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VLAN Trunk port

Can anyone explain to me what is this config means?

interface GigabitEthernet0/4

switchport access vlan 912

switchport trunk allowed vlan 915-917

switchport mode trunk

no mdix auto

no cdp enable

spanning-tree portfast trunk

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Cisco Employee

Re: VLAN Trunk port

Hi Chris,

That configuration is a configuration of a trunk port.

The first command, "switchport access vlan 912", is currently ignored, because it applies only if the port is in access mode, which is not the case right now.

The second command defines which VLANs are transported over this trunk. Currently, only VLANs in the range from 915 to 917 are transported over it. Frames from any other VLANs are not allowed on it - they will never be sent out and they will be dropped if received.

The third command sets this port unconditionally to the trunking mode.

The fourth command, "no mdix auto", turns off the automatic crossover capability on this port. With this setting, you have always to use the proper type of cable (straight-through, crossover) when connecting to another device.

The fifth command turns off the Cisco Discovery Protocol on this port.

Lastly, the sixth command allows this port to rapidly transition to the Forwarding state in the Spanning Tree Protocol, even if it is in trunking mode.

I hope this helps.

Best regards,

Peter

1 REPLY
Cisco Employee

Re: VLAN Trunk port

Hi Chris,

That configuration is a configuration of a trunk port.

The first command, "switchport access vlan 912", is currently ignored, because it applies only if the port is in access mode, which is not the case right now.

The second command defines which VLANs are transported over this trunk. Currently, only VLANs in the range from 915 to 917 are transported over it. Frames from any other VLANs are not allowed on it - they will never be sent out and they will be dropped if received.

The third command sets this port unconditionally to the trunking mode.

The fourth command, "no mdix auto", turns off the automatic crossover capability on this port. With this setting, you have always to use the proper type of cable (straight-through, crossover) when connecting to another device.

The fifth command turns off the Cisco Discovery Protocol on this port.

Lastly, the sixth command allows this port to rapidly transition to the Forwarding state in the Spanning Tree Protocol, even if it is in trunking mode.

I hope this helps.

Best regards,

Peter

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