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Frame tagging--when it occurs


Say we have a packet, Packet X. When does Packet X, in its LAN travels, actually acquire a frame tag on a Catalyst 2950 switch port that is set to statically assign a frame tag? I've heard of Ingress and Egress and understand the technologies--but which does the Catalyst 2950 use? Can a 2950 port be set to either Egress or Ingress tagging?

While on the subject of frame tags--how does a router and/or PIX deal with a frame tag? I think I remember reading that it strips off the tag, but that doesn't seem reasonable to me.

Thanks much!!!

3 Replies 3


What type of tagging are you talking about ? 802.1q, ISL, QoS ?


on a 802.1q trunk the 2950 will tag every packet leaving the trunk port with the dot1q header with the appropriate vlan id.. ( except on the native vlan )

This port will also accept 802.1q encapsulated frames. Non-encapsulated ethernet frames received on a trunk port are assumed to belong to the native vlan.

It is not a good idea to configure a trunk port and connect it to a non-trunking port. So if you configure a trunk port and the other side of the link is a router, make sure the router port is also configured as a trunk.

The 2950 abides with the 802.1q spec. So the behaviour of the 2950 is the same as all other currently selling Cisco switches.

I would strongly suggest that you review how 802.1q works. Once you understand the protocol, then you will only have to learn the commands to configure what you need.

Here are some references to get you started.

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