Incorrect DHCP Helper Address

[JustinGayheart]

hello everyone,

i have a vlan that i have been fighting with to get DHCP working. 

i finally noticed the IP DHCP RELAY ADDRESS is incorrect in the VLAN config.

interface Vlan162
description SFC-Production
no shutdown
no ip redirects
ip address 172.16.2.251/24
no ipv6 redirects
hsrp version 2
hsrp 162
name VLAN-162_HSRPv2
preempt
priority 120
ip 172.16.2.1
ip dhcp relay address 128.1.11.201 <------- Address Does not exist, i believe it was meant to be 128.1.1.201

This particular VLAN has only been used for Wireless clients before, but I want to use it on switches now as well.

SO I have a question around this:

If I change the ip dhcp relay address 128.1.11.201 will that drop or disconnect any existing clients?

This was all setup before my time in my role, so I am trying to straighten things out as best as I can. 

[MHM Cisco World]

follow

[Karsten Iwen]

The change will only affect the next clients asking for a DHCP address. Be aware that changing involves the adding of the new address *and* removing the old one as it is allowed to have multiple relay-addresses.

[JustinGayheart]

Karsten,

Thank you so much for your answer... Part of my fixing things involves not breaking what is already here!

thank you

[Richard Burts]

I appreciate your concern about "not breaking what is already here!" and along that line I am thinking about the question that you asked:

will that drop or disconnect any existing clients?

If the dhcp relay address is incorrect then how could there be clients that would be impacted?

Or if there are clients that would be impacted does that suggest that the relay address was not incorrect?

I am also thinking about this statement "This particular VLAN has only been used for Wireless clients before" If it was used for Wireless clients, then did these clients dynamically learn IP addresses? How did they do this? Did this vlan work ok for the wireless clients?

We really need a better understanding of this environment. 

[JustinGayheart · Accepted solution]

Hi Richard,

I think they are working wirelessly as the Wireless LAN Controller has a DHCP Server override on it, which is pointing to the proper server.

The Vlan interface on the Cisco Nexus Switch has the wrong IP DHCP HELPER listed.

I have attached a config from both... maybe if you looked at it you could confirm that is correct?

[Richard Burts · Accepted solution]

Thanks for the additional information. The explanation that the Wireless LAN Controller has a DHCP Server is helpful. I have looked at the configs that you posted and am more comfortable with the suggestion that the current configuration of the switch vlan interface has a typo. You should go ahead with removing the current line and replacing it with a statement that has the correct address.

[JustinGayheart]

Ok great! And just to be double/paranoid sure... none of the existing clients on the Wifi should notice anything right?

[Richard Burts]

I sympathize with wanting to be double/paranoid sure. I am confident that changing the incorrect line in the vlan interface config will have no impact on existing wifi clients.

 Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.