HSRP Limitations on Cisco Catalyst 3560 Switches

ciscocisco-catalystfhrphsrpswitch

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.

Best Answer

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.

Related Solutions

Cisco – Switch replacement procedure 3560G to 3750X

I don't think you're going to see any website with direct step-by-step instructions on how to do this. There maybe a few blogs which speak to a blogger's personal experience....however, this is how I would approach it.

I'm going to assume that your current 3560Gs have L3 links to the core and an L2 link (or portchannel) between the two switches. I will also assume that you're using interface tracking to help swap HSRP states and pre-emption and etc...

While you are adding the 3750X to the original 3560G switches. All you would require is to extend the L2 link to the 3750X stack. You can pre-provision the 2nd 3750X and even have it running and connected when you do this entire process.
Then move the L3 uplinks from the standby 3560G to the first 3750X and ensure that HSRP is configured in order to facilitate the failover.
Once that is done...begin migrating your cabling/device cable to the 3750X stack.
Then move the final 3560G L3 link to the 2nd 3750X switch. You should now also be able to remove the L2 link between the 3750'ss and 2560's and power them off.

Since you're using HSRP - the 3750X's should become active under your control (via a priority change)

Finally, after you are left with just the 3750X in a stack-formation. You will no longer require HSRP to run between the two switches since the 3750X will really be seen as one switch all together. Ultimately, the end solution should have 2x L3 uplinks to your core or router, and the VLAN interface existing solely on the 3750X stack. I would also have the each of the L3 links attached to the separate 3750X switches as well.

This final solution should give you a more robust design from preventing spanning tree and HSRP timers from delaying your network re-convergence and allowing the routing protocol to do the upstream path selection instead of HSRP.

Cisco HSRP – Configuring HSRP with Redundant Links

I would use SVIs for the L3 interfaces at core instead of assigning the IP configuration to physical ports.

This way, having VLAN 100 allowed in all links between Core1, Core2, Distribution1 and Distribution2 the Spanning Tree protocol will manage which of the available uplinks will be used to forward traffic between distribution and core.

A config sample for HSRP with SVI would be:

Core1

interface vlan 100 
  ip address 192.168.1.2 255.255.255.0
  standby version 2
  standby 100 ip 192.168.1.1
  standby 100 priority 105
  standby 100 preempt
!

Core2

interface vlan 100
  ip address 192.168.1.3 255.255.255.0
  standby version 2
  standby 100 ip 192.168.1.1
  standby 100 preempt
!

Should you need more information regarding the configuration options for HSRP in your 3560´s you can check the 3560 configuration guide for the software release you are using. For instace: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_52_se/configuration/guide/3560scg/swhsrp.html

Best Answer

Related Solutions

Cisco – Switch replacement procedure 3560G to 3750X

Cisco HSRP – Configuring HSRP with Redundant Links

Related Topic