The MPLS-OPS Archive

[Sundeep Singh <singh0de@ee.iitb.ac.in>]

On Mon, 16 May 2005, Andreas S wrote:

> 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?

R2-R3-R4 (label-B) is like a LSP within a LSP R1-R5 (label-A) . So when a 
packet enters through R1, it will have label-A (at level 1) . On R2, 
label-B (on level-2) will be pushed on it, creating a stack. Remeber only 
top most label is consulted & for LSP R2-R3-R4: R2 is the ingress & R4 is 
egress. At R4, label will be poped and there'll again be level-1 label at 
R5.

>
>
> 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?

k can be 2,3 or whatever.. lets say we have LSPs R1-R5, R2-R4 & R3-R4. In 
this case, at R3 & R4 there'll be level-3 label stack. Then at R4, label 
stack will return to level-1 (m=3 to m=1 ; k=2).



>
>
> 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...

Don't know of any advantage... but just the incoming interface is to be 
checked to distinguish whether packets with same label values came from 
same router or not.

>
>
> 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?

Yes, FTN is need only at the edge (ingress).

Sundeep.


>
>
> Feedback and clarification on any of these issues would be appreciated.
>
> Thanks for your help!
>
> Regards,
> Andreas
>
> -------
> The MPLS-OPS Mailing List
> Subscribe/Unsubscribe:  http://www.mplsrc.com/mplsops.shtml
> Archive: http://www.mplsrc.com/mpls-ops_archive.shtml
>
> MPLScon 2005 - May 16-19, NYC, NY
> http://www.mplscon.com/
>

-- 
Sundeep.

-------
The MPLS-OPS Mailing List
Subscribe/Unsubscribe:  http://www.mplsrc.com/mplsops.shtml
Archive: http://www.mplsrc.com/mpls-ops_archive.shtml

MPLScon 2005 - May 16-19, NYC, NY
http://www.mplscon.com/