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Webcast-Catalyst9k
New Member

DHCP issues experienced since ISR upgraded to IOS 15.1.(3)

Recently I upgraded all my sites’ ISRs’ IOS versions from 12.4(24) to 15.1(3). The upgrade caused no problems for all except 2 sites where Windows DHCP servers stopped being able to assign IP addresses due to IPs having already been assigned when in reality this is not the case.

The main common denominator with the 2 problem sites is that they both have a flat network whilst all the successfully upgraded sites have segmented networks.

As the ISRs are not configured for DHCP and whilst I await further diagnostic details from the remote admins, for the time being I can only assume that DHCP traffic is handled differently in IOS 15.1 running on flat networks.

Is anyone aware of any new DHCP related features introduced with IOS 15.1 that could be causing this issue? Has anyone experienced this same problem and found a workaround?

Thanks in advance for any replies.

3 REPLIES
Cisco Employee

DHCP issues experienced since ISR upgraded to IOS 15.1.(3)

Hi Paulo,

Can you please describe the network and the config of the ISRs of the affected sites more precisely? What do you mean by a "flat" network - do they have only a single LAN/VLAN? I also assume that the ISR is configured as a DHCP Relay Agent - is this correct?

How exactly does the problem manifest itself? Is it possible to provide also some logging messages from devices or from the DHCP server itself?

Best regards,

Peter

New Member

DHCP issues experienced since ISR upgraded to IOS 15.1.(3)

Thanks for the reply Peter.

Correct. By "flat" network, I mean that each site only has a single LAN/VLAN.

The ISR is NOT configured as a DHCP Relay Agent as this was not necessary whilst it ran IOS 12.2(24), but I am currently searching docs about configuring this as a possible fix as I suspect this to be the problem or solution.

The problem manifested itself as follows -> After upgrading the ISR to IOS 15.1.(3) DHCP clients could no longer obtain IPs. Nodes with already assigned DHCP IPs continued to work fine unless these released or renewed the IPs in which case they would not be assigned an IP. If the admins try to manually assign a MAC OSX node for example, a free IP from the DHCP range a message is displayed stating that the IP is already in use when in reality it is free.

Unfortunately I do not have the required detailed information to facilitate troubleshooting this issue as I only have access to the ISRs’ but not to the respective DHCP servers or other nodes at these sites. The little information I have regarding the DHCP problem is what has been transmitted to me by the respective local admins.

I understand how this lack of information makes troubleshooting the problem rather ambiguous however I was simply hoping by creating this post that it would be spotted by someone who has experienced the same issue and be able to suggest a workaround e.g. switch off the DHCP service on the ISR. In the meantime as mentioned, I am searching info on configuring the DHCP relay option in the hope this will resolve the issue.

Thanks again for your timely response.

Paulo

Cisco Employee

DHCP issues experienced since ISR upgraded to IOS 15.1.(3)

Hello Paulo,

The ISR is NOT configured as a DHCP Relay Agent as this was not  necessary whilst it ran IOS 12.2(24), but I am currently searching docs  about configuring this as a possible fix as I suspect this to be the  problem or solution.

This is very interesting. If the ISRs were not configured as relay agents in their older IOS version then there must have been some other relay agent present in the network. Otherwise, if the DHCP server was outside that network, the clients would not be able to contact the DHCP server at all.

The manifestation of the problem seems to be rather obvious - there is  no communication between your clients and the DHCP server. A lack of DHCP relay service would be a natural explanation but as you suggest the ISRs were not configured for that, I am left puzzled as to how the DHCP could have worked before the upgrade.

I therefore recommend caution when configuring these ISRs as relay agents because if there really is a different device in those networks that works as a relay agent or even as a full DHCP server, the results may not be positive.

In any case, to configure your ISRs as DHCP relay agents, enter this command on their LAN interface:

interface FastEthernet0/0

ip helper-address X.X.X.X

where X.X.X.X is the IP address of the DHCP server (assuming Fa0/0 is the LAN interface here). Also make sure you can ping the DHCP server from your ISRs using the command

ping X.X.X.X source Fa0/0

Be sure to include the source interface as well - this will verify that the DHCP server is capable of responding back to your LAN. If the ping does not work then basic routing and reachability issues need to be verified before.

Best regards,

Peter

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