[Guy Harris <gharris@xxxxxxxxxxxx>]

On Sun, Jan 23, 2000 at 01:03:40AM +0200, Paul Ionescu wrote:
> So, ethereal has to decide early in ethernet dissector if the frame
> is ISL or ethernet(802.3 or Ethernet II), and I don't think that this is
> possible because at this level the ISL packet looks like a valid ethernet
> 802.3 packet.

It *is* a valid ethernet 802.3 packet - it's an Ethernet/802.3 frame
with a length less than the Ethernet MTU, hence it's an 802.3 frame
rather than an Ethernet frame, and it has an LLC header with SNAP as the
DSAP and SSAP and an OUI that belongs to Cisco.

Unless frames *other* than ISL frames use an OUI of 00-10-7B, the way to
recognize an ISL frame is to look for frames with that OUI, and
interpret the PID appropriately.

Unless it is completely impossible to run both ISL and non-ISL traffic
on the same Ethernet, it's clearly possible to distinguish ISL frames
from non-ISL frames, and I'm *really* skeptical that it'd be impossible
to run both on the same Ethernet.