Re: A philosophical question about FA and FA-LSP

[Kireeti Kompella <kireeti@juniper.net>]

On Sat, 5 Mar 2005, dimitri papadimitriou wrote:

> > What's special?  Nothing really.  A "normal" TE link usually has a
> > (single) physical interface associated with it; an FA TE link is
> > usually multi-hop.  A "normal" TE link usually is accompanied by an
> > IGP adjacency (at least, in the packet domain); an FA TE link usually
> > doesn't.
>
> but if a couple this with your answer "Correct -- LSPs created in IGP A
> can be advertised as TE links in IGP B. This can be used (say) for
> inter-area signaling." don't you have now an adjacency accompanying the
> TE link advertisement in IGP B

Not necessarily.  Say you have:

       IGP B - X == IGP A == Y - IGP B

X and Y participate in both IGPs; X has an LSP to Y, and advertises it
into IGP B.  All's fine.

I'm guessing you are referring to the fact that IGP B appears
partitioned?  Well, to connect up the piece on the right with the
piece on the left, one can:
 a) create an adjacency over the FA LSP (only possible if LSP is PSC)
 b) have a virtual link (OSPF) from X to Y
 c) have some other "backdoor" connection between the two parts.

> - therefore why is there a need to rename
> the canonical inheritance i.e. why not just speak in that case about a
> TE link or a "hierarchical TE link" as it was the case in one of the
> earlier versions of the LSP-HIER document and keep the FA concept when
> dealing with a unique instance ?

You can call it what you like :-)  "FA" in particular refers to the
idea that the "adjacency" is primarily for forwarding, and not
necessarily for the control plane.  However, the latter is not ruled
out (as in case (a) above.)

Kireeti.
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