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Two-Tier Active/Active Multichassis Link Aggregation with IRB and VRRP

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Article ID: KB32506 KB Last Updated: 06 Mar 2018Version: 1.0
Summary:
Active/Active Multichassis Link Aggregation (MC-LAG) can be used with an integrated routing and bridging (IRB) interface and the Virtual Router Redundancy Protocol (VRRP) to provide a network with multiple gateways for redundancy and scalability. Maintaining reachability to each router in a two-tier topology requires two pairs of VRRP for proper MAC address learning.
Symptoms:
A two-tier MC-LAG topology is a full-mesh of four nodes: R1, R2, R3, R4.
Topology

Each node will be reachable from the end clients. The configuration would look like:
R1# set interfaces irb unit 100 family inet address 10.0.100.2/24 vrrp-group 1 virtual-address 10.0.100.1
R2# set interfaces irb unit 100 family inet address 10.0.100.3/24 vrrp-group 1 virtual-address 10.0.100.1
R3# set interfaces irb unit 100 family inet address 10.0.100.252/24 vrrp-group 2 virtual-address 10.0.100.254
R4# set interfaces irb unit 100 family inet address 10.0.100.253/24 vrrp-group 2 virtual-address 10.0.100.254

Note that each node has a unique address but are paired by vrrp-group.
Solution:
VRRP is required on R3 and R4 when they are not set to be a gateway for other devices on the LAN because it enables MAC address synchronization between R3 and R4. Alternatively, MAC's could be synchronized if both IRB interfaces had the same address, but then the end clients could not distinguish which node to reach. By using a unique VRRP group, you do not risk disruption to the primary gateway address, but still acheive MAC synchronization while also maintaining reachability to R3 and R4.

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