EtherChannel Tutorial

EtherChannel is a technology that bundles several physical links between switches (or routers) into a single logical connection, treating them as one combined link. Here’s a simple example that shows why that’s useful.

Say your company has two switches connected by a single FastEthernet link (100 Mbps):

As the company grows, that single link isn’t enough — more than 100 Mbps needs to move between the switches. Simply plugging in more cables between them won’t help, because Spanning Tree Protocol (STP) will immediately block any redundant links to avoid a loop:

At this point there are two ways to add capacity:

• Buy two new 1000 Mbps (1 Gbps) interfaces, or
• Bundle the existing links together using EtherChannel.

The first option means new hardware on both switches, which gets expensive fast. EtherChannel, by contrast, just needs a few more unused ports on the switches you already have:

EtherChannel combines the physical links into one logical link carrying their combined bandwidth. Because STP sees that bundle as a single link, none of its member links get blocked. EtherChannel also load-balances traffic across its member links automatically, and if one of those links fails, the traffic it was carrying simply shifts over to the remaining links — as long as at least one member link is still up, the logical EtherChannel link stays up too.

EtherChannel works just as well for router connections:

Whenever an EtherChannel is formed, a logical interface representing it is created on the switches or routers involved, and that logical interface can be configured however you’d configure any other interface — assigning it access/trunk mode on a switch, or an IP address on a router, for example.

Note: an EtherChannel can bundle up to 8 Fast Ethernet or 8 Gigabit Ethernet ports.