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DHCP snooping problems on C2970 and C2960

Dear All,

We recently rolled out a Catalyst 2970 aggregation switch with 10 Catalyst 2960 access switches connected to it and have run into DHCP snooping problems.

DHCP snooping is enabled for VLAN's 1 to 4094 on each of the switches. Each access switches' uplink port as well as all of the link

ports (including the physical and port-channel uplink interfaces to the campus distribution from the aggregation switch) are all DHCP snooping trusted. Each access VLAN interface on the distribution switch is correctly configured with the correct ip helper address.

However, if a DHCP client is connected to any of the access switches, or to an untrusted access port on the aggregation switch it doesn't get an IP address (and the DHCP server doesn't see any traffic from this client) until DHCP snooping is disabled for that access VLAN on the relevant access switch.

All of the switches are running IOS Version 12.2(25)SEE2.

I've checked the bugs database but can't find any existing DHCP snooping related bugs that match the symptoms I'm experiencing.

We're running this switch configuration on at least one other site without this problem - however, this site differs slightly in that:

a) we are using VLAN's between above 256 and ran into CSCse03859 which effectively disables DHCP snooping on all of our access VLAN's (so perhaps if we were using VLANs below 256 we'd hit this same problem)

b) and, the building aggregation C2970 in this case is running a different IOS (12.2(25)SEB4) and we never tried connecting any DHCP clients to it.

I'd welcome any insights into this problem - though it feels like an IOS (rather than a configuration issue) to me.



New Member

Re: DHCP snooping problems on C2970 and C2960


On the Vlan interface that has the IP that the ip helper points to you must configure:

ip dhcp relay information trusted



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