[c-nsp] HSRP Groups on ASR1k

[Benjamin Lovell]

I think I might have gotten this a little mixed up as far as it altering the MAC itself. The group ID does map to a MAC. It's been a while since I looked at this and am not remembering the details but it has to do with hardware platforms have two possible problems with HSRP. a) limit to number MACs it can program for diff group IDs b)a problem with not being able to hardware switch traffic if the same HSRP mac lives in more than one place as the hardware then can't map things based on MAC alone. 

Here is a link about the 3550's that illustrates one of these problems. These same limits would not apply to other platforms but give you an idea of the nature. 

http://www.cisco.com/en/US/products/hw/switches/ps646/products_qanda_item09186a00801cb707.shtml


-Ben



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          .            .          Benjamin Lovell
          |            |          AS Video Practice
         |||          |||         Cisco Customer Advocacy
       .|||||.      .|||||.       Research Triangle Park, NC
    .:|||||||||:..:|||||||||:.    Email:  belovell at cisco.com
             cisco            desk:919.392.8255 cell:203.509.1562
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On Sep 28, 2010, at 2:08 PM, Phil Mayers wrote:

> On 09/28/2010 06:31 PM, Benjamin Lovell wrote:
>>> 
>>> Is this right?
>>> 
>>> Isn't the HSRP MAC the same for a given group number, regardless of
>>> which sub-int?
>>> 
>>> We run all our interfaces (not ASR1k though) in "standby group 0"
> 
>> BL> It varies by platform. The 3K series switches for example also use
>> the VLAN ID and will give you 256(i think) unique MACs even if using the
>> same group ID. Either way using the same or no group ID is defiantly not
>> Cisco's recommendation.
> 
> That's interesting. We don't see that on the (few) 3750s we've got routing - the one routing our subnet is 00:00:0c:07:ac:00 but the SVI is (obviously) vlan tag.
> 
> Why do Cisco recommend against using the same group ID? What's the problem?
> 
>>> 
>>>> 
>>>> This could be a software limitation that was addressed or is planed
>>>> to be addressed in later code releases or it could be a hard limit of
>>>> the NIC used on the SPAs. I would open a case with the TAC to have
>>>> them talk to the devs about this and see if it will be important to
>>>> you.
>>>> 
>>>> BTW - not clear on the part where you said you are using HSRP groups
>>>> 1 and 2 on the customer sub-ints. You should use a unique standby
>>>> group for each HSRP instance. If you are not this *may have something
>>>> to do with your problem.
>>> 
>>> Why? Using a different standby group per sub-int will surely
>>> definitely run you over the mac receive filter size limit? What's the
>>> problem using the same group number on different interfaces?
>> 
>> BL>It's certainly possible/likely but this is really a matter of
>> platform knowledge about how the ASR 1K does MAC filtering / HSRP MAC
>> assignment. I am not that deep into the ASR 1K platform and unless one
>> is it would not be wise to jump to conclusions.
> 
> Fair point.
> 
> However, on the 6500 under 12.2(33)SXI, the standby group is limited from 0-255. This means it could only do HSRP on 255 subnets; we do way more than that, and I'm sure customers elsewhere do. If there's a problem lying under the hood re-using the group ID, it would be good to know what it was.
> 
>> 
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