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Cisco Switch Only allowing One trunk native vlan

Posted on 2014-08-04
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Last Modified: 2014-08-06
As per guidance from Network admin at head office ,
At my site office on a Cisco Switch ( which is connected to head office Core switch ),
i configured a  port to be Native vlan trunk port, to connect and use a "Cisco Access Point"  on that port.

Everything was fine,  until i configured another port on same switch,  to be " Native vlan trunk port " to use another access point.

Well, when i done the same , old port came in to default vlan (vlan1)...!!

and the port on which i did the configuration , came in to native vlan, and now Access point is not working in old port but is working in the last port i have configured

Dear all IT Geek....What does that mean...?
Does it mean that in Switch only one" native vlan trunk port" will be allowed....?
or if no....is there any mistake i have done...? like below is the steps i followed

Cisco SW(config)#default interface f0/22
Interface FastEthernet0/22 set to default configuration
Cisco SW(config-if)# switchport mode trunk
Cisco SW(config-if)# switchport  trunk native vlan 505
Cisco SW(config-if)# shutdown
Cisco SW(config-if)# no shut down

also please indicate,  if i have to ask or inform Network admin at head office  to do something from their end,  to make work my switches with multiple ports in "  native vlan trunk port "  mode ,  so that i can use  multiple access point on same switch.
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Question by:FiruIT
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LVL 28

Expert Comment

by:mikebernhardt
ID: 40239579
Please post the output of "show version" and also the current running configuration of both ports- "show runn interface f0/22" and "show runn interface f0/[the other port]"
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Expert Comment

by:max_the_king
ID: 40240712
Hi Firuit,
you need to get the difference between native vlan and default vlan, as they are different concepts.
you may want to read the following:
https://supportforums.cisco.com/discussion/11700441/what-difference-between-default-vlan-and-native-vlan

basically, the native vlan will go untagged on trunk, which is what you need for the access point to work.

hope this helps
max
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Author Comment

by:FiruIT
ID: 40240814
thanks Max that was good guidance ,
Dear all,
Please testify if my below conclusion is correct or wrong ,
 (if its wrong please provide me hint , i will google and will find where i  went wrong)

" IF we configured "native vlan trunk mode"  on a Cisco Switch Port for a Wireless AP to function and latter if we disconnect AP from that Port  ....  , then that port will start appearing under  Vlan 1   when we run
 "Show Vlan Br"    and so will not appear in Native vlan , that we see when we run     "show int trunk"
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Assisted Solution

by:max_the_king
max_the_king earned 705 total points
ID: 40240831
Hi,
Native VLAN ID numbers must match on both ends of the trunk.
A trunk port can carry untagged packets simultaneously with the 802.1Q tagged packets. When you assign a default port VLAN ID to the trunk port, all untagged traffic travels on the default port VLAN ID for the trunk port, and all untagged traffic is assumed to belong to this VLAN. This VLAN is referred to as the native VLAN ID for a trunk port. The native VLAN ID is the VLAN that carries untagged traffic on trunk ports.
The trunk port sends an egressing packet with a VLAN that is equal to the default port VLAN ID as untagged; all the other egressing packets are tagged by the trunk port. If you do not configure a native VLAN ID, the trunk port uses the default VLAN.
In your specific case, you seem to want to configure a native VLAN ID different from default (the one used by access point) and that will not mess with default VLAN. However you must match the native vLAN id you chose on the other side of the trunk as well.

Further recommended reading is the following:
http://www.networkworld.com/article/2234512/cisco-subnet/tagging-the-native-vlan.html

max
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Accepted Solution

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mikebernhardt earned 705 total points
ID: 40241800
In answer to your question, you are correct: If you disconnect the AP from the trunk port, the port will look exactly as you described. An unconnected trunk port shows vlan 1 (which is, as Max said, the default access vlan, not the native vlan) and it will not show up in "show int trunk"

So if that was your original basis for thinking something was wrong, don't worry about it. It's working as it should. If the AP was working and suddenly stopped when you configured a 2nd trunk port, that's a different story.
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Author Closing Comment

by:FiruIT
ID: 40243076
Dear max_the_king

your guidance was very nice  and helpful , thanks a lot.

Dear mikebernhardt ,

thanks very much for answering specifically to my doubt it helped me a alot.
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