Yes, it's true, .1Q adds 4 bytes. The TPID and TCI fields look like this:
Tag Protocol Identifier (TPID) 2 bytes
- fixed value of 0x8100 16 bits
Tag Control Information (TCI) 2 bytes
- User Prioroty 3 bits
- Canonical FFormat Indicator (CFI) 1 bit
- VLAN Identifier 12 bits
An Ethernet frame of maximum size (1518 bytes) that gets tagged with .1Q
VLAN info is now 4 bytes too big (1522 bytes). This frame is called a "baby
giant". Switches can handle the frame correctly, but it may also generate an
error statistic. This might not be a big deal unless you have lots of them
and you have certain alarm thresholds set in your management system...
One other thing - ISL will only encapsulate frames that exit the switch on a
trunk port, while .1Q tags get added to all frames (as I recall... somebody
might want to check me on this).
Dale
[=`)
>From: Ole Drews Jensen <[EMAIL PROTECTED]>
>Reply-To: Ole Drews Jensen <[EMAIL PROTECTED]>
>To: "'Tom Walstrom'" <[EMAIL PROTECTED]>, Ole Drews Jensen
><[EMAIL PROTECTED]>, [EMAIL PROTECTED]
>Subject: RE: 802.1Q or ISL
>Date: Fri, 8 Sep 2000 07:40:46 -0500
>
>Thanks for your reply Tom,
>
>However, according to the book I'm reading, the 802.1Q DOES change the
>frame
>size by adding 4 bytes into it.
>
>Take care,
>
>Ole
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Ole Drews Jensen
> Systems Network Manager
> CCNA, MCSE, MCP+I
> RWR Enterprises, Inc.
> [EMAIL PROTECTED]
> http://www.insync.net/~drews/ccnp
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>
>
>-----Original Message-----
>From: Tom Walstrom [mailto:[EMAIL PROTECTED]]
>Sent: Friday, September 08, 2000 6:30 AM
>To: Ole Drews Jensen; [EMAIL PROTECTED]
>Subject: RE: 802.1Q or ISL
>
>
>Ole,
>
>ISL encapsulates the frame adding, as you said, a 26 byte header and a 4
>byte CRC trailer. 802.1Q frame-tagging does not change the frame size
>(hence its interoperability, because it appears as a standard ethernet
>frame
>to non-802.1Q devices), but does modify the existing frame with VLAN
>identification information.
>
>I would suspect that the real reason to deploy ISL is that it runs one
>spanning tree per vlan where 802.1Q runs only one spanning tree. Also ISL
>allows bonding into etherchannels. Seems like this would be more likely to
>make a difference on the network. I would think you would only do 802.1Q
>where interoperability was the issue, like with a Catalyst 4000 which I
>believe only supports 802.1Q. Maybe some switch guru could further
>illuminate this issue.
>
>Regards,
>
>Tom
>
>-----Original Message-----
>From: Ole Drews Jensen [mailto:[EMAIL PROTECTED]]
>Sent: Thursday, September 07, 2000 1:38 PM
>To: '[EMAIL PROTECTED]'
>Subject: 802.1Q or ISL
>
>
>Just a thought. - Digging through another book towards the light at the end
>of the tunnel, I have now added VLAN Trunc Links to my knowledge. That has
>brought this question up in my mind, so I would like to hear some feedback
>on this subject.
>
>I know that much of this depends on the average frame sizes, so lets say
>that I have analyzed my network, and the average frame size is 800. Lets
>also say that we are only dealing with Cisco Catalyst switches in this
>scenario.
>
>The question is, what would be least resource-waste to use as a trunking
>link : ISL or 802.1Q???
>
>The 802.1Q has to break the frame open to modify it, but it adds only 4
>bytes to each frame.
>
>The ISL does not have to break the frame open because it simply
>encapsulates
>it into a new one, but it adds 30 bytes to each frame.
>
>Any comments on this?
>
>Thanks,
>
>Ole
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~
> Ole Drews Jensen
> Systems Network Manager
> CCNA, MCSE, MCP+I
> RWR Enterprises, Inc.
> [EMAIL PROTECTED]
>~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>___________________________________
>UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html
>FAQ, list archives, and subscription info: http://www.groupstudy.com
>Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
>
>**NOTE: New CCNA/CCDA List has been formed. For more information go to
>http://www.groupstudy.com/list/Associate-Announcement.html
>_________________________________
>UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html
>FAQ, list archives, and subscription info: http://www.groupstudy.com
>Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
_________________________________________________________________________
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.
Share information about yourself, create your own public profile at
http://profiles.msn.com.
**NOTE: New CCNA/CCDA List has been formed. For more information go to
http://www.groupstudy.com/list/Associate-Announcement.html
_________________________________
UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html
FAQ, list archives, and subscription info: http://www.groupstudy.com
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
