The MPLS-OPS Archive

["M. ELK" <elkou141061@hotmail.com>]

Hi Andrea

AFAIK

Pls find reply within U msg .

Brgds

>From: Andreas S <dg6gai@gmail.com>
>Reply-To: Andreas S <dg6gai@gmail.com>
>To: mpls-ops@mplsrc.com
>Subject: [MPLS-OPS]: Newbie Questions
>Date: Mon, 16 May 2005 17:50:55 +0800
>
>Hi,
>
>I am quite new to MPLS and have some questions on the FAQ on this
>website and the RFC3031 that might sound stupid to you, but I couldn't
>answer these for myself.
>
>
>A.   LSPs
>
>The FAQ gives an example of a LSP as follows (4f):
><http://www.mplsrc.com/faq2.shtml#MPLS Components>
>
>|------| 1                                    1 |-----|
>| R1   |--\                                 /---| R5  |
>|------|   \   2          2          2     /    |-----|
>             \|------|   |-----|   |-----| /
>              | R2   |---| R3  |---| R4  |/
>              |------|   |-----|   |-----|
>
>It states that we have two LSPs: R1-R5, and R2-R3-R4.
>
>According to the formal description of the RFC3031 (section 1.15), a
>LSP is defined as follows (I quote the corresponding section
>directly):
>
>"In other words, we can speak of the level m LSP for Packet P as the
>sequence of routers:
>1. which begins with an LSR (an "LSP Ingress") that pushes on a level m 
>label,
>
>2. all of whose intermediate LSRs make their forwarding decision by
>label Switching on a level m label,
>
>3. which ends (at an "LSP Egress") when a forwarding decision is made
>by label Switching on a level m-k label, where k>0, or when a
>forwarding decision is made by "ordinary", non-MPLS forwarding
>procedures."
>
>According to my interpretation of the above, the R2-R3-R4 path should
>start at R1 and finish at R5, as R1 is the LSR that "pushes on a level
>m label" (m=2, requirement 1), and R5 is the LSR where "a forwarding
>decision is made by label switching on a level m-k label" (requirement
>3).
>
>Did I get this wrong in some way?
>
i guess the explanation is bit confusing .
consider either :
A) R0 connected to R1 .
     we have one LSP ,say LSP1  as R0,R1 and R5 .
     we have another LSP , say LSP2 as R1,R2,R3,R4,R5 .
   when R1 receive label packet over LSP1  , it swap the label and push the 
labeled packe inside
   LSP2 to reach R5 . so R1 will transmit the packet with 2 label , the top 
is for LSP2 and the
   bottom is for LSP1 .

  This is exacty what the example mean as it state
   quote
When R1 receives the packet P with a label that is 1 level deep, it will 
swap that P's label with a corresponding label that will be used by R5.
unquote

Or

B) Consider we have targeted LDP between R1 and R5 , or even BGP+label 
between R1 and R5
    for certain FEC say FEC_1.
    still we have 2 LSP . LSP1 between R1 and R5 .
    LSP2 between ,R1,R2,R3,R4 and R5.
    For R1 to send packet part of FEC_1 to R5. It create label stack with 
depth 2 .
    the bottom label is for LSP1 and the top is for LSP2 .



>
>B.   k > 0
>
>This leads me to my next question:
>A mentioned above, the LSP ends when a forwarding decision is based on
>a m-k level label, whereas k>0. As far as I understand, k should be
>always 1, which still satisfies the condition of k>0, but in which
>situations can k be 2, 3 or whatever?
>

i will agree that always  K=1 , may be some one in the list could
clarify this point .
may be for the case where the LSR push 2 label , where the bottom of the 2
is the explicit null or the router alert label .

>
>C.   Same label for different FECs at one LSR
>
>The RFC3031 (3.1) states that a LSR may only accept the same label for
>different FECs from different routers, if it can tell from which
>router the packet came (so far so good). Is there any advantage in
>doing so (apart from saving label values) and is there any
>implementation out there that uses it in this way? To me, it just adds
>avoidable complexity...
>

the label could be drawn from the global space (one per router) , or to have
interface space (one per interface).
If the LSR use one space per interface , so for the same FEC it could 
advertize
label L1 to neighbor1 and label L2 to Neighbor2 .
my guess that interface space was meant initially for ATM interface where 
the label will
occupy the space of VPC and VCC in the ATM header . This was in the early 
day where
it exist ATM switch upgraded with MPLS and IP routing capacilities ,My guess 
that this method is not used any more .

>
>D.   FEC-to-NHLFE Map (FTN)
>
>I was wondering if there is any use for the FTN other than in MPLS
>Ingress LSRs? Or, need/have interior LSR a FTN at all?
>

the FTN only needed to switch/route unlabeled packet .
Even for the P , we still need the FTN to switch/route traffic originated 
from
the P itself (supervision ....etc ).

>
>Feedback and clarification on any of these issues would be appreciated.
>
>Thanks for your help!
>
>Regards,
>  Andreas
>
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