From: Scott Morris (swm@emanon.com)
Date: Thu Dec 01 2005 - 09:42:35 GMT-3
ISL has 15 bits as you noted, but recently, Cisco seems to have ratcheted
this down to 12 bits to match in line with the dot1q settings. Both support
4094 vlans. Perhaps there's some thought of "compatibility" or
"similarity"? Who knows.
It's odd though, because some docs/OS versions note it as 10 bits like you
mentioned which brings it more in line with VTP capabilities. Although
switches I've seen seem to use the capability of 4096 as you see. Who knows
where Cisco is going with it. Chances are, these days, your trunking will
be 802.1Q anyway...
http://www.cisco.com/en/US/tech/tk389/tk390/technologies_tech_note09186a0080
094665.shtml
Scott
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
sholy augustine
Sent: Thursday, December 01, 2005 2:45 AM
To: ccielab@groupstudy.com
Subject: ISL Vs 802.1Q
Hi,
I have some confusion regarding the difference between ISL and dot1q
encapsulation. As to my knowledge ISL uses only 10bits (although 15 bits is
reserved) for VLAN ID giving total of 1024 VLAN IDs and dot1q uses
12 bits giving a total of 4096 VLANs.
When I configure a trunk between 2 3550 switches, I don't see any difference
between them both in show outputs and in functionality
For both ISL and dot1q the allowed VLAN on the trunk is up to 4096. This is
an expected behaviour for dot1q, but for ISL why is it giving 4096, should
it not be 1024 only
Also for ISL encapsulation, native VLAN ID is provided
Kind Regards,
Sholy
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