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DHCP forwarding issue with 122-53.SG

We recently upgraded a Catalyst 4506 switch from

cat4000-is-mz.121-13.EW.bin to


The switch has multiple VLAN's with catalyst 3550 switches connected to it.

We have numerous Linux, Windows, Printers that require DHCP to boot. The switch has a helper address to forward those requests to a machine running ISC's DHCP version 3.x server.

That worked previously and continues to work now. Devices on all VLANs whether directly connected to the 4506 or connected to a 3550 continue to receive DHCP replies.

In addition we have some embedded systems that also use DHCP though it's possible it's technically a bootp request. Those systems no longer receive replies even though devices on the same network and 3550 secondary switch do receive replies.

We can see in the DHCP logs that the requests for these boards are received and historically valid replies are sent back.

Has their been some change in the 122-X train how forwarding of DHCP or bootp replies (not requests which work fine) are handled ?

From what we can see the boards are either not receiving the replies or the replies are wrapped in such a way that they can't be successfully unpacked.

This environment is very remote, we're working toward getting better information through packet dumps etc but the time frame for accomplishing that is on the order of when we'd have to decide to revert the IOS.

Thank you very much in advance for any insight.

James Robnett

edit: Bah I don't how this ended up here, too much of a hurry I guess. I'll repost in LAN routing and switching. Apologies.

  • WAN Routing and Switching
Hall of Fame Super Silver

Re: DHCP forwarding issue with 122-53.SG

Hello James,

answered in LAN forum.

let's go on there

Best Regards