Disadvantages of using thousands HSRP groups

[mohit rawat]

Hi all,

I have query that, is there any disadvantage or any performance related issue if we use 1000-2000 hsrp groups in switch. as

per cisco in HSRP version 2 we can create 4095 hsrp groups. 

Thanks 

[Georg Pauwen]

Hello,

according to the documentation:

--> HSRP version 2 permits an expanded group number range, 0 to 4095, and consequently uses a new MAC address range 0000.0C9F.F000 to 0000.0C9F.FFFF. The increased group number range does not imply that an interface can, or should, support that many HSRP groups. The expanded group number range was changed to allow the group number to match the VLAN number on subinterfaces.<--

So memory and CPU utilization are still to be kept in mind.

[rasmus.elmholt]

The max HSRP groups are 4095 but there is a systemwide max depending on the hardware. You need to take a look at the datasheet for you device.

This is an example from the 6500 Series

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/29545-168.html

The default hello timer for HSRP is 3 seconds. With 4000 groups it will generate a lot of traffic.

[mohit rawat]

Thanks Rasmus,

yes..but as per 6500's documents it says max 256 group can be created? getting confused

[rasmus.elmholt]

No need to get confused.

HSRP version 2 supports 4096 group numbers, but most of the devices I've seen only support 256 active at one time.

This is from the 3560 where it supports only 32 instances.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_52_se/configuration/guide/3560scg/swhsrp.html

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.

[mohit rawat]

Thanks,

it is 256 per interface or only 256 groups is  final limitation of device?  what if we wand to use HSRP for 1000 vlans.

thanks

[Georg Pauwen]

Mohit,

the limitation is the hardware, which can only hold a certain amount of well known HSRP MAC addresses. I think they extended the group numbers available just so that the numbers can match those of subinterfaces.

[rasmus.elmholt]

The design recommendation is to use the same group number as the vlan number on your SVI fx. So it makes sense to have a 12 bit group number like our 12 bit vlan id like gpauwen says.

[mohit rawat]

 thanks gpauwen,rasmus,

can you please clarify one more thing, that limit(256) is for WS-SUP32-GE-3B. whats about SUP 2T? I could not find any related documents for SUP 2T. what is the limit of HSRP groups for vlans in switch having SUP 2T.    

[rasmus.elmholt]

Hi

I cannot fint anything either, but a guess would would be 256 as well.

You are not able to use VSS instead, then you dont need FHRPs like HSRP?

[rasmus.elmholt]

It is system wide. So 128 on one interface and 128 on another. Or 64 on 4 interfaces.

Hope this helps