Subject: ifStackTable layering in L2TP

From: Klausberger Walter <[email-masked]>
Date: Wed, 29 Nov 2000 11:05:31 +0100

Hi everybody,

I started to read the new L2TP-MIB draft and I already have several
questions...

As I understood stacking should be made visible up to the 'forwarding
layer'(e.g. IP). When looking at common ifStackTables I never saw an IP
layer until yet. So there should be no layer above Ethernet when
transporting IP packets...

As an example in <draft-ietf-l2tpext-l2tp-mib-01.txt> the tunnel interface
in the LNS is set on top of an Ethernet interface. But L2TP is never set
directly above Ethernet, but will be transported via UDP/IP. So there is no
direct context. The connection depends on the routing table.

Assume there is an LNS with an Ethernet and an unnumbered Frame Relay
interface. First the tunnel is setup via Ethernet IF. Due to the entries in
the routing table, the tunnel IF is set on top of the ethernet IF. Now a
problem occurs in the backbone and the connection is switched over to the
unnumbered Frame Relay IF (same IP address). Would this need a change in the
ifStackTable?

It is possible to transport L2TP directly over ATM or via UDP/IP/ATM. Would
there be no difference in the ifStackTable?

In case of LAC it is even more unclear to me how the ifStackTable will look
like. In former L2TP-MIB drafts it was stated that the tunnel IF sits on top
of the interfaces that forward the PPP packets (e.g. DS0). Now there is a
reference to the tunnel MIB (RFC 2667) and everything seems to be different
now. Do we have a problem here with unnumbered IP interfaces, because...
   Tunnels are handled by creating a logical interface (ifEntry) for each
tunnel. These are then correlated, using the ifStack table of the Interfaces
MIB, to those interfaces on which the local IPv4 addresses of the tunnels
are configured.
...or do I have to take the numbered Ethernet IF for the ifStackTable, even
if the tunnel is connected via Frame Relay IF?

Or am I totally wrong?

just some questions, I hope someone can answer...

with best regards
Walter