Why ISL or DOT1Q does not tag frames from native Vlans?

[saishreevj]

Why ISL or DOT1Q does not tag frames from native Vlans?

[Richard Burts · Accepted solution]

Perhaps a little background will help. When we trunk on an Ethernet interface we need a way to identify vlan membership. That is what tagging the Ethernet frame does - it identifies the vlan membership of that frame. But there is a possibility that some devices in the network may not understand vlan tagging. The basic purpose of the "native vlan" is to provide a vlan that does not tag its frames which provides compatibility for those devices that do not understand vlan tagging.

[Georg Pauwen · Accepted solution]

Hello,

in addition to Richard's remarks, a lot of 'control' traffic such as VTP traffic requires a native, untagged Vlan to be able to send packets across a trunk.

[Richard Burts]

I am glad that our explanations have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

[Joseph W. Doherty]

BTW, in addition to what the other posters have noted, I believe (?) other vendor dot1.Q trunks, at least by default, do not provide an untagged VLAN.

I suspect the reason for this difference, Cisco was an early VLAN provider (with ISL) and |expected some mixed hosts within the same VLAN (I recall Cisco trunks will also accept tagged frames for the native VLAN too).  Conversely, by the time other vendors started supporting VLANs, and dot1.q, the thinking was hosts on a VLAN tagged media should all understand the tags.  (Of course, a more recent example of having hosts on a media with both tagged and untagged frames is on an access port with a "voice" VLAN too.  On old Cisco switches that don't support "voice" VLANs, the port can be configured as a trunk with a native VLAN and just one tagged VLAN.))