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Newbie Questions

  • From: Andreas S <dg6gai@gmail.com>
  • Date: Mon, 16 May 2005 17:50:55 +0800
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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?


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?


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...


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?


Feedback and clarification on any of these issues would be appreciated.

Thanks for your help!

Regards,
 Andreas

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