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Cisco IP Phone 7931 DHCP release

Posted on 2011-02-22
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Last Modified: 2012-05-11
Hello everybody!

 

We've got 7931 Ip phones which release their DHCP for voice VLAN and start getting addresses from access VLAN. Other types of phones work fine. I've configured SPAN to track the packets coming from the phone, and I see that IP phone starts sending DHCP requests all of a sudden.

 

As far as know, IP phones receive voice vlan information through CDP, may be it has anything to do with CDP exchange between the phone and the switch.

 

Thank you in advance!
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Question by:fgasimzade
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by:Ernie Beek
ID: 34950847
Is there anything different in the configuration of the switchports to which those phones are connected?
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by:fgasimzade
ID: 34951181
No, nothing, I just found its a bug CSCsu70484
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by:Ernie Beek
ID: 34951196
Always great when things get buggy :-~

Anyway, glad you found it.
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by:fgasimzade
ID: 34951273
Ooops, sorry, that bug is for 7937 Ip Phones..
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by:fgasimzade
ID: 34958436
The issue is still persists.. Not only 7931, 7911 and 7965 act the same way.. They get IP from data vlan and then change it to voice vlan
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Ernie Beek earned 500 total points
ID: 34958877
Ok, allthough it's not a (documented) bug did you try upgrading to the newest firmware? Or do you allready that.
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by:fgasimzade
ID: 34959775
Yes, I have upgraded the firmware on a number of devices, looks like it works fine now. Will watch these devices for a couple of days.. However, I noticed the same behavior for 7911 phones on different switch (so it is not a switch issue) but on the same DHCP server. Can it has anything to do with DHCP server?
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by:Ernie Beek
ID: 34960196
Could be. Is the DHCP server serving both scopes (voice and access)?
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by:fgasimzade
ID: 34960434
Yes.

Another thing I noticed is that Ip Phones occasionally obtain IP addresses from data VLAN and then immediately  from voice vlan. But they dont release data vlan ip addresses since I can see their mac addresses in data vlan pool on the DHCP server

Can you somehow explain me ip address obtaining process for Ip Phones. Do they initially receive data vlan ip and change it to voice vlan when they receive CDP from the switch?
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by:Ernie Beek
ID: 34961229
Could you show me an example of how a switchport (for a phone) is configured?
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by:fgasimzade
ID: 34961393
yeah, sure

interface FastEthernet1/0/5
 switchport access vlan 2
 switchport mode access
 switchport voice vlan 160
 srr-queue bandwidth share 10 10 60 20
 srr-queue bandwidth shape  10  0  0  0
 mls qos trust device cisco-phone
 mls qos trust cos
 auto qos voip cisco-phone
 spanning-tree portfast
end

There was no config change before this issue arised
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by:Ernie Beek
ID: 34968416
Ok, just checking.

So it still looks like a server issue.

About the CDP:

When a Cisco phone is powered up, you will see the mute, headset and speakerphone indicators light up. Next, the phone and switch exchange Cisco Discovery Protocol (CDP) data. CDP is a Cisco proprietary protocol that lets interconnected Cisco devices learn about each other. The switch uses CDP to tell the phone which VLAN will be used for voice traffic. (Note: if you are not using separate VLANs for voice and data traffic, we need to talk!) During this process the phone will display "Configuring VLAN."

Once the phone knows which VLAN to use, it will tag all of its data frames with the appropriate 802.1q tag and request an address from the DHCP server. The phone will display "Configuring IP." The DHCP server will offer an IP address as well as the address of the TFTP server.


Got this from: http://www.enterprisenetworksandservers.com/monthly/art.php?1158

So before anything else the phone is told which vlan to use. That should mean that the DHCP server is giving out addresses from the wrong scope (or both as you told).

I can remember a similar case. There the DHCP server also gave out addresses in the wrong scope. If I remember correctly, the issue there was solved by giving the switches voice vlan interfaces an ip address as well (in the voice range but outside the scope ofcours). That way the DHCP server was able to see what scope to use.
Perhaps that's something to look at.
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by:fgasimzade
ID: 34968745
Thank you for your response!

We have IP address configured on voice vlan interface.

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by:Ernie Beek
ID: 34969243
Just so I don't misunderstand:

You allready had the vlan interface configured with an ip address on every switch?

Like
Interface vlan160
   ip address x.x.x.x
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by:fgasimzade
ID: 34969601
No, no, just on the switch connected to our DHCP server. Voice vlan interface with ip helper-address configured
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by:Ernie Beek
ID: 34970018
Well, if you have some addresses left on the voice vlan it might be an idea to implement that on all the switches.
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by:fgasimzade
ID: 34970710
The thing is that IP phones with this behavior are connected to the switch DHCP servers is connected to
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by:Ernie Beek
ID: 35083234
Ehr, wasn't that what I suggested along the way (ID: 34958877)?
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