Re: [mpls-linux-general] about tunneling.

[James R. L. <jl...@mi...>]

On Tue, Jun 26, 2001 at 04:21:43PM +0530, Srinivas Reddy wrote:
> hello all ,
> 
> 	i am getting confussion about tunneling. practically in which case we get 
> tunneling. when we get tunneling we have to send Targetted Hello only(please 
> correct me if i am wrong). Can some body guide me in which cases we will 
> create tunnel especially in the case if we use BGP .

I'm not sure if I understand your question but here is an attempt to
answer it :-)

BTW I hate the term "tunnel" (as in an MPLS tunnel) but since you've used
it, I will use the term and explain what I mean by it.


If we have a signalling protocol set up an LSP to a remote router AND
the signalling protocol associats the remote router ID with the LSP,
then we have an instance where "tunneling" is feasible.  In this case
"tunneling" means that I will treat the remote router as a directly connected
peer (via the LSP) for the purpose of another protocol (signalling OR routing).

Stepping back.  Signalling protocol that setup LSPs and associate router
IDs are:

RSVP-TE (user is responsible for assigning a "destination" address,
in this case the destination address should be the remote router ID)

CR-LDP (the FEC could be the remote router ID /32 or the user is responsible
for assigning a "destination" address, same as for RSVP-TE)

LDP (the FEC is the remote router ID and is normally sent via unsolicited
mode (ala Juniper))

In any of these cases the result will be an LSP which can be associated with
the remote router ID (how the association is made varies).

Once that assocaition is made then the remote router is can be considered
as directly connect (as you would say, a "tunnel" has been created to the
remote router).

Now a second level of protocols can use this "tunnel".  Examples are:

LDP (requires "tunnels" in both directions and then uses targeted hellos
to establish an adj.  The resulting session needs to be associated 
with a "tunnel" for the label mappings to be used)

RSVP-TE (the "tunnel" is treated as one hop in an explicit hop list)

BGP (requires "tunnels" in both directions and each side must advertise their
router ID as the next hop for each NRLI thus BGP can resolve the nexthop via
the "tunnel")

One note about the second level signalling protocols:

The resulting label allocation made by second level signalling protocols are
only known by the end point of the LSP.  Thus packet traversing the LSPs
created by second level signalling protocols have 2 labels on them.  The
top label is for the "tunnel" LSP and the bottom label is for the
LSP created by the second level signalling protocol.

I hope this answers your question :-)

Jim

PS In the above example you can replace router ID with "network wide unique
address that is 'known' to be assocaited with the remote router and is still
valid even when re-routing occurs due to link failures".  :-)

> 
> thanks in advance 
> regards
> lsr
> 
> _______________________________________________
> mpls-linux-general mailing list
> mpl...@li...
> http://lists.sourceforge.net/lists/listinfo/mpls-linux-general

-- 
James R. Leu