Single vlan dhcp problem

[Sheridan]

This is doing my head in now - I'm trying to setup a single vlan (we have several already in place which are routed) that is seperate and independent from all other existing ones, and in the setup there is a dhcp server that is physically connected only to that vlan to provide addresses without any dhcp helpers etc

 

So the switch path is like this:

 

Client device--Edge Switch--Stack of switches (3)--Core switch--dhcp server

 

Now, if I plug in a device a the edge switch and set it to request a dhcp address it doesn't work. However, if I configure a spare port on the edge or ANY of the stack of switches and watch the traffic with Wireshark I can see the DHCP Discover packets passing all the way through so I know the vlan is configured correctly. The problem is that the stack of switches doesn't seem to be passing the dhcp request up to the core switch and therefore the dhcp server doesn't see the request!

 

Now, in my testing I have found:

1) Manually setting IP addresses on devices attached to the edge switch can ping the dhcp server, so vlan must be correct!

2) Plugging into a spare port in the core switch configured to the same vlan gets a dhcp address, so vlan and dhcp are working at the core switch

3) The dhcp request from the client at the end switch can be seen at every switch apart from the core

4) With wireshark on the core switch I can see various packets arriving from the client (filtering by mac address) for services such as the antivirus, so again it seems to suggest the dhcp packets are being blocked rather than any vlan issue

 

So I can only conclude that the stack of switches uplink to the core is blocking the dhcp request somehow, yet I cannot see where or how! We don't have ACLs in place, and there is no need for any dhcp helpers as the whole thing is effectively one single flat vlan.

 

Its all netgear switches by the way and netgear couldn't help me fix this either, apart from suggesting a firmware update which I did to no effect.

 

Any ideas before I lose the plot completely!

Edited April 19, 2017 by Sheridan

[keithu]

Some Netgear switches have a feature called DHCP Snooping that blocks 'rogue' DHCP servers. I'm sure it's configurable, but I couldn't tell you exactly how.

[Sheridan]

Some Netgear switches have a feature called DHCP Snooping that blocks 'rogue' DHCP servers. I'm sure it's configurable, but I couldn't tell you exactly how.

 

This was already disabled, however I did find that the dhcp l2 relay does need to be enabled even though its not crossing any vlan boundaries, so far so good - now to test it properly!

[Julian]

HP switches have a feature called "ip helper address", it is used to get DHCP requests from one vlan, to the vlan on which the DHCP server sits. The "ip helper address" is set to the DHCP server address. You will have to do something similar to get your DHCP requests, to jump vlans.