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New Member

Switchport nonegotiate and DTP frames

CCNP Switching 642-813
Official Certification Guide
978-1-58720-243-8

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I have a question with information provided on page 79-80. The following example is detailed for trunk port configuration.

(config )# interface gigabitethernet 2/1
(config-if)# switchport trunk encap dot1q
(config-if)# switchport trunk native vlan 100
(config-if)# switchport allowed vlan 100-105
(config-if)# switchprot mode dynamic desirable
 

I understand that the native vlan(untagged) traffic will be vlan 100. However, if the port negotiates to be used as a trunk due to DTP frames – vlans 100-105 will be tagged going over the trunk link.

But, what I do not understand –  “what happens if the port does not negotiate as a trunk link”? If the latter occurs, will the port come up as an ‘access’ port, part of VLAN 100 since it is ‘native’ being ‘untagged’ traffic? If this is the case, then the port is part of vlan 100 as an ‘access’ port, since vlan access ports are untagged?

Thank you
JJ

  • LAN Switching and Routing
3 REPLIES

If the the interface fails to

If the the interface fails to negotiate a trunk, the operational mode will be access and none of your trunk commands will apply. With this configuration, the interface will be on the default VLAN (1) if a trunk isn't negotiated. If you want to make sure that the port uses a native VLAN of 100 regardless of which mode is negotiated, you'll need to add "switchport access vlan 100" to your interface configuration so that VLAN 100 is selected either way.

New Member

Hi Jody,Thank you for the

Hi Jody,

Thank you for the details – you brought up a great point about the default vlan. What would be the best way to have the interface fallback to VLAN100 if the interface is not negotiated as a trunk?

 

What would the implications or impact be – if I assign the interface as ‘switchport access vlan 100’ and ‘trunk port’? Would this cause any problems?

 

Thank you

JJ

Using the "switchport access

Using the "switchport access vlan 100" will set the native VLAN if the interface doesn't negotiate as a trunk. If it does negotiate as a trunk, the command will have no effect at all.

If your interface is configured as follows:

interface gigabitethernet 2/1
 switchport access vlan 100
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 100
 switchport allowed vlan 100-105
 switchport mode dynamic desirable

The interface will have the following behaviour:

Trunk:

Encapsulation 802.1q, native VLAN 100, allowed VLANs 1,100-105. (You can never really get rid of 1 on a trunk.)

Access:

VLAN 100

The "switchport access" and "switchport trunk" commands don't define the mode of the interface (the "switchport mode" command does that), only how the interface will behave when it is in that mode.

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