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Cisco switch trunking commands

Posted on 2010-09-08
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Last Modified: 2012-05-10
If I set the "switchport mode trunk" command on an interface, but don't specify the "switchport trunk allowed vlan X,X" command as well, what vlans are allowed through?  Also, if this same interface has the command set on it "switchport access vlan 2", does that do anything or is it a non factor since the interface is set to trunk mode?
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Question by:jpletcher1
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rfc1180 earned 168 total points
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>but don't specify the "switchport trunk allowed vlan X,X" command as well, what vlans are allowed through?

all of them are allowed

> Also, if this same interface has the command set on it "switchport access vlan 2", does that do anything or is it a non factor since the interface is set to trunk mode?

non-factor
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by:Gunter17
Gunter17 earned 168 total points
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By default a trunk link carries all VLANs that exist on the switch. This is because all VLANs are active on a trunk link; and as long as the VLAN is in the switch's local database, traffic for that VLAN is carried across the trunks.

On a Cisco switch, ports are assigned to a single VLAN. These ports are referred to as access ports and provide a connection for end users or node devices, such as a router or server.
...
For the IOS device, you must first select the port (or port range for integrated IOS) and then use the switchport access vlan command followed by the VLAN number.

This is all from ciscopress.com.

To answer your question outright:
1. If a switchport trunk allowed vlan is not specified and the port is in trunk mode, the port belongs to all VLANs.
2. If the switchport access vlan 2 command is given, but the port is in trunk mode the command will be ignored.

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by:Jimmy Larsson, CISSP, CEH
Jimmy Larsson, CISSP, CEH earned 164 total points
ID: 33629672
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by:jpletcher1
ID: 33629695
Thanks guys, this was the info I was looking for.
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