RE: Switching Advice

From: Jim Warner (warner@cats.UCSC.EDU)
Date: Sat Dec 29 2001 - 01:57:43 EST


Nimesh asked:

> yes but I guess the question is why? What is the function of the Native
> Vlan that requires it to be not dot1q encapsulated. Does Cisco just opt
> for it or is there a logical reasoning behind it... spanning tree seems to
> ring a bell but am not sure what about spanning tree requries it to be
> dot1q encapsulation.

Yes. Spanning tree is it. The IEEE spec permits a single unencapsulated
instance of spanning tree to protect the topology against loops. The
STP packets are _not_ encapsulated. They run on the the unencapsulated
vlan. There are a few other control packets that must run on the
unencapsulated lan. Half/full duplex negotiation cannot be encapsulated.
802.1x port athentication must also run native. I think flow control
must also run on the default vlan -- if you believe in L2 flow control.



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