Trying to understand why VLANs need to be created on intermediate switches

Posted by Jon Reeves on Server Fault See other posts from Server Fault or by Jon Reeves
Published on 2011-01-15T17:27:35Z Indexed on 2011/01/15 17:55 UTC
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I'm currently studying for the Cisco switching exam and having trouble understanding exactly how 802.1q tagging works.

Given three daisy chained switches (A,B, and C) with trunk ports between them and VLAN 101 defined on both end switches (A and C), I'm not sure why the VLAN also needs to be defined on the middle one (B)?

Note that I am not disputing that it does need to be configured, I'm just trying to understand why exactly.

As I understand it, traffic from VLAN 101 on switch A will be tagged as it goes through the trunk to switch B. According to the documentation I have read, trunks will pass all VLANs by default, and the .1q tag is only removed when the frame leaves through an access port on the relevant VLAN. From this I would expect switch B to simply forward the tagged frame unchanged through the trunk to switch C.

Can anyone shed some light on how switch B processes this frame and why it does not get forwarded through the other trunk ?

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