Trunk and access VLANs

Unanswered Question
Oct 25th, 2009

Hi,

It may sound very basic. But most of Engineers in my surrounding unable to clarify this. I tried in real switches also but not able to find the reason

Trunk and access VLANs in the same Port

INT FA 0/0

SWITCH MODE TRUNK

SWITCH ACCESS VLAN 100

SWITCH TRUNK ENCAPSULATION 802.1Q

Here what is the meaning and purose of giving SWITCH ACCESS VLAN 100 command in the TRUNK link. I think it is no way going to limit the number to allowed VLANs etc.

Please share your ideas.

sairam

I have this problem too.
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glen.grant Sun, 10/25/2009 - 12:28

Switchport access vlan 100 is doing nothing in that config and does not need to be there . The state of the port is determined by the switchport mode trunk command . If you wanted it as access then you would use switchport mode access. You can also use switchport mode dynamic desirable which means if the far end is auto or desirable it will become a trunk if it is not then it would be a access port , the vlan determined by the switchport access vlan Xx " command . In the command that you show all vlans are allowed on the trunk . to restrict the vlans across the trunk you need "switchport trunk allowed vlan xx,xx,xx , where xx is the vlans you want across the trunk.

Edison Ortiz Sun, 10/25/2009 - 16:15

Sairam,

Agree with Glen 100%.

Another misconception is assuming Vlan 100 serves are the native vlan on the command above. On the trunking mode, the native vlan is declared with the 'switchport trunk native vlan' command and based on the output above, the native vlan is '1'.

Regards

Edison.

trdheeraj Tue, 10/27/2009 - 17:53

Sairam,

When you configure a switchport as trunk,it can carry a tagged packet and untagged packet simultaneously.If the switch receives a untagged packet on trunk port,it forwards it to native or VLAN 1 of the switch. If you want to change this to Vlan 100,then use command #switchport trunk native vlan 100.

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