When using ISL or 802.1q trunking, the sizes of the frames is only increased
inside the switches. Once the frames exit the switches they are reduced back
down to their original size by removal of the coloring info. If a frame
must traverse multiple VLAN capable switches, each switch will reapply its
own coloring info to the frame as it enters that switch and remove it as it
leaves.
In you are unfamiliar with the coloring concept, it is a way to describe how
the VTP domain defines individual VLANs. By thinking of each VLAN as a
color, it is helpful in understanding what is being done during the inter
VLAN processes etc. Example: a switch receives a frame from VLAN 2 destined
for VLAN 3, the switch identifies each VLAN as colors blue and red
respectively. By adding a red field to the incoming VLAN 2 frame header it
can easily identify the destination VLAN.
This is a crude example of the frame tagging process but I hope it shed a
little light.
>From: srclevenger <[EMAIL PROTECTED]>
>Reply-To: srclevenger <[EMAIL PROTECTED]>
>To: Anthony Awatefe <[EMAIL PROTECTED]>
>Subject: Re: Question about ISL and 802.1Q encapsulation
>Date: Wed, 27 Sep 2000 14:28:00 -0700
>
>Anthony,
>
>I agree with you but have taken this a step further. If you have 802.1Q
>tagging
>applied to interfaces, Standard ports will not know what to do with it
>because
>it is tagged special (unless we are talking vlan aware / hybrid ports, then
>they
>take on both ends of the world in switching.) Therefore the packet drops at
>a
>standard port if not 802.1q aware. Right? Will you will see this as the
>mini-giant? If it is aware of the 802.1q tag at each end will you not see
>the
>frame just pass thru the switch and not register the error? Could you have
>a
>code bug on the switch then if you see the pass thru 802.1q tag as an
>error? I
>understand that switches should adjust to different tags so that you do not
>see
>them as errors or have I been given a smoke screen? Could there be a
>configuration error in the setup of the switch, ie a combo of the 802.1q
>and
>hybid?
>Understand that this is the sort of thing I am still learning for the BCMSN
>test
>in October, so I maybe in the same boat as you are. So please let me know
>what
>you find out.
>
>regards,
>Steve Clevenger
>
>
>Anthony Awatefe wrote:
>
> > Normal Ethernet frames sizes can not exceed 1518 bytes. When 802.1q
>tagging
> > is used frame sizes can increased to a maximum of 1522 bytes. This is
>known
> > as a mini giant, switches have no problem handling these frame types,
> > although it my register on the switch as an error.
> >
> > With ISL Encapsulation the switch identifiies the frame as an ISL frame
>type
> >
> > Regards
> > Anthony
> >
> > ----- Original Message -----
> > From: "Dove" <[EMAIL PROTECTED]>
> > Newsgroups: groupstudy.cisco
> > To: <[EMAIL PROTECTED]>
> > Sent: Wednesday, September 27, 2000 7:28 AM
> > Subject: Question about ISL and 802.1Q encapsulation
> >
> > > Hi,
> > >
> > > I have a question about ISL and 802.1Q encapsulation/tagging. As I
>know,
> > ISL
> > > will encapsulate a frame with adding 26 bytes to header and 4 bytes to
> > > trailer; and 802.1Q will modify the existing header and add 4 bytes to
>it.
> > > My question is: if the frame is already in its maximum size (i.e.
>1,518
> > > byte), after ISL or 802.1Q encapsulation/tagging, the frame size will
>be
> > > greater than the ethernet maximum size. Will it cut the frame into 2
>parts
> > > or ignore the frame size checking?
> > >
> > > Regards.
> > > dovelet
> > >
> > >
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