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Arista Warrior by Gary A. Donahue

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Chapter 12. MLAG

Multichassis Link Aggregation (MLAG), is the Arista term for linking a port-channel 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 less likely to fail in colorful, job-threatening ways.

MLAG Overview

The term LAG is an abbreviation for Link Aggregation, which is a non-Cisco 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.

A LAG connects multiple physical links on the same switch. MLAG is designed to allow two or more links on multiple switches into a single logical link. Why would you want to do this? Let’s take a look.

With a traditional network design, interconnecting three switches results in a loop. Loops are bad, so Spanning Tree Protocol (STP) blocks the interface on the link farthest from the root. An example of this is shown in Figure 12-1.

Traditional STP-blocked network loop

Figure 12-1. 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 ...

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