DHCP Relay Option 47 error

[erikreig1]

Hi,

We configured recently a DHCP Relay (ip helper-address) in a Cisco 1921. It doesn't work because the router discards replies coming from the DHCP server to the clients. It complains that the package have an option malformed.

The DHCP server is the same for other corporate networks where we also have ip helper-address configured in the corresponding gateways, and they didn't give any problem.

The errors were detected by putting a debug on the router, as follows (where 10.50.0.3 is the router  and  IP 10.10.2.25 is the corporate DHCP server)
November 5, 09 : 20: 49,257 : DHCPD : Finding a relay interface for client 0100.2264.b41a.3c on Vlan50 .
November 5, 09 : 20: 49,257 : DHCPD : giaddr setting to 10.50.0.3 .
November 5, 09 : 20: 49,257 : DHCPD : BOOTREQUEST from 0100.2264.b41a.3c forwarded to 10.10.2.25 .
November 5, 09 : 20: 49,805 : DHCPD : option 47 is malformed .
November 5, 09 : 20: 49,805 : DHCPD : invalid DHCP options - Unable to parse

I captured and analyzed the packets sent by the DHCP server with option 47 field informed and see no problem.
If the DHCP server config is changed to not pass any value in option 47, the router stops complaining and correctly forwards response from the DHCP server to the client that requested IP.

We need option 47 because we already use NETBIOS over TCP/IP:

The router runs C1900 Software ( C1900 - UNIVERSALK9 -M ) , Version 15.2 ( 1) T1 , RELEASE SOFTWARE ( fc1 )

c1900-universalk9-mz.SPA.152-1.T1.bin
The exact model of router CISCO1921 / K9

Thanks for all!!

[Richard Burts · Accepted solution]

If the DHCP server is using option 47 with other networks without problem and if the packet capture looks normal then it suggests that there is an issue with the version of code that is running on the router (and 152-1.T1 is a pretty early version so is more likely to have some issues). So I would suggest trying a different version of code in the router and see if that improves the problem.

HTH

Rick