[c-nsp] HSRP and dhcp-relay question

Mihai Tanasescu mihai at duras.ro
Wed Dec 27 02:49:47 EST 2006


Phil Mayers wrote:

> On Sat, Dec 23, 2006 at 12:44:53PM +0100, Gert Doering wrote:
>
>> Hi,
>>
>> On Sat, Dec 23, 2006 at 11:00:23AM +0200, Mihai Tanasescu wrote:
>>
>>> Because of the 2 ip helper-address statements I get 2 DHCP requests 
>>> that are identical.
>>
>>
>> Yes, unavoidable (<insert plea for 'make HSRP standby interface inactive
>> regarding all other IP functions'>).
>
>
> Yes and yes.
>
> BE AWARE however, that I have seen some DHCP clients e.g. PXE ROMs 
> take the gateway as the relay IP (which is the real IP) and not the 
> virtual IP embedded in the DHCP gateway field.
>
> If the client happens to receive the response from the non-master and 
> that router has somehow compromised connectivity e.g. it has moved to 
> a low-bandwidth wireless link then your DHCP client may experience 
> problems.
>
> This is not a cisco fault, though HSRP does make the fault show up.
>
>>
>>> Is there any way to make a single request happen and to have it 
>>> forwarded with SRC IP == standby IP ?
>>
>>
>> As far as I can tell, no.
>>
>> OTOH, in our own experience, this doesn't seem to to harm - we've run
>> this with ISC DHCPD and with MacOS X Server DHCPD, and in both cases,
>> both replies are identical, and the clients don't seem to mind.
>
>
> It works just fine with every setup I've ever tried too.


Thank you all for the replies and explanations.

I'll be testing the setup after the 1st of January as everyone has 
entered on vacation now.

Happy holidays.



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