how to create redundancy for existing stack switch?

[rrairineo1]

Hi. how can i introduce redundancy to avoid failover of services running in two different stack switches? Before, one of the stack switches failed and some of the services in our network were affected. how do i introduce redundancy to this current setup to avoid point of failure. please see attached file for diagram. 

[devils_advocate]

In terms of physical links, the only redundant you are missing is between the Access Switches and the Stack, each Access switch only has a single uplink and would benefit from two, one to each stack switch, preferably in a portchannel so you can load balance.

In terms of L3 redundancy etc, we would need to know more about the setup. 

[rrairineo1]

Hi. thank you for response. i have thought of these (refer to the attachment). is this an ok setup or do you have any suggestions to improve the design. (i am new at this network designing thing) Need to learn more. 

[devils_advocate]

Are the 3750 stacks purely distribution for the LAN?

Do any hosts plug into them or are they purely for uplinks etc?

I prefer the following design as it has less switches and provides full redundancy:

snip20160318_8.png

You will probably still need something like HSRP in this design on the two stacks.

The other option would be to replace the 3750's with something that can do VSS like a 6504 or a 4500x but this depends on budget and how many ports are needed as the distribution level.

[Jon Marshall]

I don't have a lot of experience with 3750s but as I understand it you can't create an etherchannel from an access switch that terminates on two different stacks, only to switches within the same stack.

The uplinks would have to be just trunks to each switch stack.

Have I got that wrong ?

Jon

[devils_advocate]

You can't use PAGP between stack members but you can use LACP :)

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/69979-cross-stack-etherchannel.html

[Jon Marshall]

I may be misreading the diagrams but I thought it was two separate stacks ie. you are not creating an etherchannel to members of the same stack but creating an etherchannel that terminates on different stacks.

Jon

[devils_advocate]

Ahhh yes, I missed that bit Jon!

I guess STP will take care of the redundant links to the second stack, as long as the Root bridge and HSRP priorities are aligned properly. 

[Jon Marshall]

No problem, I thought maybe I was misunderstanding how it worked.

As you say it is difficult to give answers without more information and it's not clear whether using separate trunk links to each stack with STP/HSRP etc. or perhaps combining the separate stacks together would make more sense.

It may be that the stacks are in separate locations in which case they would have to be trunks to each stack.

Jon

[rrairineo1]

Thank you guys for responding to my questions. ill take into consideration all your inputs. 

[rrairineo1]

yes john. do you think it can work or do you have better idea?

[Jon Marshall]

As already suggested if these are two separate stacks then you need to connect each access switch to each stack but not with etherchannel unless you have enough links to do that ie. each access switch would need two connections to each stack.

Assuming you don't then each access switch connects to each stack with a trunk link and per vlan one of the uplinks would be blocking.

You would then run HSRP between the stacks and match the STP root switch to the HSRP active switch and you can balance the vlans between the switch stacks.

Jon

[Joseph W. Doherty]

You have it correct.

You can only port-channel to the same device, and a 3750 stack works as a single device.

[rrairineo1]

how can i use hsrp on the two stacks (referring to the snip20160318_8.png option)? ill take note on 6504 and 4500x. ill read more on that but what's with the 6504 and 4500x that makes it a better option? thank you for suggestions. 

[devils_advocate]

Assuming the 2x 3570 stacks are going to be your distribution switches and host the default gateways for your LAN subnets then you just configure HSRP as per the guide.

Each LAN subnet will need 3 IP addresses for HSRP, one for each SVI and a virtual IP which will be the hosts default gateway. You would need to ensure STP is configured correctly to make sure the root bridge followed the HSRP Active node in the event of a failure. 

The benefit of using VSS is that both Uplink ports from the Access switches can be used for forwarding, STP will not block any ports.

However this means only two VSS switches to replace the 4x 3750's so it may not be what you want. You could have two sets of VSS switches but arguably this may not be better than two 3750 stacks so it depends on your end goal.