Chapter 18. MLAG

Multichassis Link Aggregation (MLAG) is the open-standard (and thus, Arista) term for linking a port-channel or Link Aggregation Group (LAG) to multiple switches instead of just one. The technology accomplishes the same basic goal as Cisco’s Virtual Port Channel (vPC), although, in my experience, MLAG is simpler to configure and more forgiving when used.

MLAG Overview

As just mentioned, the acronym LAG is an abbreviation for Link Aggregation Group, which is an open-standard way of describing the bonding of multiple physical links into a single logical link. In Cisco parlance, this technology is called Etherchannel. Different vendors use different terms for similar solutions, but the term LAG has become a cross-vendor acceptable way of describing the idea. Why would you want to do this? Let’s take a look.

With a traditional network design, interconnecting three switches at Layer 2 (L2) results in a loop. Loops are bad, so Spanning Tree Protocol (STP) blocks the interface on the link farthest from the root. Figure 18-1 shows an example of this.

Traditional STP-blocked network loop
Figure 18-1. Traditional STP-blocked network loop

In this scenario, there is a LAG connecting switch A to switch B. Switch C connects to both A and B switches, forming a loop. STP has blocked the interface on switch C that leads to switch B in order to break said loop. This design will allow for failover if the link between ...

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