MC-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 CPE/access node is connected with multiple links toward a redundant pair of Layer 2/3 access aggregation nodes such that both link and node level redundancy is provided. By using a multi-chassis LAG protocol, the paired Layer 2/3 aggregation nodes (referred to as the redundant-pair) appear to be a single node that is utilizing LACP toward the access node. The multi-chassis LAG protocol between the redundant-pair ensures a synchronized forwarding plane to and from the CPE/access node. It is used to synchronize the link state information between the redundant-pair nodes and provide correct LACP messaging to the CPE/access node from both redundant-pair nodes.
To ensure SLAs and deterministic forwarding characteristics between the CPE/access and the redundant-pair node, the multi-chassis LAG function provides an active/standby operation toward/from the CPE/access node. LACP is used to manage the available LAG links into active and standby states so that only links from one aggregation node are active at a time to and from the CPE/access node.
MC-LAG has the following characteristics:
Selection of the common system ID, system-priority, and administrative-key are used in LACP messages to ensure that partner systems consider all links part of the same LAG.
The selection algorithm is extended to allow the selection of the active subgroup.
The subgroup definition in the LAG context is still local to the single box. Consequently, even when subgroups configured on two different systems have the same subgroup-id, they are still considered two separate subgroups within the specific LAG.
The configuration of multiple subgroups per PE in an MC-LAG is supported.
If there is a tie in the selection algorithm, for example, two subgroups with identical aggregate weight (or number of active links), the group that is local to the system with lower system LACP priority and LAG system ID is selected.
Providing an inter-chassis communication channel allows the inter-chassis communication to support LACP on both systems. The communication channel enables the following functionality:
It supports connections at the IP level that do not require a direct link between two nodes. The IP address configured at the neighbor system is one of the addresses of the system (interface or loop-back IP address).
The communication protocol provides heartbeat mechanism to enhance robustness of the MC-LAG operation and detect node failures.
It supports operator actions that force an operational change on nodes.
The LAG group-ids do not have to match between neighbor systems. At the same time, multiple LAG groups between the same pair of neighbors is also allowed.
It verifies that the physical characteristics, such as speed and auto-negotiation are configured and initiates operator notifications (traps) if errors exist. Consistency of MC-LAG configuration (system-id, administrative-key and system-priority) is provided. Load-balancing must be consistently configured on both nodes.
Traffic over the signaling link is encrypted using a user-configurable message digest key.
The MC-LAG function provides active/standby status to other software applications to build reliable solutions.
Figure: MC-LAG L2 dual homing to remote PE pairs and Figure: MC-LAG L2 dual homing to local PE pairs show different combinations of supported MC-LAG attachments. The supported configurations can be divided into the following subgroups:
dual-homing to remote PE pairs
both end-points attached with MC-LAG
one end-point attached
dual-homing to local PE pair
both end-points attached with MC-LAG
one end-point attached with MC-LAG
both end-points attached with MC-LAG to two overlapping pairs
The following figure shows dual homing to remote PE pairs.
The following figure shows dual homing to local PE pairs.
The forwarding behavior of the nodes is governed by the following principles. Note that the logical destination (actual forwarding decision) is primarily determined by the service (VPLS or VLL), and the following principle apply only if the destination or source is based on MC-LAG:
Packets received from the network will be forwarded to all local active links of the specific destination-sap based on conversation hashing. If there are no local active links, the packets will be cross-connected to the inter-chassis pseudowire.
Packets received from the MC-LAG sap will be forwarded to the active destination pseudowire or active local links of destination-sap. If no such objects are available at the local node, the packets will be cross-connected to inter-chassis pseudowire.