The World of Junos' MC-LAG and its Significance

In the world of networking, redundancy and high availability are key factors that ensure the smooth operation of a network. One of the ways to achieve this is through the use of Multi-Chassis Link Aggregation (MC-LAG). In this blog post, we will delve into the world of Junos' MC-LAG, its significance, and how it is configured.

What is MC-LAG?

MC-LAG, or Multi-Chassis Link Aggregation Group, is a type of link aggregation where a client can use a standard LACP to form a logical link aggregation interface with two different physical devices. This technology allows for the creation of redundancy and high availability in the network by providing two active paths for traffic.

Why is MC-LAG Significant?

MC-LAG plays a crucial role in network design for several reasons:

  1. Redundancy: MC-LAG provides network redundancy by allowing traffic to be distributed across multiple physical links. If one link fails, traffic can still flow through the other active links.

  2. Load Balancing: MC-LAG allows for efficient utilization of bandwidth by distributing traffic across all available links. This ensures that no single link is overwhelmed with traffic.

  3. Simplicity: MC-LAG simplifies network design by allowing a client to form a logical link aggregation interface with two different physical devices using standard LACP.

Configuring MC-LAG in Junos

Now, let's look at how to configure MC-LAG in Junos. For this example, we will assume that we have two Junos switches, Switch 1 and Switch 2, and we want to configure MC-LAG between them.

First, we need to configure the Inter-Chassis Control Protocol (ICCP) which allows the two switches to communicate and synchronize their states. On both switches, we would configure the ICCP like this:

set protocols iccp peer
set protocols iccp group 1 peer-ip
set protocols iccp group 1 mode active

Next, we configure the MC-LAG interface on both switches:

set interfaces ae0 aggregated-ether-options lacp active
set interfaces ae0 unit 0 family inet address

Finally, we configure the MC-LAG on both switches:

set chassis mc-lag peer
set chassis mc-lag iccp-group 1

With these configurations, we have successfully set up MC-LAG between the two switches.


MC-LAG is a powerful tool in the world of networking that provides redundancy, load balancing, and simplicity. It is a crucial concept for anyone studying for the JNCIS-ENT certification and is widely used in real-world network designs. By understanding and mastering MC-LAG, you will be one step closer to becoming a Junos networking expert.

Remember, practice makes perfect. So, don't just read about MC-LAG, try configuring it in a lab environment to get a feel for how it works. Happy studying!

© Ben Jacobson.RSS