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I have read all of the documentation so many times that I put myself into a headspin. Now the sentences I read don't make any sense.

What exactly is the HSRP limitation on Cisco Catalyst 3560 switches? I have read that there is a limit of 32 instances of HSRP GROUPS.

What if I have instances on group 1? Can I have 32 instances on group 1 as well as 32 instances on group 0?

What if I have SVI's on group 0 running on another switch that shares a VLAN ID with the switch in question? Will that count against the limit? I have read elsewhere than it can, but now I am not so sure.

Is there some literature with clarification that somebody can point me to? Because I'm not willing to take anybody's word on anything at this point.

Referenced from here

You can configure up to 32 instances of HSRP groups. If you configure the same HSRP group number on multiple interfaces, the switch counts each interface as one instance:

For example, if you configure HSRP group 0 on VLAN 1 and on port 1, the switch counts this as two instances.

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An HSRP group on an interface is an instance. Each layer-3 interface could have an HSRP group 1, but each is a different instance, and, indeed, are actually different HSRP groups, despite having the same number. The HSRP group is local to the interface, so HSRP group 1 on one interface is not the same HSRP group 1 on a different interface..

For example, you could have 32 interfaces, each with one HSRP group, 16 interfaces, each with two HSRP groups, one interface with 32 HSRP groups, or any combination, not to exceed 32 groups.

The "group" for HSRP is a group of routers (or layer-3 switches) on the same LAN that share an HSRP group number, not a group within the router. The router group will exchange HSRP messages on the LAN to determine which is the active router and which is the standby router for that group.

There are all types of documents that explain how HSRP works, and if you really understand how HSRP works, then you will understand that the groups on each interface are independent from the groups on any other interface. HSRP is configured on layer-3 interfaces, and each layer-3 interface in a router is in a different network, but HSRP must communicate with other HSRP routers on that network, and the standby address is only for that network. Remember that for your layer-3 switch, you can only create one SVI for each VLAN, and if you try to assign multiple interfaces in the same, or overlapping, network, you will get a configuration error.

RFC 2281, Cisco Hot Standby Router Protocol (HSRP) is one source of information, and it defines a standby group:

Standby Group - the set of routers participating in HSRP that jointly emulate a virtual router

As you can see, the group is the group of routers on a network, not a group of interfaces inside a router.

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  • But I found this documentation about 3550 which directly contradicts what you just stated: Shouldn't the 3560 be more robust than the 3550 in every way?! cisco.com/c/en/us/support/docs/switches/… Does this mean that the 3560 can actually support 512 (32 x 16) HSRP interfaces? – Octavian Jan 26 '19 at 1:14
  • No, what I explained is exactly what that document you linked is telling you. – Ron Maupin♦ Jan 26 '19 at 1:15
  • Q. What is the maximum number of HSRP interfaces supported? A. Each of the 16 unique group numbers can be used by 16 consecutive Layer 3 interfaces, which gives a total maximum of 256 HSRP interfaces. The total number that is recommended is 64, but this number depends on the routing protocols and features that are configured on the box. If you configure too many HSRP interfaces, you could cause the CPU load on the switch to become too high, which can have unexpected side effects. – Octavian Jan 26 '19 at 1:17
  • That directly states 256 HSRP interfaces – Octavian Jan 26 '19 at 1:21
  • So please explain exactly what the word 'exactly' means to you :) – Octavian Jan 26 '19 at 1:22
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Well I found an exact answer with documentation to match. The answer is 32. HSRP can be configured on 32 interfaces. (Documentation to provide an exact answer was what I had asked for, not various interpretations of the documentation I had already provided.)

From here, page 34-4:

HSRP Configuration Guidelines Follow these guidelines when configuring HSRP:

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

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