subscriber’s traffic must flow in both directions through the multi-chassis active BNG node.
In the upstream direction, this is always true as traffic is steered to the active BNG (SRRP master state) node just by the virtue of SRRP operation.
In the downstream direction which represents bulk of traffic, SRRP cannot be relied up on to steer traffic through the active BNG (SRRP master state). This poses a problem in a very common environment where IP subnets are shared over multiple group interfaces with SRRP enabled. A particular subnet is advertised to the network side from both active and standby BNGs. Natural routing on the network side determines which BNG node receives subscriber’s traffic in the downstream direction. If the standby BNG (SRRP backup state) node receives the traffic, it cannot simply send the traffic directly to the access network where the subscriber resides by just inserting the source MAC address of the SRRP instance in the outgoing packet. This would break the operation of SRRP. Instead, the standby BNG must send the traffic to the active BNG through a redundant interface. The active BNG then forwards traffic directly to the subscriber. Source MAC address of this traffic is the MAC address of SRRP instance. This traffic shunting over the redundant interface can result in a substantial load on the link between the two BNGs.
The increase in shunted traffic can quickly become an issue if the redundant BNG nodes are not colocated. To minimize the shunt traffic, more granular routing information must be presented to the network core. This leads to more optimal routing where downstream subscriber traffic is directed toward the active BNG, without the need to cross the redundant interface. The disadvantage of this approach is that this further fragments the IP address space within the network core. In the extreme case where /32 (subscriber) IP addresses are advertised, the churn that /32s cause in the core routing can be unsustainable. In this case, routing updates in the core are triggered by subscribers coming on/off-line.
Optimal operation calls for the shunt traffic to be eliminated and at the same time, a high IP route aggregation on the network side is achieved. The existence of the shunt traffic stems from the fact that routing protocols advertise subscriber subnets into the network with no awareness of the SRRP master or backup state. To address this problem along with better aggregation of advertised subnets, two SRRP enhancements are introduced:
SRRP fate-sharing
SRRP aware routing
Both of these concepts are described in SRRP enhancement.
Traffic destined for or from the subscriber is forwarded under the condition that the subscriber-interface is operationally UP. This applies also to shunting of downstream subscriber traffic from the standby (SRRP backup state) to the active (SRRP master state) node. It is always necessary to keep the subscriber-interface operationally UP by configuring a dummy group interface with a oper-up-while-empty command under it. This is especially true for the MC-LAG which causes the messaging SAP on the STANDBY node always to be in the INIT state. In case that MSAPs are used on such group interfaces, the group interfaces would be also operationally DOWN, causing the subscriber-interface to be operationally DOWN.