Correct Failover / DHCPOFFER functionality

Claus Holm Christensen iscdhcp at
Tue Jun 30 17:46:39 UTC 2009

Robert Hille skrev:

Please note the "via x.x.x.x" extension of the log message.  It shows 
that one DISCOVER was forwarded by the .254 router, the other DISCOVER 
by the .253 router.  The OFFER is returned to the client through both 

Since the client don't have an IP address yet, the REQUEST goes out by 
broadcast as well, both routers forward the request and the server 
answer through both routers as well.  This isn't exactly correct from 
your logs, but I have a feeling that you may have anonymized the log a 
little and made a small mistake on the last two lines :-)

Later the client will try to renew the lease with unicast REQUEST/ACK 
messages, but this time the exchange shouldn't be duplicated in the logs.

> This doesn't occur on all subnets, and from the reading I'm doing I'm 
> beginning to wonder if it might be network related, maybe something with 
> the helper addresses on the routers?

I think you're right on suspecting the network, I have exactly the same 
pattern in my logs, and this always happend on the VLANs with two 
routers attached, running some sort of failover.  I don't remember it on 
the VLANs with only one router alone.

Claus Holm Christensen

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