[mpls-linux-general] Hierarchical LSP set up

[Ramanjaneyulu Y T <yt...@cs...>]

Hi,

   I have questions on how to establish LSPs . pls go through following
mail and answer to those.

       Thanks in advance,




  The following is the scenarios to establish hierarchial LSps.

Method1
-------





    ---------						 ---------
   |	     |						|	  |
   | LER11   |						|  LER12  |
   |	     |						|	  |
    ---------					 	 ---------
	|						     |
	.       	  Y		      Z	   	     .
	.       --------       -------		  ------   T .
        |__X___|        |     |       |          |      |____|
	       | LER1   |-----|       |-----.....|LER2  |
	       |	|     |       |          |      |
		--------       -------	          ------
            
at LER1                                       at LER2
label look up table 				label look up table
  		
 X -- FWD Y					Z - FWD T

NHLFE    					NHLFE 	

 Y -- PUSH Y 					T -- PUSH T





Note:
----
 PUSH y  -- PUSH label Y
 FWD  y  -- check entry 'Y'
            at NHLFE table    

  The output entry it having separate  operations. In this case it just
forwads to soecified interface in table.
                  

   ---
    Initially there is no LSP existing between LER1 and LER2. When request
comes from LER11 , PATH message is sent to LER12 . When LER12 responds
with
RESV message with label for previous hop.When RESV message reaches from
upstream LSR to LER2 , it allocates new label and binds with label from
upstream LSR and propagates RESV message to previous hop. The same process
is
repeated at all LSRS and LER1 also. Now LSP is established between from
LER11
to LER12 through LER1 and LER2. Note that this is not hierarchical LSP. 

  In this case , all data transfers occurs normally. 

Consider the following scenario.

  


    --------						 ---------
   |	     |						|	  |
   | LER11   |						|  LER12  |
   |	     |						|	  |
    ---------					 	 ---------
	|						     |
	. 		  Y		  Z		     .
	.       --------       -------		  ------     .
        |__X___|        |     |       |          |      |__T_|
	       | LER1   |-----|       |-----.....|LER2  |
	 ______|	|     |       |          |      |_____
	|A	--------       -------	          ------      | 	
	|						   B  |
	. 						      .	
	.						      .
							      |	
	|						 ----------
     --------						|	   |
    |	     |						|  LER 22  |
    | LER 21 |						|	   |
    |	     |						 ----------
     -------- 					



In this scenario, we want to set up an LSP between LER21 and LER 22
through
LER1 and LER 2 which is a heirarchical LSP (in the sense that the same LSP
wil
be used by both the traffic flows from LER 11 and LER 21)
					
Note:
----
  LER2 generates label 'C' and informs to LER1 for LER21.  



at LER1                                       at LER2

label look up table 				label look up table
  		
 X -- FWD Y					Z - FWD T
 A -- PUSH C , FWD Y                            C - FWD B

NHLFE    					NHLFE 	

 Y -- PUSH Y 					T -- PUSH T
						B -- PUSH B	







   If LER21 wants to establish LSP to LER22. It sends PATH request to
LER22.
LER22 responds to PATH message with RESV message with label for previous
hop.
when RESV message reaches to LER2 , it generates a new label for LER1 ,
LSP
is already existing between LER1 and LER2. In core LSRS between LER1 and
LER2
, just bypasses this message. At LER1 , it generates a new label and
places
PUSH LER2 label operation in newly generated label and forwards it to
previous hop.


    My question is , how LER2 differentiate MPLS data packets whether to
forward
to LER12 or LER22 by seeing on top label ( here 'Z'). 

Method2
-------
    If we establish one LSP between LER1 and LER2, then we can use this
LSP to
establish LER11 and LER12 as hierarchical LSPs. same way for LER21 and
LER22.
If this is the case at LER2 , data packet contains label stack of depth 2
.
So POP top label at LER2 and based on next label one can forward the
packet. 
But for this one is required to establish LSPs statically and those 2
nodes
should  know each other. But this approach is not suitable, i think , due
to
these constraints.


       During LSP establishment between LER11 and LER12 , the LER2
generates
new label and informs to LER1. This is same for second LSP also.

    suppose LER2 generates M for LER12. The table update is as follows.

at LER1                                       at LER2

label look up table 				label look up table
  		
    						Z - POP
 A -- PUSH C , FWD Y                            C - FWD B
 X -- PUSH M , FWD Y  				M - FWD T

NHLFE    					NHLFE 	

 Y -- PUSH Y 					T -- PUSH T
						B -- PUSH B
								


  So at LER2 , the MPLS pkt conatins M,Z or C,Z. So by looking on tables ,
it
learns POP top label and looks on next label. This is works fine in
hierarchical environmant , but it has constraints.

Questions:
---------

1. Is above methods are correct ?

2.    Other than these 2 methods any other methods avaialble to achieve
hierarchial LSPs ?.













                                         
                                         Regards
                                        Ramanjaneyulu Y.T. 
--  
 " A real friend is one who walks in when the rest of the world walks
 out."
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