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2582 Views 8 Replies Latest reply: Mar 1, 2011 11:06 AM by Jeroen RSS

Currently Being Moderated

Enable TX force untag

Feb 28, 2011 8:57 PM

Jeroen 5 posts since
Feb 28, 2011

Hi everyone,

 

 

I'm looking for opinions about what this option does. This pertains a Cisco smart switch and hence the dubious terminology. Please refer to a full description at the bottom of my post.


"When this option is enabled, all egress frames from this port become untagged"

 

Egress? As in, a host sends a tagged frame to the ingress port of the switch, and when it egresses from that switch port it will become untagged?

 

Or

 

Egress, as in, when a switch receives a tagged frame on a port (ingress), it will remove the tag before it sends the frame to the host (egress).

 

 

 

A) Suppose the option is enabled:

 

Possible answer 1:

 

it is an option that prevents a host from ever sending tagged frames.

 

Possible answer 2:

 

it is an option that prevents a host from ever receiving tagged frames.

 

 

 

B) Suppose the option is disabled:

 

Possible answer 1:

 

it is an option that allows a host to send tagged frames (except frames tagged with a VLAN ID equal to the PVID)

 

Possible answer 2:

 

It is an option that allows a host to receive tagged frames (except frames tagged with a VLAN ID equal to the PVID)

 

Many thanks for your thoughts!

 

 

Enable Tx Force Untag 

When this option is enabled, all egress frames from this port become untagged. The default value is Disable. When this function is disabled, only frames with the VLAN ID equal to the PVID will become untagged, otherwise, frames are sent with a VLAN tag.

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    1. Feb 28, 2011 9:22 PM (in response to Jeroen)
    Re: Enable TX force untag

    Back in 2001, when I went to my first CCIE lab at building C in San Jose, I ate at the cafeteria.

     

    There is a location there, in the cafeteria where we can return the trays, plates, glasses, flatware, etc.   At that location, there were two openings, one said ENTER and the other said EXIT.

     

    Enter, with regards to that opening, means ingress.    Exit, with regards to the other opening, means egress.

     

    Egress are the bits leaving the switchport, heading towards the wire.    Ingress are bits coming in from the wire towards the port.

     

    The feature you are refering to will NOT tag outbound frames (from the switch perspective).

     

    Best wishes,

     

    Keith

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    3. Mar 1, 2011 6:15 AM (in response to Jeroen)
    Re: Enable TX force untag

    Hello Jeroen.

     

    Normally, egress frames (from the switch perspective) will include the VLAN Tag in the 802.1q header, as the switch places the frame on the wire.

     

    When the switch has a frame that belongs to the native VLAN, it will not tag the frame, but rather simply send it without a tag, and the other side (using the same native VLAN) would understand that the untagged frame belongs to the native VLAN.

     

    Now for your command.

     

    When we use the command Enable Tx force untag, based on the description given for it, a switch will NOT tag any frames, but rather just send them, very similar to the way it sends frames for the native VLAN.

     

    And now for the why.   It was probably a corner case, where someone had a need, and Cisco provided the feature.   Such as a protocol analyzer that didn't have the ability (decades ago) to understand a dot1q header, or something similar, so they wanted the egress switch to not include them.

     

    In normal operations, without the tags, dot1q would be very disfunctional to the downstream switch that receives all the untagged frames, as it would believe they all belong to the native VLAN.

     

    Thanks for the interesting question.   I am happy to supply my opinion.   I am also open to any other input people may have regarding how they would use the untagged option. 

     

    Best wishes,

     

    Keith.

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    5. Mar 1, 2011 8:24 AM (in response to Jeroen)
    Re: Enable TX force untag

    Yes, frames sent egress on access ports are not sent with tags, so this command would only have an effect on 802.1q tagged frames.

     

    What is your application for this?  Do you have a specific function you want to set up?   That may be a better approach to this.

     

     

     

    Keith

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    7. Mar 1, 2011 9:17 AM (in response to Jeroen)
    Re: Enable TX force untag

    You don't want to route to your management VLAN, with some access-lists that control who comes in and out of the management VLAN?

     

    Keith

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