Access/Hybrid/Trunk 端口模式及区别
2010-10-27
在工作和交流中，总会有人为Hybrid和Trunk的概念和区别争吵，前几天面试的时候还和面试官争论了起来， 现做一个总结和概念涔清。
首先看下 IEEE-820.1Q-1998中 Annex D： Backgroud of Vlan 中的定义，
Trunk Links
All frames, including end station frames, on a Trunk Link are VLAN-tagged.
Access Links
All frames on an Access Link carry no VLAN identification;
Hybrid Links
A Hybrid Link can carry both VLAN-tagged frames and other (untagged or priority-
tagged) frames. It must be borne in mind that, for a given VLAN, all frames transmitted by a given
Bridge on a given hybrid link must be tagged the same way on that link. They must be either
a) All untagged; or
b) All tagged, carrying the same VLAN ID.
All, A Hybrid link in which all frames are VLAN-tagged is a Trunk Link. Conversely, a Hybrid Link that has no
VLAN-tagged frames is an Access Link. The distinction between Access and Trunk becomes less important
也就是说如IEEE-802.1Q 1998-2003中定义的， Trunk端口只能接收和发送带Tag报文； Access只能接收和发送Untag报文；Hybrid可以同时处理Tag和Untag报文；
但是， 随着市场和各芯片厂家的实现区别， 有些在Trunk上也加入了Pvid的概念， 所以在2005版的IEEE-802.1Q时， 放弃了对端口模式的严格定义， 如下原文，
Annex D
(informative)
Background to VLANs
In IEEE Std 802.1Q, 2004 Edition, and in previous revisions of this standard, the text in this annex contained
tutorial material that related the terminology and concepts introduced IEEE Std 802.1Q to the terminology
and concepts that were current in the VLAN market at that time. As that material is now of only historical
interest, and has no particular relevance to present-day usage, it has been removed.
This annex will be removed in the next revision of this standard.
所以， 争论Trunk和Hybrid区别已经没有意义， 标准已经放弃了定义， 各厂家实现也不一样， 但是要明白其中的处理流程才是最重要的。