RBFS Stateless Redundancy for PPPoE Subscribers

The BNG nodes configured as a redundancy pair maintain a redundancy session over an TCP connection through an IP/MPLS link. TCP connection can be formed using the directly connected interfaces or via an IP/MPLS path through core network. This communication channel is used by the standby node to continuously monitor the health state of the active node. The redundancy session remains established as long as the TCP connection is available. The session between the nodes is used to send 'keepalive' messages to check the node health. In the event of a failure on the active node, the standby node detects the failure through this channel and initiates the failover.

Every redundancy session is represented by a redundancy session ID. When you define a value for the redundancy session ID, this ID should be unique within the redundancy group or domain and must be configured with the same value on both nodes (active and standby) participating in that redundancy session. When two nodes get the same session ID, they recognize each other as the peer nodes for a particular redundancy session.

From the perspective of the OLT, there is only one active-standby LAG that is distributed across multiple BNGs. This means that the OLT sees one logical link, even though it is physically connected to two separate BNGs.

The following architectural diagram provides a high-level view of PPPoE redundancy mode. It shows two RBFS nodes, paired for redundancy, deployed in an active-standby node cluster, with their interfaces are connected with an TCP connection.

Both nodes are connected to an OLT device on southbound (access) side, through which they receive and forward subscriber traffic. On the northbound side, the nodes are connected to the core network for upstream and downstream communication.

PPPoE Redundancy can mitigate the following types of failures:

Multiservice Edge nodes have different states for various redundancy sessions.

Active: All subscribers are served by active node in the RBFS active-standby node cluster. One node which is active for a redundancy session can be a standby node for a different session. The priority values that you specify for the redundancy nodes determine the roles of active and standby. The node that receives the higher priority value for the session ID assumes the role of active for that subscriber group.

Standby: Standby node keeps communication with the active BNG node to monitor node health status using the 'keepalive' messages. The node that gets the lower priority value for the redundancy session ID assumes the role of standby. Standby node for a subscriber group does not perform any subscriber services for that group unless or until the active node encounters an outage.

Down: When a node becomes inactive due to an outage, it is considered as 'down'. In the event of a node outage, it is completely down and cannot perform subscriber services and any communication with its peer node. But in the case of a LAG (between the node and access node) failure, the node cannot perform subscriber services, but it can communicate with the peer node through the RD TCP connection.

Stand Alone: When the active node goes down, the switchover occurs and standby takes over the subscriber service. In this scenario, the serving node is in 'stand alone' state (for that redundancy session) as it has no peer node for redundancy.