Subject: Ingress LER in LDP

From: "Santosh Gupta" <[email-masked]>
Date: Thu, 1 Jun 2000 11:33:50 +0530

Hi
    Can someone tell me how to decide if a router is supposed to act as an Ingress LER for some particular FEC in MPLS domain ?

Santosh Gupta

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From: "James R. Leu" <[email-masked]>
Date: Thu, 1 Jun 2000 08:04:49 -0500

>     Can someone tell me how to decide if a router is supposed to act as an Ingress LER for some particular FEC in MPLS domain ?

In general if the next hop for FEC is an LDP peer this LSR _may_ be ingress
for the FEC.  An LSR may be ingress for a FEC (if it initiates a label request
for it's own use) AND transit for a FEC (if an upstream neightbor made a label
request for FEC).

Hope this helps,

Jim
-- 
James R. Leu

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From: Eric Rosen <[email-masked]>
Date: Thu, 01 Jun 2000 10:27:39 -0400

Santosh> Can someone tell me how to decide if a router is supposed to act as
Santosh> an Ingress LER for some particular FEC in MPLS domain ?

People keep asking  this question, and I'm never quite sure  just what it is
they are asking, because there isn't much of a mystery here.

If a router received an unlabeled  packet which belongs to a particular FEC,
and the next hop for that packet supports MPLS, and the router is configured
such that it is allowed to do allow label imposition for packets in that FEC
with that next hop, then the router should impose the label assigned to that
FEC by that next hop.

Does that answer your question?

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From: "Santosh Gupta" <[email-masked]>
Date: Wed, 2 Jun 1999 14:52:12 +0530

Hi
    Please see comments inline.

> If a router received an unlabeled  packet which belongs to a particular
FEC,

There are 2 modes in which an ingress LSR could operate,
1. The LSP formed is made when an unlabelled packet comes to it
2. The LSP should be made in the very beginning as it anticipates that data
is bound to come.
If an LSR is running in the second mode and Ordered-DownstreamOnDemand mode
then how does decide that it should initiate an LSP for the particular FEC.
An LSR could very well act as an LSR for some FEC and LER for some other
FEC.

    A possible solution to this could be that it reads all routes from
Routing Table and if a route has been added by BGP then it knows that it is
supposed to act as an Ingress LER for the particular FEC into which the
Destination falls.
    Other possible solution could be to configure an SLA at each Ingress LER
(either manually or thru some centralized means like COPS) . If LDP finds
some SLA locally then it implies that it is supposed to act as an Ingress
LER and then it will initiate LSP for each FEC.

Is there any better method?

santosh

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From: "Santosh Gupta" <[email-masked]>
Date: Wed, 2 Jun 1999 14:57:19 +0530

Hi
    check the comments inline.

> >     Can someone tell me how to decide if a router is supposed to act as
an Ingress LER for some particular FEC in MPLS domain ?
> In general if the next hop for FEC is an LDP peer this LSR _may_ be
ingress

Actually an LSR needs to know if it is an Ingress LSR for the FEC or not.
Depending on this decision, it has to make a binding for forwarding
(FEC---->label) in the forwarding module. It would be very inefficient to
make this binding in core routers which never do any forwarding but just
switching. Hope you got my question.

Santosh

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From: "James R. Leu" <[email-masked]>
Date: Fri, 2 Jun 2000 07:47:27 -0500

> make this binding in core routers which never do any forwarding but just

The LDP spec only gives you guide lines as to what actions to take when and LSR
is acting as ingress for a FEC.  Acting as ingress for a FEC means that the
LSR has need of being an LER for traffic destinged to that FEC.  It is an
implementation detail as to what external information it uses to make that
descision. Your comments in the previous e-mail about using BGP or an SLA are
a testament to this.

Jim
-- 
James R. Leu

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From: Eric Rosen <[email-masked]>
Date: Fri, 02 Jun 2000 11:32:28 -0400

Santosh> Actually an LSR needs  to know if it is an Ingress  LSR for the FEC
Santosh> or not.  Depending  on this decision, it has to  make a binding for
Santosh> forwarding (FEC---->label)  in the  forwarding module. It  would be
Santosh> very inefficient to  make this binding in core  routers which never
Santosh> do any forwarding but just switching.

I think the only  case where this could possibly be an  issue is the case of
an ATM-LSR which doesn't support VC  merge.  Is that the case you're focused
on?  

Jim> It is an implementation detail  as to what external information it uses
Jim> to make that decision.

I think Jim  Leu's answer is correct.  I would  probably say "product design
decision" rather than "implementation detail".

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From: "Santosh Gupta" <[email-masked]>
Date: Mon, 5 Jun 2000 10:09:25 +0530

Hi Eric

> an ATM-LSR which doesn't support VC  merge.  Is that the case you're
focused

Actually this doesnt depend on VC merging capability of an ATM-LSR. The
issue is whether I should have any entry in the ATM-LSR for FEC->>Label
mapping. Let me depict it.

A---->B----->C----->D ....            E
                  /
                Customer

Suppose an LSP is formed from A to E with A as Ingress LER and E as Egress
LER. The LSP passes thru C. There is yet another customer who wishes to
connect at C and send data to for the same FEC. So there is a need for the
FEC--->Label mapping at A and C. Whether C merges the 2 LSPs or sets up
seperate LSP doesnt matter.

>It is an implementation detail  as to what external information it uses

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From: Eric Rosen <[email-masked]>
Date: Mon, 05 Jun 2000 10:56:25 -0400

In an ATM-LSR  without VC merge, ingress functionality  for a particular FEC
requires a cross-connect  table entry (i.e., hardware resource)  that is not
otherwise necessary.  In other  cases, an efficient implementation makes the
issue  moot, in  my  opinion.  In  any  event, the  answer  to the  original
question ("how does one decide whether label imposition for a particular FEC
is required?") is still the same.