I haven't thought through the STP part yet, but the first section I
understand. In ISL, *all* frames are tagged. However, in 802.1q, the
native VLAN is not tagged. The switch (or router) must assume that all
untagged frames belong to the native VLAN.
If Switch A has VLAN1 as its native VLAN, but Switch B has VLAN2, the
traffic from these two VLANs would be intermixed, causing all sorts of
mayhem and confusion.
This also creates in interesting router configuration for 802.1q trunk
links. The native VLAN information must be configured on the major
interface and the other VLANs must be placed on subinterfaces, unlike
ISL where all VLANs are on subinterfaces. Supposedly they've fixed this
issue so that you can place the native VLAN on a subinterface in recent
versions.
Regards,
John
>>> "Priscilla Oppenheimer" 7/23/01 1:04:42 PM
>>>
At 08:53 AM 7/23/01, Remmert Veen wrote:
>Hi Sammi,
>
>Indeed, ISL is Cisco propietary, so should you consider other
vendor's
>switches in your network, now or in the future, I'd recommend 802.1q.
Beware
>however, dot1q has some drwabacks with regards to loops. Check out
>http://www.cisco.com/univercd/cc/td/doc/product/lan/c2900xl/29_35wc/sc/swgvlans.htm#xtocid1196639
>for the details.
I'm trying to understand what that link is saying:
"Make sure the native VLAN for an 802.1Q trunk is the same on both ends
of
the trunk link. If the native VLAN on one end of the trunk is different
from the native VLAN on the other end, spanning-tree loops might
result."
What's a native VLAN? Is that the one that sends STP, VTP, etc.? How is
it
different from the management VLAN?
If the native VLAN were set differently on the two ends of a trunk, I
guess
STP traffic wouldn't flow correctly, so loops could result? Why
wouldn't
this also be a problem for ISL?
The link also says this:
"Disabling STP on the native VLAN of an 802.1Q trunk without disabling
STP
on every VLAN in the network can potentially cause STP loops. We
recommend
that you leave STP enabled on the native VLAN of an 802.1Q trunk or
disable
STP on every VLAN in the network. Make sure your network is loop-free
before disabling STP."
That one seems sort of obvious. Disabling STP on any trunk could result
in
problems, I would think. What are they getting at?
Thanks,
Priscilla
>If not (so your network is all Cisco) you might wanna consider ISL.
Since it
>is Cisco propietary, it's obviously fully supported by Cisco and has
some
>minor benefits.
>
>Hth,
>Remmert
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=13419&t=13325
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