Problem with trunk port that loses config

Answered Question
Apr 7th, 2010
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Hello,


I have two switches which I have linked together with a Trunk port.


On both switches i have putted several ports in a vlan. On each switch I have put a device in the vlan and they can ping each other.

Only after some time (approx 2-3 minutes) I get a request timed out.


After some search I have noticed that on the 2948 switch, the port deletes it's trunk configuration.


This is the configuration when I have enabled the trunk with the command


set trunk 2/1 desirable dot1q



c2948_01> (enable) show trunk

Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
2/1   desirable    dot1q       trunking      1
2/49     desirable    dot1q          trunking      1
2/50     desirable    dot1q          trunking      1

Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
2/1      1-1005
2/49     1-1005
2/50     1-1005

Port      Vlans allowed and active in management domain
--------  ---------------------------------------------------------------------
2/1      1-5,10,20
2/49     1-5,10,20
2/50     1-5,10,20

Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
2/1      1-5,10,20
2/49     1-5,10,20
2/50     1-5,10,20
c2948_01> (enable)


after the 2 - 3 minutes when I do a new show trunk I get this

even after a write terminal, it loses it's trunk config and the port get's it's original configuration back


c2948_01> (enable) show trunk

Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
2/49     desirable    dot1q          trunking      1
2/50     desirable    dot1q          trunking      1

Port      Vlans allowed on trunk
--------  ---------------------------------------------------------------------
2/49     1-1005
2/50     1-1005

Port      Vlans allowed and active in management domain
--------  ---------------------------------------------------------------------
2/49     1-5,10,20
2/50     1-5,10,20

Port      Vlans in spanning tree forwarding state and not pruned
--------  ---------------------------------------------------------------------
2/49     1-5,10,20
2/50     1-5,10,20
c2948_01> (enable)


any idea what might be wrong?


thank you

Correct Answer by glen.grant about 7 years 2 months ago

  The switch is not losing the config , all that display is telling you is that port is no longer trunking whether it be a connection issue or a config issue .  That being said  if the 2948 is setup as desirable then the 2960 port  needs to be setup as "switchport mode dynamic desirable "  , it should not be hardcoded on unless you are going to hardcode both sides as on .  I would check to see if the link itself is down when  2/1  disappears with a "show port 2" command on the 2948. To me it sounds like you have a bouncing link.  Also on the 2960 you should not have spanning tree portfast trunk on that connecting link .

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Tulsi Ram Puttaraju Wed, 04/07/2010 - 12:53
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Check the status of your VTP configuration, sh vtp status  and let us know the sh ver of both switches.


May be one of your switches is not accepting the configs of the other switch.

yogesh.suryawanshi Wed, 04/07/2010 - 21:09
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Hi,


Convert this switch in to vtp transparent mode first.

Then do the required configuration & check


Regards

Yogesh

jjalexander401 Wed, 04/07/2010 - 19:53
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I've always considered it best practice to not put ports in desirable mode.  I like to clearly "state" what it is that i want them to do.  Try them all with switchport mode trunk instead.


What are your ports 49 and 50?

redsnapperke Wed, 04/07/2010 - 21:30
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Switch B's ports 49 & 50 are fibre uplinks who go to another side of the plant to switch C.


Switch A is a switch without fibre uplinks, on this switch we have configured several vlans and the idea is that devices in vlan x can communicate through the trunk via switch B with switch C



attached you can find the putty.log of switch B with the fiber uplinks. Don't have the possibility to log on remotely to switch A.


Switch A is a catalyst 2960 with this configuration on the trunk port


switchport trunk encapsulation dot1q

switchport mode trunk

switchport trunk allowed vlan x,y,z

spanning-tree portfast trunk

Attachment: 
redsnapperke Thu, 04/08/2010 - 11:33
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Attached is the config & version of both switches.


On both switches I have put port 1 in a Trunk to make communication between them possible.

Except on switch A it looses it's trunk config.


VTP is on both server & VTP version is 2

they are both the same vtp domain

vlan database is the same


switch A has two fiber trunks to another switch C port 49 & 50

Tulsi Ram Puttaraju Thu, 04/08/2010 - 11:43
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Make the switch B as transparent and edit the configuration as required it wont loose the configs. Here both the switches are becoming server and deleting the other switch configs.

Correct Answer
glen.grant Thu, 04/08/2010 - 12:50
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  The switch is not losing the config , all that display is telling you is that port is no longer trunking whether it be a connection issue or a config issue .  That being said  if the 2948 is setup as desirable then the 2960 port  needs to be setup as "switchport mode dynamic desirable "  , it should not be hardcoded on unless you are going to hardcode both sides as on .  I would check to see if the link itself is down when  2/1  disappears with a "show port 2" command on the 2948. To me it sounds like you have a bouncing link.  Also on the 2960 you should not have spanning tree portfast trunk on that connecting link .

redsnapperke Mon, 04/12/2010 - 05:14
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changed on the 2960 the port to dynamic disarable and disable spanning tree port fast


it's working now

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