Cisco Switches: One Switch Stack vs. Two Switch Stack

Many of the Cisco Switches people put into their data centers are the 3750 and 3850 switches. One of the main advantages of these switches is the fact that they can stack. Stacking reduces management of the switches and allows for easy first hop redundancy of all virtual interfaces. So why not always use one stack? Why waste the time with Hot Swappable Routing Protocol (HSRP) or Virtual Router Redundancy Protocol (VRRP)?

The answer is uptime and availability:

Most of the time, you will not need to bring down an entire switch stack. But for critical uptime availability, you plan for those exceptions that are outside of mostly. For example,  “Mostly the power works, so we don’t need any backup power.” This has never been heard of in a data center. “Mostly the circuit is up, so why would we need an alternative route?” To have a high availability, you have to plan for the time outside of mostly.

An issue that illustrates the need for multiple switch stacks instead of one would be where you want to increase the MTU on a Cisco switch to the Jumbo frames. To increase to jumbo frames, the command is rather easy:

1. Set the command: 3850(config)#system mtu jumbo 9000
2. Reload the switch (or entire switch stack)

The first command is easy to run, the second command will not work unless the entire switch stack is reloaded. Typically you can reload an individual switch in the stack by using the command.

• Reload slot (switch-stack-number)

This will reload the switch, but because the entire stack still has an MTU of 1500, so does the switch when it comes up: you have to take down the entire stack.

This is more of an issue in today’s environment, where the switches connect the storage SANs to the virtual hosts and servers. When you take down the switch, you have to take down the entire environment. Because shutting down an entire site, just to make one simple change is silly; a little planning ahead of time will save many problems. If you want maximum uptime, create two switch stacks in your data center and have your hosts and storage attached to each.

Jason Howe, PEI