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3750 Maximum HSRP instance limitation?

I worked on a small project recently where we used a pair of 3750-12S (running the IP Services image) as a collapsed distribution/core layer with 2960's and 2950's for the access layer. In total we had 11 access-layer switches each connected to each 3750 over dot1q GigE links, each switch with 4 VLAN's, these VLANs were terminated on the 3750's.

The intention was to use HSRP on each VLAN but we hit this limitation on the 3750 where we could only have a maximum of 32 HSRP instances. When I attempted to add the 33rd HSRP instance an error message was logged stating something like 'platform only supports 32 HSRP groups'. Since we used the same HSRP Group ID (standby 1) I thought we wouldn't hit this issue.

Are there any workarounds or is this just a platform limitation with the 3750? We were running 12.2(25)SED1, I had a look in the 12.2(25)SEE release notes and didn't see anything related to HSRP being added.




Re: 3750 Maximum HSRP instance limitation?

32 HSRP instances do come from a Hardware limitation and that there is no way to overcome or apply a software workaround that should increase the number of HSRP groups as like in the Cat3550 or the Cat6k.


Re: 3750 Maximum HSRP instance limitation?

You are correct on the 3750's you can only run hsrp on up to 32 vlans , it is right in the documentation . So I guess if you have that many vlans you really should be using a bigger switch like a 4500 or 6500 .

a little blurb from the 3750 hsrp config info.

HSRP Configuration Guidelines

Follow these guidelines when configuring HSRP:

* HSRP can be configured on a maximum of 32 VLAN or routing interfaces.

* In the following procedures, the specified interface must be one of these Layer 3 interfaces:

Re: 3750 Maximum HSRP instance limitation?

The wording in the documentation is a little vague and this is why I assumed this was the same limitation you have with HSRP on the 6500 Sup2/MSFC2. This is where you are limited to 16 unique HSRP groups due to the number of MAC's available. You can get around this by re-using the same HSRP group number on each interface.

The vague bit of documentation was:

If HSRP is enabled, the switch can recognize 32 additional MAC addresses, each associated with a set of VLANs or routing interfaces.

I agree once I looked at some other documentation I saw:

HSRP can be configured on a maximum of 32 VLAN or routing interfaces

I was slightly dissapointed that the 3550 does not have this limitation since the 3750 is supposed to be the logical upgrade path and with its stacking capability it can easily be placed in the distribution layer terminating lots of VLAN's.

The customer does not have the cash to install catalyst 4500's since the difference in price is just too much.

Thanks for the replies.



Re: 3750 Maximum HSRP instance limitation?

I would agree with you if the limitations were not there on the 3550's , seems like they should have at least that capability in the 3750 . The only thing I can say is to look at the setup and determine if every single vlan absolutely needs hsrp backup or not .