DHCP Relay

Twitch
A model citizen

DHCP Relay

Morning everyone. I'm seeing some weirdness with DHCP Relay. We have a Cisco 4331 router connected to Meraki MS210-48P switches running firmware version 15.18. The 4331 router handles all of the layer 3 functions for the network, and has two subinterfaces configured - one for VLAN 1 (0/0/1.10) and one for VLAN 40 (0/0/1.40). The VLAN 40 subinterface has ip helper-address 10.0.10.7 pointing to the domain controller, which handles DHCP for both subnets.

 

The problem is clients in VLAN 40 are not getting an IP address.

 

The screenshot below was captured on the DHCP Servers & ARP page under Switching. It gives me the impression that the DHCP server is offering an IP address to the client, but it appears that the client never receives that offer, and ultimately times-out with a message stating that the DHCP server could not be found.

 

Twitch_0-1687787201476.png

 

The 4331 is forwarding the DHCP messages to the server, but the responses are not making it back to the client.

 

Has anyone experienced this issue? Is Layer 3 Routing required for Meraki switches to properly handle the DHCP relay? (I would think the switches are simply receiving a unicast message destined for the domain controller from the router due to the DHCP relay config and no special configuration would be required on the switches to simply pass the traffic out of the correct port to the domain controller)

 

Thanks!

 

Twitch

 

 

 

6 Replies 6
ww
Kind of a big deal
Kind of a big deal

You have allowed the dhcp server/mac on that same page?

Or are you allowing any dhcp server 

 

Maybe you can take a client side(dashboard switchport packet capture) to see  if the offer is actually send to the client or not.

Twitch
A model citizen

Morning ww - we are allowing DHCP servers, though I am not sure why the description for the server has a different name for VLAN 40 considering that it's the same server that is listed for VLAN 1. For some reason, Meraki seems to think that the DHCP server for VLAN 40 is the LAN interface of the 4331 router (the listed MAC is for the Gig 0/0/1 interface on the 4331, but I'm assuming this has something to do with the relay config.)

 

Twitch_0-1687790307423.png

 

ww
Kind of a big deal
Kind of a big deal

Looks fine,

 

You could do a dashboard packet capture on the switchport to a client not getting a IP to check if the offer is send to the client.

PhilipDAth
Kind of a big deal
Kind of a big deal

Is the MS using current stable firmware (or better)?

cmr
Kind of a big deal
Kind of a big deal

15.18...

Twitch
A model citizen

Correct. There is an update available, but I have not had a chance to look into which version the update will take us to.

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