DHCP Relay Bug?

[Gumby]

Has anyone encountered DHCP relay issues?

Running mostly 225's and 250's with DHCP relay across our WAN, latest stable firmware (14.32).  It seems that when clients do a renewal request, which is unicast to the DHCP server, the switches are intercepting this and sending a second relay request to the DHCP server.

The requests from the meraki relay generally hits first, so it is the one that the server replies to.  Unfortunately the switch isn't then forwarding this onto the client, so the client gets a timeout error.

Happening on both windows clients and cisco phones.  Repeatable every time and easily viewed with wireshark.

As a test I configured a linux box with dhcp relay on one vlan and it operates as you would expect, only relay's dhcp discover broadcast traffic.

[Gumby]

Example below.  Client (172.21.150.160) sends a renew request direct to the server (10.14.1.30).

On the server we see the 4 requests come in.  On each request it comes from the client and from the dhcp relay on the meraki switch (172.21.150.254).  As the switch request is first, the server is replying to it.  This never makes it back to the client.

I've done other tests where the request from the client does manage to beat the request from the relay and it is returned to the client as expected.

[Gumby]

Updated a switch at one site to the 15.9 beta, doesn't exhibit the same issue.  The switch isn't attempting to intercept DHCP renewals.

Below is the trace on the server.  Above the red line is the initial DHCP request being relayed through the switch.  Below the line is the renewals coming direct from the client:

[Gumby]

Another update, I claimed success to early.

It seems if the switch performing the DHCP Relay is also the default route, it will intercept and resend DHCP renewal requests.  If I run the DHCP Relay on another switch which is not the default route, everything behaves as normal.

[PhilipDAth]

I've had some weird behaviour with DHCP relay on MS225s.  It is hard to pin down.  The last one I looked at only affected 1 VLAN using DHCP relay (out of maybe 4 using DHCP relay), and then only one class of devices (VoIP devices).

I also found DHCP packets not being properly forwarded in packet captures.  I couldn't find anything interesting between working and non-working packet captures.

I ended up changing that one VLAN to use DHCP server on the switch to get everything going again.

[Sysad]

I had a similar issue after installing MS 14.32. Have you contacted support to see if a newer stable version of firmware resolved the issue with the DHCP relay bug?

[Frank-NL]

We just had this issue on 15.20. There seems to be some kind of relevance to a large L2 domain I think.

This was MS210