Overview - Alcatel-Lucent 7750 SR OS Interface Configuration Manual

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:
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7750 SR OS Interface Configuration Guide
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.
→ The LAG group-ids do not have to match between neighbor systems. At the same
time, there can be multiple LAG groups between the same pair of neighbors.
Interface Configuration
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