Cisco 3945 - Max HSRP Groups?

[bg_27]

I am trying to find out what the maximum amount of HSRP/Standby groups a Cisco 3945 will support.

I found this link that I think says 256 http://www.cisco.com/en/US/customer/tech/tk648/tk362/technologies_q_and_a_item09186a00800a9679.shtml#q5

Can anyone validate this?

Thanks

[Ivan Krimmel]

Hi,

256 HSRP groups(0-255 group ID's) is a usual capability of many platforms(I doubt there's a more capable one), so you are right.

The important point here is the fact, that this is the number per interface, i.e. one could reuse the same ID's on other interfaces as well actually having much more HSRP groups on a box in the end. So in theory there's no software limit, however please do keep in mind, that at some point in time your HSRP sessions will start to flap - because of what? right, the hello packets being exchanged to keep the session up. With subsecond hellos you won't even reach a dozen of groups per box.

I hope this explains.

Regards,

Ivan.

[Peter Paluch]

Hello Ivan,

Just to add a little more detail, the HSRPv2 introduced support for group numbers in the range 0-4095, in order to allow matching the group numbers to the VLAN numbers. It is not expected, though, to run so many groups concurrently on a single device.

Also, recent HSRP developments in 12.4T allow for so-called HSRP Multiple Group Optimization where a particular group (a slave group) can inherit the state of another HSRP group (a master group). All elections are performed only by the master group, the slave groups will subsequently follow the state of the master.

Nevertheless, the exact number of HSRP groups running concurrently is difficult to specify. In my personal opinion, it is in the order of tens, maximally hundreds.

Best regards,

Peter

[Ivan Krimmel]

Hi Peter,

good point, thanks.

I am not sure whether this is the only numerical enhancement(0-4095), or we indeed could configure 4096 HSRP groups. I think the current limitation is still 256 groups, though I might have missed the latest announcements ;)

Regards,

Ivan.

[Peter Paluch]

Hello Ivan,

I am not sure about the maximum concurrent count of the HSRP groups myself. It obviously has to do also with the hardware capabilities of the Ethernet controllers on the respective platform, as they must support listening to additional unicast MAC addresses (the vMAC addresses of individual standby groups). And certainly, the software limitations also play their role.

In any case, I can try creating roughly 4000 subinterfaces and groups on them, provided the 2811 I have in my lab supports that many subinterfaces The TCL for {} cycle will undoubtedly be of great help here

Best regards,

Peter

[Ivan Krimmel]

Peter,

I also miss a good TCL script to verify some edge conditions :)

just wondering - why would you need 4k subints, as the maximum number limit should be linked to a single interface. As I said, even without this enhancemen(0-4095)t there's no maximum number of groups being specified, as long as one uses the same ID's on different interfaces.

BR,

Ivan.

[Peter Paluch]

Hi Ivan,

just wondering - why would you need 4k subints, as the maximum number limit should be linked to a single interface

Umm... Because you originally wrote:

I am not sure whether this is the only numerical enhancement(0-4095), or  we indeed could configure 4096 HSRP groups. I think the current  limitation is still 256 groups, though I might have missed the latest  announcements

So I wanted to see if I can exceed this limit on an interface, and hence, on a platform.

Best regards,

Peter