Subject: Hierarchical LSPs and RSVP-TE

From: Ramanjaneyulu Y T <[email-masked]>
Date: Thu, 7 Dec 2000 22:27:19 +0530 (IST)

HI,
   If this issue has already been discussed please provide pointers to
mail archives.  

The question here is the establishment of Hierarchal LSPs by using
RSVP-TE signalling.

We consider the following scenario.

                         (virtual pipe of LSP)
LER1-----LSR2-----LSR3-.-.-.-.-.-.-.-.-.-.-.-.LSR4-----LSR5-------LER2
                    \                        /
                     \                      /
                      \____lsr11.....lsr1n-/

We have the above topology of the network. We want to establish an LSP
between LER1 and LER2.

Note that we don't have a direct path between LSR3 and LSR4 . But  they
are
connected through  lsr11 .... lsr1n.(which are opaque to the endpoints
LER1
and LER2)

LER1 initiates a PATH message with ERO Object as:
     LSR2  LSR3  LSR4  LSR5 LER2

So as Path message propagates and comes to LSR3 it will find that it does
not
have a direct path to LSR4. so it will add more sub objects to ERO
specifying
the nodes lsr11 ... lsr1n so that the Path message traverses through these
routers and finally comes at LSR4. Now LSR4 will forward the message to
LER2
through LSR5. and Resv message will appropriately flow backwards and
complete the LSP setup.

Now if there are more requests for LSPs which have paths through LSR3 and
LSR4
then each time LSR3 will be generating independent labels which will
perhaps
use a lot of labels. Since the path between LSR3 and LSR4 is common for
many
LSPs we can setup a single LSP between LSR3 and LSR4 on which we can build
hierarchical LSPs through this LSP. 

There is also an issue of Management of LSPs.
   Since now we have a point to point LSP between LER1 and LER2  we would
not
like these two endpoints to know about the actual route traversed between
the
two routers LSR3 and LSR4. So even if anything happens to the Path between
LSR3 and LSR4, It will be automatically rerouted by LSR3 (if alternative
path is
available) without the actual endpoints (LER1 and LER2) being aware of
such a
change. 
But for this we require to have a common LSP between LSR3 and LSR4.

Now this will require an LSP  to be either already present between LSR3
and
LSR4  which is installed statically or it can be setup  dynamically.
   If we have to setup the intermediate LSPs dynamically  then how do we
do
it ??
 
So one possible solution will be  something like this:

 When a Path for a new LSP set up starts from LER1 , reaches LSR3 (we
don't
have any LSP between LSR3 and LSR4) then we delay this path request and
start
a new path message from LSR3 to LSR4 through lsr11...lsr1n which will
establish LSP between LSR3 and LSR4.Once this is established we can allow
the
delayed Path message to be forwarded to LSR4 through the intermediate LSRs
and then finally to the destination. The resv message will also propagate
upstream to the sender.
   But in this procedure when do we reserve the resources for the
LSP(between
LSR3 and LSR4)???

One more solution is to establish the inner LSP (between LSR3 and LSR4)
statically.

Which one is better?? Or is there any other method of setting up of such
hierarchical LSPs.??

Thanks in advance,

                                         Regards
                                        Ramanjaneyulu Y.T.