DHCP Snooping with existing leases

[#1]

Hoping somebody can help.

I recently introduced a new switch into production that was running DHCP snooping. In the lab this appeared to work as inteded and prevented untrusted DHCP servers. In production the behaviour was much different, a handful of devices got IP addresses, but many of them failed to aquire one.

I'm unsure as to why this was the case and wonder if it has anything to do with existing leases. My understanding it that DHCP snooping maintains it's own snooping bindings of DHCP offers. I did however try deleting the DHCP lease of one machine, but this failed to produce the intended results still.

Any advise would be much appreciated.

Cheers,

Neil.

[Kyle McKay]

Hi Neil,

Would you be able to post your configuration?

The Switch relies on observing the entire DHCP process from client to server. It populates its binding table based on that information. If you were to enable DHCP snooping on a switch, the binding table would be completely empty to start. The switch needs to observe the DORA DHCP process (Discover, Offer, Request, Acknowledge) for each connected device in order to successfully populate that binding table.

If devices are failing to acquire a DHCP-assigned IP address, there are many different reasons for it. The most common problem I have seen in initial implementation pertains to the DHCP Snooping information option. Without actually seeing your configuration or your debugs, I would go out on a limb and suggest turning that off (no ip dhcp snooping information option).

Hope this helps!

[paul driver]

Hello
when you enable dhcp snooping on an axitve swith the snooping database will NOT get populated with existing leases only when the client renews will this occur.

Also make sure if you have dhcp clients in additional swithes then snooping will be required on them also.

lastly make sure you trust the switches interconnects otherwise your clients will ne dropped

res
Paul

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