Multi-Chassis Etherchannel (MEC)

What is rationale and benefits behind this?

Let's start with some basics. We have an user connected to access switch. Which is connected to some distribution switch upstream.

This works until distribution switch or link fail. How do we make it better? Use Etherchannel.

While we still depend on failure of distribution switch, we are now protected from single link failure and bandwidth available to access switch has been almost* doubled.
Benefits of current topology are:

- STP consider etherchannel as one link - will NOT engage and recalculate L2 topology after single link failure

- Practically zero switchover time

- Both links are used at tue same time
But still couple of issues are not resolved:

- failure of distribution switch kills connectivity

- completely equal utilization of both links is very hard to achieve; having doubled bandwidth by having number of links doubled is not possible

Now let's consider different topology with paralel connections to two distribution switches.

It's better for sure, as right now we don't rely on only one distribution switch. Should one of them fail, we can still have our network access through other one. This is also resilient to single link failure.

Drawbacks? Yes, a couple:

- Spanning tree must be enabled in order to make sure there are no L2 loops in the network

- One of uplinks will be permanently blocked, so we pay for two connections and use only one

- after failure of active link, connectivity could be interrupted for amount of time needed by STP to recalculate network topology.

Multichassis etherchannel is a hybrid topology which allows to get the best of dual homing and bundling separate connections together at the same time.

Resilient to single switch or link failure. Both links used at the same time, and single link failure does not trigger topology recalculation.

Although it seems perfect, we still need to remember that there are some limitations, too. First of all, it adds complexity to environment, especially when control plane of distibution switches are separated like Cisco vPC (Virtual Port Channel available on Nexus switches) does. In other implementations they can be joined together like Cisco VSS (Virtual Switching System available on Catalyst 6xxx switches) which basically turns two different switches into one.

Problem	Single homed	Dual homed	Ether Channel	MEC
Switch failure breaks connectivity	YES	NO	YES	NO
Link Failure breaks connectivity	YES	NO	NO	NO
Cannot use all connections	NO	YES	NO	NO
Link failure triggers topology change	YES	YES	NO	NO