[Isis-wg] How should we encode TRILL IS-IS to distinguish it from layer 3 IS-IS?

The TRILL WG would like to use IS-IS for RBridges to have a routing
protocol among themselves.

RBridges encapsulate packets with a TRILL header as they are forwarded
across the RBridged-cloud (called a "campus").

Which means that layer 3 IS-IS, if routers are connected via a "campus", 
will look
like:

inner frame: exactly as the first IS-IS router transmits it, i.e., an 
Ethernet header
with Ethertype="IS-IS", Ethernet source address=IS-IS router that 
transmitted it,
Ethernet destination address usually="all-Level1-routers" or 
"all-Level2-routers", but
sometimes unicast (like in the case of a PSNP).

And this frame will be TRILL-encapsulated while travelling across the
campus between the RBridges, but will be stripped off at the egress 
RBridge so
layer 3 routers won't know it happened.

The layer 3 IS-IS packet, when TRILL-encapsulated look like:

outside the inner frame is a "TRILL header", which identifies the 
ingress and egress
RBridges, and a hop count, and (luckily, because I think we'll need it) 
some reserved bits.

outside that is an ordinary looking Ethernet header (in case there are 
bridges
between two RBridges), with Ethertype=TRILL.

***********
So far OK.

But now -- how do we encode TRILL IS-IS packets so they are not confused 
with
layer 3 IS-IS, and also not confused with TRILL-encapsulated data packets?

TRILL will use "area address=0", which I'm assuming is not a legal area 
address for
layer 3. Is this assumption correct?

Layer 3 IS-IS routers will not ever see a TRILL IS-IS packet because it 
will be
TRILL-encapsulated. The Ethertype will be TRILL rather than IS-IS, and 
RBridges
will not use the IS-IS layer 2 multicast addresses.

But the question is---how does an RBridge know the difference between a 
layer 3
IS-IS packet that has been encapsulated for transit across the campus, 
and a TRILL
IS-IS packet?

Both will have TRILL headers.

There's lots of solutions -- I just want to know what will be most 
convenient for
IS-IS implementers.

a) We could use an "illegal" inner Ethertype, say Ethertype=0, that 
would never
appear in a data packet. Then RBridges would interpret Ethertype=0 as a 
TRILL IS-IS
packet.

b) We could use a flag in the TRILL header (steal a reserved bit) to 
mean "This is
a TRILL IS-IS packet"

Any other ideas or advice?

Thanks,

Radia

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