Computers flicker to receive IP address dynamically after switching office switches.

[Rafael Santos]

Dear,

I have an office where the phones are IP and receive address through a Voice VLAN and the computers are in vlan default 1.

Users' computers and IP phones receive IP dynamically through a DHCP server located in the Array.

We switched the switches from this WS-C2960-24-S to the WS-C2960-24PC-L, maintaining exactly the same configuration and position of the cables on the ports.

Since switching the switches some local computers are having difficulty receiving the IP address of the DHCP Server and for these computers to receive an IP address we have to remove the network cable from the computer and connect again or else apply the release renew command so that the same get IP address dynamically.

No changes were made to the DHCP servers.

Could you help me with this problem? The problem is intermittent, time does not happen. The network cabling of these micros has also been certified.

Thankful,

Rafael Santos

[Deepak Kumar]

Hi,

Run a command on all of the Ports which are connected to Hosts or Phones.

"Spanning-tree portfast"

It will resolve your issue.

Regards,

Deepak Kumar

[Rafael Santos]

Hello,

Spanning-tree portfast is already applied to the interfaces.

The strange thing is that I noticed that the micros that present this problem, are connected to IP telephones that are working correctly, however the computer door is totally erased. Only when I remove the cable and connect again does it activate and the micro picks up the ip address.

Here is the interface configuration of one of the problem computers:

interface FastEthernet0 / 12
  description ## CONNECTION WITH DESKTOP / VOIP / NOTEBOOK ##
  switchport voice vlan 4
  srr-queue bandwidth share 10 10 60 20
  priority-queue out
  qs trust device cisco-phone
  mls qos trust cos
  auto qos voip cisco-phone
  spanning-tree portfast
  spanning-tree bpduguard enable
  service-policy input AutoQoS-Police-CiscoPhone

[georgehewittuk1]

You should be statically setting port as an access and the data vlan tag.

Can you run a show interfaces ----- switchport for this one?

[Deepak Kumar · Accepted solution]

Hi,
Add a Command "Switchport mode access" and remove the bpuguard enable command for a testing purpose.

Regards,
Deepak Kumar

[Rafael Santos]

Hello Deepak kumar,

I added the switchport mode access command on the interfaces that connect the deskotps.

I found a problem in the infrastructure where two servers were DHCP sources from the same range of IP addresses as the local IP phones. As a precaution and following best practices, I deactivated one of these sources and left only one.

I will follow up for a few days and check if the problem will appear again. I will inform you the result here.

Thank you,
Rafael Santos

[Rafael Santos]

Hello,

After a few weeks since I set the switchport access vlan parameter I no longer have the problem of receiving DHCP on the stations. I do not understand how this could be influencing the problem, since even if the parameter was not set the interfaces in question were using the VLAN default, that is, VLAN 1.
Anyway the problem has been solved and thank you for the help.

Thank you,
Rafael Santos

[Georg Pauwen]

Hello,

on a side note, check to make sure you know which switch is the root in your network, using the command 'show spanning-tree vlan 1'.

The root should only have designated ports and typically is located in the logical center of your network. When you add new switches, one of them might inadvertently have become the root..

[pieterh]

the new switch added PoE negotiation to the ports

together the negotiation of PoE / speed+duplex / access-vs-trunk, takes too long for the DHCP process on the micros.

the micro may stop sending DHCP packets before the port forwards data-packets

at least until a timer expires then it should restart DHCP negotiation, but this timer can be longer than you desire.

this adds to the intermittent behavior of the problem.

I agree with Deepak to add "switchport mode access" to the interface configuration

you already have "spanning-tree portfast enabled", but this only works after the switch has detected to operate as an access port, if you manually configure as acces port you speed up the process for port-activation.

other improvement may be gained by

- updating network drivers on these micros

- checking if speed/duplex is fixed (and match the setting fixed/auto with the switchport)

- disabling PoE ("power inline never") on these ports

- last resort: fixed ip-adresses on the micros.