difference between DHCPREQUEST sent through rely on rebinding and renewing

[Nick Urbanik]

Dear Folks,

I have been asked what the difference is between the DHCPREQUESTs as
seen through a DHCP relay by the server.

The reason for the question is that the number of instances of
DHCPREQUESTs for clients in the rebinding state are reported to be
about five times higher than the DHCPREQUESTs for the clients in the
renewing state.

That seems so wrong.  So is there a way to distinguish between these
two types of DHCPREQUESTs from the server's point of view?  Or could
this report be correct, and if so, is the DHCP server having trouble,
or could there be a reason for this being true and the DHCP server
working correctly?

Note that the relay is sending both unicast to the DHCP server, as far
as I understand.
--
Nick Urbanik http://nicku.org 808-71011 [hidden email]
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[Simon Hobson]

Nick Urbanik <[hidden email]> wrote:

> I have been asked what the difference is between the DHCPREQUESTs as
> seen through a DHCP relay by the server.
>
> The reason for the question is that the number of instances of
> DHCPREQUESTs for clients in the rebinding state are reported to be
> about five times higher than the DHCPREQUESTs for the clients in the
> renewing state.

If you are looking only at requests that have been relayed, then I am not surprised. A client renewing any existing lease should not be using the relay at all as it should be unicasting the request to the server. If a lot of renewal requests are being received via the relay then that indicates that there may be a network problem preventing renewal by unicast and so the clients are (eventually) falling back to broadcasts which the relay agent can pick up.

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[perl-list]

---

From: "Simon Hobson" <[hidden email]>
To: "Users of ISC DHCP" <[hidden email]>
Sent: Thursday, December 8, 2016 7:07:48 AM
Subject: Re: difference between DHCPREQUEST sent through rely on rebinding and renewing

Nick Urbanik <[hidden email]> wrote:

> I have been asked what the difference is between the DHCPREQUESTs as
> seen through a DHCP relay by the server.
>
> The reason for the question is that the number of instances of
> DHCPREQUESTs for clients in the rebinding state are reported to be
> about five times higher than the DHCPREQUESTs for the clients in the
> renewing state.

If you are looking only at requests that have been relayed, then I am not surprised. A client renewing any existing lease should not be using the relay at all as it should be unicasting the request to the server. If a lot of renewal requests are being received via the relay then that indicates that there may be a network problem preventing renewal by unicast and so the clients are (eventually) falling back to broadcasts which the relay agent can pick up.

This is not necessarily true in an ISP environment.  Some types of equipment (Cisco ASR 9k for example) will not send the IP of the DHCP server to the client.  The client can only communicate with the Relay Agent (what Cisco is calling a dhcp proxy on the 9k).  Other types of ISP equipment I have seen will grab the unicast renews and so forth and relay them anyway thus not allowing direct communication between the client and server.  So everything being relayed is certainly possible in an ISP environment and operating as intended.

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