Subject: doubt in LDP Label Mapping procedure.

From: Abhijit <[email-masked]>
Date: Wed, 2 Aug 2000 16:25:15 +0530 (IST)

Hi all,

In the ldp-06 draft in Appendix A, in procedure of label mapping message,

LMp. 14 says is the LSR ingress for FEC?

How an LSR can know beforehand that it is ingress for a particular FEC? A
particular LSR can be egress for a particular FEC, but whether it is
ingress or not is dependent upon the source from where packet is entering
MPLS domain. 

Consider

         A-----------B-------------D
                     |
         C___________|

Here let a set of 4 routers A, B, C, D be the MPLS domain. When packet
enters this domain through A , A is the ingress and when it enters through
C, C is the ingress, If there is also a link between A & C and
shprtest path to B from A is via C, C will be ingress for a set of packets
but it won't be ingress for other set of packets, So how can you decide 
beforehand that C is ingress or not?

-abhijit

---

From: Alok Singh <[email-masked]>
Date: Thu, 3 Aug 2000 10:48:41 +0530 (IST)

Hi abhijeet,
           
regarding your question :
How an LSR can know beforehand that it is ingress for a particular FEC? A
particular LSR can be egress for a particular FEC, but whether it is
ingress or not is dependent upon the source from where packet is entering
MPLS domain.                                    
..................
I think we have to do an explicit configuration. It seems that is the only
way.. As far as your doubt that " an LSR acting as router depends upon the
source from where packet is entering the MPLS domain" i can say something.
An ingress router will have some interfaces related to MPLS domain. And
some other interfaces which will talk to the outside world. Whereas a
general LSR will have all interfaces talking to other nodes in the MPLS
domain only. This is one criterion from which we can differentiate b/w the
two. So i think somebody has to decide which interfaces belongs to a
particular MPLS domain and which to the outside world..
 
WOULD ANYBODY OUT THERE LIKE TO PUT MORE LIGHT ON THIS ISSUE ?

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