Multi-Chassis LAG
Page 140 7750 SR Interface Configuration Guide
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 towards 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 towards the access node. The multi-
chassis LAG protocol between redundant-pair ensures a synchronized forwarding plane to/from
the access node and is used to synchronize the link state information between the redundant-pair
nodes such that proper LACP messaging is provided to the access node from both redundant-pair
nodes.
In order 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
towards/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 does not support LACP, the power-off option can be used to
enforce 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:
• Selection of the common system ID, system-priority and administrative-key are used in
LACP messages so partner systems consider all links as the part of the same LAG.
• Extension of selection algorithm in order to allow selection of active sub-group.
→ The sub-group definition in LAG context is still local to the single box, meaning that
even if sub-groups configured on two different systems have the same sub-group-id
they are still considered as two separate subgroups within given LAG.
→ Multiple sub-groups per PE in a MC-LAG is supported.
→ In case there is a tie in the selection algorithm, for example, two sub-groups with
identical aggregate weight (or number of active links) the group which is local to the
system with lower system LACP priority and LAG system ID is taken.
• Providing inter-chassis communication channel allows inter-chassis communication to
support LACP on both system. This communication channel enables the following:
→ Supports connections at the IP level which 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 detecting node failures.
→ Support for operator actions on any node that force an operational change.
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