Cisco Wireless & DHCP

Posted by robd on December 02, 2020
Cisco, Wireless

Had a very frustraiting issue recently where our Zebra RF Scanners werent getting DHCP addresses on certain Cisco Access Points.

Only the scanners were not working, everything else seemed fine!

So I checked a heap of things:

Data Rates

Some of RF scanners are OLD, so its important to find out what data rates they require and then match your RF profile.

I suggest you profile the scanner using sometime like a WLANPi first just so you dont have to enable any older data rates.

Or use:

Read more about old data rates here.

Port Config

We run FlexConennect so was every port in a Trunk and did every port have have the correct vlans tags?

Trunk

Where all the vlans trunked up to the core switch?

DHCP Server

Rebooted it and everything seems fine, lots of DHCP requests from other devices etc.

To be sure I did run wireshark and there were no requests from the scanners while on the “broken” APs.

Debug, Debug, Debug

I then started these debugs and waited forced the client to join again:

So the results showthis:

When it works it looks like this:

DHCP request,

DOT11 Auth

DOT11 Association

ARP

DHCP Request

DHCP ACK

When it doesnt, everything looks good until the end, no ACK from DHCP:

So what does this tell us? The DHCP requests are not getting to the DHCP server.

The Fix

So based on the above, I doubled checked the switches.

Trunks and ports were fine BUT I had missed something!!

Showed me I hadnt actually added the sodding vlan on the switch…… 🙁

Why did other devices work?

Well we use one SSID and Cisco ISE moves RF scanners to a different vlan when they’ve authed. Other devices dont use our special RF scanners VLAN.

The Lesson

Its never Wireless, its always something else!

 

Tags: , , ,

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Copy Protected by Chetan's WP-Copyprotect.