Overview

Multi-chassis LAG is a method of providing redundant Layer 2/3 access connectivity that extends beyond link level protection by allowing two systems to share a common LAG end point.

The multi-service access node (MSAN) node is connected with multiple links toward a redundant pair of Layer 2/3 aggregation nodes such that both link and node level redundancy, are provided. By using a multi-chassis LAG protocol, the paired Layer 2/3 aggregation nodes (referred to as redundant-pair) appears to be a single node utilizing LACP toward the access node. The multi-chassis LAG protocol between a redundant-pair ensures a synchronized forwarding plane to and from the access node and synchronizes the link state information between the redundant-pair nodes such that correct LACP messaging is provided to the access node from both redundant-pair nodes.

To ensure SLAs and deterministic forwarding characteristics between the access and the redundant-pair node, the multi-chassis LAG function provides an active/standby operation to and from the access node. LACP is used to manage the available LAG links into active and standby states such that only links from 1 aggregation node are active at a time to/from the access node.

Alternatively, when access nodes do not support LACP, the power-off option can be used to enforce the active/standby operation. In this case, the standby ports are trx_disabled (power off transmitter) to prevent usage of the LAG member by the access-node. Characteristics related to MC are:

Figure 1 and Figure 2 show the different combinations of MC-LAG attachments that are supported. The supported configurations can be sub-divided into following sub-groups:

Figure 1. MC-LAG Layer 2 dual homing to remote PE pairs
Figure 2. MC-LAG Layer 2 dual homing to local PE pairs

The forwarding behavior of the nodes abide by the following principles. Note that logical destination (actual forwarding decision) is primarily determined by the service (VPLS or VLL) and the principle below applies only if destination or source is based on MC-LAG: