ip helper-address BROKEN in Catalyst 3750?

[gerryobrien]

Using ip helper-address on a Catalyst 3750 LAN SMI no longer seems to work. In the attached config we are trying to get DHCP requests passed from VLAN 5 to VLAN1 (the default). Sniffer running on both sides tell us that the UDP packets are not passed through.

It seems unrelated to platform as it happens with both Windows & Linux clients. The DHCP server works fine on the default VLAN. This type of setup worked finr on a 2948G-L3 which only had bridge group virtual interfaces.

Does a full DHCP server need to be setup on a 3750 to get relaying to work? The CISCO documentation is unclear about this point. Perhaps someone knows the answer? Has anybody else had a similar experience? Any pointers would be gratefully recieved.

Thanks, Gerry

[aashish.c]

Hi,

access-list 105 deny ip any 134.226.60.0 0.0.0.255

this access list is blocking the traffic from vlan 5. have u tried removing this. I have also faced the same problem, but I found a wierd solution to it. I checked the MAC address table and there was a loop . 1 MAC address was learned by different ports. it was also showing the port was wasn`t connected to any devices, left idle. I shutdown that port and looping stopped and DHCP requrests started going and PCs were getting the IP .

Try this out and update me.

[aashish.c]

Hi,

access-list 105 deny ip any 134.226.60.0 0.0.0.255

this access list is blocking the traffic from vlan 5. have u tried removing this. I have also faced the same problem, but I found a wierd solution to it. I checked the MAC address table and there was a loop . 1 MAC address was learned by different ports. it was also showing the port was wasn`t connected to any devices, left idle. I shutdown that port and looping stopped and DHCP requrests started going and PCs were getting the IP .

Try this out and update me.

[gerryobrien]

Hi,

The access-list in on a VLAN that does not have the helper address. Traffic from clients that do not use DHCP is routed out of the VLAN fine. Anyway, we had the problem before we places that access-list.

Thanks,

[gturek]

Gerry,

Here is the config we are using. This allows DHCP requests for PCs to traverse the channels successfully. IP phones receive ip from the switch DHCP service. I am not sure it will be a great help for your particular issue.

[gerryobrien]

Found a fix for the problem of routing DHCP requests between VLAN using ip helper-address. The source of the problem was the "deny ip any any" at the bottom of ACL lists 101 attached to VLAN 7 in the attached config file. The strange thing is that the DHCP forwards are between VLANs 3, 4, 5, 6 and the server on Vlan1 and had NOTHING to do with Vlan 7 except that is where the default routes go. The fix was to put in the second last line in the ACL 101 list which allows BOOTP through. IS THIS A BUG? If so, then does anybody at Cisco know about this? If not, then to say the least, the documentation is unclear. We have spent a lot of time trying to sort this out