Catalyst 3750 Switch Stack


I like using Catalyst 3750 stack for user access switch. Don’t get me wrong, I’d rather use chassis based switches (Catalyst 4500 or 6500) but as we all know that money is usually a concern. 3750 stack is way less than a dual sup and fully populated 4500 or 6500 so if you’re in a budget then 3750 for user access is the way to go, in my opinion. Yes, you don’t have to get dual sup, but you don’t get the redundancy and it is still more money than a 3750 stack. With the 3750 stack, you get the redundancy since all other switches in the stack can take over as the master switch – equivalent to active switch (VSS) or active sup (dual sup chassis based switch).

3750’s StackWise/StackWise Plus/StackPower, 2960’s FlexStack, and VSS, which was mentioned here, are pretty much the same thing – device pooling/stackable switch technologies. With StackWise/StackWise Plus, you can stack up to nine switches. StackPower and FlexStack allows you to stack up to four switches. The last time I checked, the VSS (Virtual Switching System) allows you to stack only up two chassis based switches. If I remember correctly, VSS used to be a 6500 only technology but now the newer 4500s chassis can support VSS as long as it is equipped with the right supervisor – like Sup 7E.

Stacking Catalyst 3750 does not need any manual configuration. All you have to do is connect the StackWise cables (included when you buy it), boot it up and the stack will automatically form. When you rack and stack nine 3750s with 1U space in between, you’ll need one 3M (~9ft) StackWise cable. The one that comes with the switch is only 50CM (~1ft) which is enough to connect all switches except from the top to the bottom switch with 1U space between switches. If you don’t need the 3M then you can also buy the 1M (~3ft) which is good for 3 – 5 switch stack configuration with 1U space between switches, if I remember correctly. Make sure you plan accordingly when ordering your 3750 stack.

Once all the StackWise cables are connected and the switch powers up, it’ll go through its POST sequence, master election, and etc. What I am concern here is the master election. I like my switch stack to be predictable. As what Jeremy Cioara would say, auto = ought not to use it. Though in QoS video, he admitted liking the AutoQoS. Having said all that, I want to know exactly who should take over as a master when it boots up or when the master fails. As mentioned, the switch stack goes through a master election and is described here. In all the scenarios that I’ve experienced, there are only two things that I need to remember and they are the priority and MAC address.

MAC address is only important when you’re working on a new stack or a stack which has no config in it. The switch with the lowest MAC address wins the master election when the priority is set by default (value of 1). See the example below. The stack below is fresh out of the box.

As you can tell, I only have two switches in this stack. The switch 2 won the master election since the switch priority is a tie and has a lower MAC address than the switch 1. Switch priority is the first one to be checked when it boots up. That’s not what the Cisco’s documentation says, but again I only consider switch priority and MAC address in the election process. Higher switch priority wins the master election and when all the switches have the same priority then the one with the lowest MAC address wins, as already mentioned.

There maybe some network professionals out there that do not care about who the master is in the stack, but I am not one of them. The scenario in this post is physically laid out as switch 1 being the bottom switch and switch 2 as the top switch. I like my switch stack organized, so whenever I set one up I try to match the logical and physical view of the switch stack. In addition, I like my master to be on the top of the switch stack, physically and logically, so I have to alter my configuration to match the way I want it to look like. The configuration below shows you how to alter the switch stack to match the logical and physical view of the stack.

The first step I do is to configure one of the switches to have a higher priority so I know exactly who the master is on a switch stack. In this scenario, I picked switch 2 to have the highest priority. I don’t have to change the priority on this stack since I know this stack will never grow more than two. However, as best practice, I like to configure the priority value of the switches.

If you recall, switch 2 is placed on the top of the stack (physically) and as mentioned earlier I want both physical and logical to match so I have to configure that as well to reflect it. Below is how to configure it to match physical and logical view of the stack.

As you may have guessed, you can’t have two switches with the same number so you have to change the remaining switches in the stack. If your stack contains nine switches, then knowing the switch number is important so you can renumber the right switch. To do this easily, you press the mode button of the switch until the stack LED lights up. You’ll now notice that the LED of the port numbers are blinking as well. Take note of those numbers so you can rename the proper switch with the right number. I normally use the top to bottom approach when I number my stack. In this scenario, I only have two switches so it’s easy to do but if you have you nine switches it’ll most likely be jumbled up. Your switch 3 (physically) may be numbered as 9, so make sure to write it down. Anyway, below is to finish what I’ve started.

Since I only have two switches and I know that this stack is not going to get another switch, I didn’t change the switch priority on the other one. I normally change the priorities of all the switches in the stack whenever I configure a stack from scratch except for the bottom switch. This way, I know which switch will be the next master switch when the original master fails. Obviously, this is a matter of preference so you don’t have to follow my method.

Once everything has been renumbered, it’s time to reboot the stack.¬†When the stack is fully booted, verify the switch stack to make sure that everything is what you desire it to be.

If you compare the previous show switch output earlier, the switch with a493.4cd6.5b80 is now showing up as switch 1 and has a priority of 15. With the priority set to 15, this helps determine who will be the master of the switch during election. Again, this does not guarantee that it’ll always be the master, however, since when there’s a tie in priority the next one to check is the MAC address.

Hope this has been helpful and thank you for reading!

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About Andrew Roderos

As an IT professional, I have a strong passion for technology and a desire to learn more about it. Technologies that I am mostly interested in are computer networking, network security, virtualization, and programming. Outside of the information technology world, I enjoy traveling, reading science fiction books and manga, watching movies, and photography.