Alcatel-Lucent 7750 SR OS, Interface Configuration Manual, Page: 77 / 444

Interface Configuration
7750 SR OS Interface Configuration Guide Page 77
Multi-Chassis LAG
This section describes the Multi-Chassis LAG (MC-LAG) concept. MC-LAG is an extension of a
LAG concept that provides node-level redundancy in addition to link-level redundancy provided
by “regular LAG”.
Typically, MC-LAG is deployed in a network-wide scenario providing redundant connection
between different end points. The whole scenario is then built by combination of different
mechanisms (for example, MC-LAG and redundant pseudowire to provide e2e redundant p2p
connection or dual homing of DSLAMs in Layer 2 TPSDA).
Overview
Multi-chassis LAG is a method of providing redundant Layer 2 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 aggregation nodes such that both link and node level redundancy, are provided. By
using a multi-chassis LAG protocol, the paired Layer 2 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.
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.
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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.
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Multiple sub-groups per PE in a MC-LAG is supported.