The MPLS WG Archive

[Kireeti Kompella <kireeti@juniper.net>]

> "some other LSRs" (let's say A in the example below) would signal 
> RSVP/CRLDP messages (towards H) that pass thru a "head-end" LSR (C) who in 
> turn would establish a FA-LSP to F.

So far, so good.  Note that the path the A computes is ABCDEFGH.

> A--B--C(head-end)====D====E====F--G--H

See section 4.3.2. (LSP regions) that explains how C determines
that an FA is needed to F.

> How does the A know to send an RSVP/CRLDP message thru C if the FA-LSP 
> (C===F) does not already exist? Shouldn't the FA-LSP appear first as a link 
> in A's topology database which would imply it has already been setup?

Well, as mentioned above, the path that A computed is ABCDEFGH; but
C intercepts this, and creates an FA to F (C is the head end of this
FA-LSP), then strips the FA from the original ERO, and sends the path
message for the A->H LSP directly to F.

C also advertises the CF FA-LSP so that others can see it and use it.

Kireeti.