DHCP Relay agent not forwarding messages to the client

[Friesen, Don MTIC:EX]

Yes, this is the expected and normal behavior.  Each server will reply with an address from its half of the pool. The machine that is receiving the offers will take the first offer to arrive and ignore the other.

We have deployed remote DHCP served by relay for 20 years, and this is how it functions.

The only annoyance with remote relay DHCP is that the NAK message is a local broadcast.

Don Friesen

From: dhcp-users-bounces at lists.isc.org [mailto:dhcp-users-bounces at lists.isc.org] On Behalf Of Patrick Trapp
Sent: Tuesday, June 16, 2015 6:17 AM
To: Users of ISC DHCP
Subject: RE: DHCP Relay agent not forwarding messages to the client

You'll want more experienced answers, I'm sure, but we are running a fair-sized network for our DHCP - I believe around 45000 devices and roughly 70 subnets. All of those devices find the DHCP servers via relay agents and we are using failover.

We do generally see two DHCP offers - one from each server - when a device requests an address. I expected that behavior and saw no reason to be concerned with it. I still don't - I am under the impression that this is how the system is designed to work.

It has worked fine for us in this fashion for years. We deployed this particular solution in 2007, but I believe it was used for other devices prior to that, so we have used this through several iterations of ISC-DHCP.

Patrick
________________________________
From: dhcp-users-bounces at lists.isc.org<mailto:dhcp-users-bounces at lists.isc.org> [dhcp-users-bounces at lists.isc.org] on behalf of Gero Palacio [gero.palacio at gmail.com]
Sent: Monday, June 15, 2015 5:17 PM
To: Users of ISC DHCP
Subject: Re: DHCP Relay agent not forwarding messages to the client
Hi Simon,

Thanks for the reply, I did not know if it was a bug or the correct behavior.

If I may, let me ask you a follow up question. Does DHCP relay works well with a pair of DHCP servers in failover mode?

I've configured the relay agent described in my previous email to forward the messages to the pair of DHCP servers. Now everytime the relay agent forwards a discovery message from the host, I receive 2 dhcp offers, one from each server. As I've configured split to 128, only 1 server should reply with an offer.

If I test the same configuration with hosts in the same subnet as the DHCP servers, the behavior is as expected. The DHCP servers responds or ignores the discovery messages according to the result of the hash based on the MAC address of the hosts.

Any idea what might be the problem? Is it even a good idea to run a relay agent and a pair of DHCP servers in failover?

Thanks!
Gerónimo.

On Mon, Jun 15, 2015 at 6:18 PM, Simon Hobson <dhcp1 at thehobsons.co.uk<mailto:dhcp1 at thehobsons.co.uk>> wrote:
Gero Palacio <gero.palacio at gmail.com<mailto:gero.palacio at gmail.com>> wrote:

> CLIENT <---> [eth0] (relay agent) ROUTER-1  [eth1] <---> ROUTER-2 <---> DHCP Server
>
> The relay agent runs ONLY on the interface that faces the client, which is eth0: "dhcrelay -4 -i eth0 10.0.1.10"
...
> Can anybody tell me if I'm doing something wrong? Is this a known bug?

It's a known feature. You must also listen on eth1 in order to get the replies from the server - it's just the way the relay agent works.

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