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Re: Does ISL Support...........? posted 11/13/2006
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cheers, mate! :-)

Venkatesh Venkatesh wrote:
All frames should be tagged in ISL bit not the case is 802.1Q


"In an ISL trunk port, all received packets are expected to be encapsulated
with an ISL header, and all transmitted packets are sent with an ISL header.
Native (non-tagged) frames received from an ISL trunk port are dropped.

An IEEE 802.1Q trunk port supports simultaneous tagged and untagged
traffic. An 802.1Q trunk port is assigned a default Port VLAN ID (PVID), and
all untagged traffic travels on the port default PVID. All untagged traffic
and tagged traffic with a NULL VLAN ID are assumed to belong to the port
default PVID. A packet with a VLAN ID equal to the outgoing port default
PVID is sent untagged. All other traffic is sent with a VLAN tag.

- Venkatesh

On 11/13/06, Daniel_Steyn@xxxxxxxx <Daniel_Steyn@xxxxxxxx> wrote: does LET you configure a native vlan on ISL.  Having said
that, ISL will ALWAYS tag egress traffic - even the "native" vlan.  My
belief is that (correct me if I am wrong, please) the native VLAN will
mark INGRESS UNTAGGED traffic with the specified native vlan value.  I
do understand that between 2 switches running ISL, there should be no
untagged traffic (they tag all VLANs), however, some NICs do allow you
to run ISL in which the possibility of untagged traffic is present.  But
even if that were the case...traffic would be tagged upon the return, so
I'm not sure to be honest.  We'll have to fire it up in a lab and test.

switch> (enable) show trunk 1/3
* - indicates vtp domain mismatch
# - indicates dot1q-all-tagged enabled on the port
$ - indicates non-default dot1q-ethertype value
Port      Mode         Encapsulation  Status        Native vlan
--------  -----------  -------------  ------------  -----------
1/3      on           isl            trunking*     400

Port      Vlans allowed on trunk

Brian/Scott or some of the other serious gurus may be able to provide us
with the answer on this one.


From: John Jones [mailto:acer0001@xxxxxxxxx]
Sent: Monday, November 13, 2006 10:05 AM
To: ccielab@xxxxxxxxxxxxxx; Steyn, Daniel
Subject: Does ISL Support...........?

I would like info on this as well. I ask because I always thought that the concept of ISL and native VLANs didn't mix. I haven't tested this though. See below:

ISL also encapsulates the entire frame, increasing the network overhead.
Dot1q only places a header on the frame, and in some circumstances,
doesn't even do that. There is much less overhead with dot1q as compared
to ISL. That leads to the third major difference, the way the protocols
work with the native vlan.

The native vlan is simply the default vlan that switch ports are placed
into if they are not expressly placed into another vlan. On Cisco
switches, the native vlan is vlan 1. (This can be changed.) If dot1q is
running, frames that are going to be sent across the trunk line don't
even have a header placed on them; the remote switch will assume that
any frame that has no header is destined for the native vlan.

The problem with ISL is that is doesn't understand what a native vlan
is. Every single frame will be encapsulated, regardless of the vlan it's
destined for.



On 11/13/06, Alexei Monastyrnyi <alexeim@xxxxxxxxxxxxxxx > wrote:

and any details/URL to read on this?

       Daniel_Steyn@xxxxxxxx wrote:
       > ISL does support a native VLAN in which ingress untagged VLANs
are then
       > tagged with the vlan specified.
       > -----Original Message-----
       > From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On
Behalf Of
       > Alexei Monastyrnyi
       > Sent: Monday, November 13, 2006 9:19 AM
       > To: Rajiv
       > Cc: ccielab@xxxxxxxxxxxxxx
       > Subject: Re: Does ISL Support...........?
       > Hi.
       > As per Q1, since ISL adds its specific L2 header to each
frame, all
       > frames have to have it, otherwise the frame should be
considered as
       > invalid. In this sense there should be no untagged frames for
       > As per Q2, if I remember right, both protocols support up to
4095 VLANs.
       > HTH
       > A.
       > Rajiv wrote:
       >> Hi all,
       >>   I want to ask that Does ISL supports the processing of
       > frames?
       >>   Also does 802.1q supports fewer VLANs than ISL?
       >>   Regards,
       >>   Rajiv
       >> ---------------------------------
       >> Everyone is raving about the all-new Yahoo! Mail beta.
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