I found the answer in the cisco site
http://www.cisco.com/warp/public/473/27.html
"Cisco developed PVST+ to allow running several STP instances (even over a
802.1q network) by using a tunneling mechanism."
************************
CCSI:21130
XuJing
************************
""CCSI"" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> I use 2 c4003 and 1 2924-xl create a lab just like the rack in bcmsn
course.
> I set the trunkand vtp to 802.1q. I use the "set spantree root " command
> make the first 4003 become the root bridge for vlan 1&2, use the same
> command make the second 4003 become the root brigde for vlan 3. I found
it's
> successful. But in the BCMSN book, it says:
> The IEEE 802.1q standard uses an approach to vlans referred to as MONO or
> Common spanning tree. The STP runs on vlan1 , which is the default vlan.
all
> switch elect a single root bridge and form a association with that root
> bridge.
>
> So I cannot explain why the system can display different root bridge
> election on vlan 1&2 and 3.
>
> who can give me the explaination?
>
> I think after v4.1, the switches is PVST+, so internal is ISL and external
> (on the trunk) is 802.1q.
>
> 2924xl
> +
> + +
> + +
> + +
> 4003-1++++4003-2
>
>
> Thanks for your help!!!
>
>
>
> ************************
> CCSI:21130
> XuJing
> ************************
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http://www.groupstudy.com/form/read.php?f=7&i=7324&t=7299
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