Actually, there are at least two L3 VPN scenarios that result in
different stack depths:
(1) L3 VPNs running only over RSVP (or only over LDP) result in a label
stack depth of two. In this topology:
<---VPN label sent---<
CE1---PE1---Px---Py---Pz---PE2---CE2
|------RSVP LSP------|
For the packet flow from CE1 to CE2, PE1 pushes two labels on (e.g., it
does a "double-push"). The first label (inner label) is the
VPN-specific label, assigned by PE2 and signaled to PE1. The second
label (outer label) is assigned by RSVP or LDP.
Px, Py, and Pz forward packets per the outer label. Pz is the
penultimate router in the PE1-PE2 LSP and pops the outer label. PE2
receives the packet with only the inner label, forwards it according to
that label (e.g., out the interface to CE2), and pops that label off
such that CE2 receives only the unlabeled packet.
(2) L3 VPNs where LDP is tunneled over RSVP result in a label stack
depth of three. In this topology:
<---VPN label sent---<
CE1---PE1---Px---Py---Pz---PE2---CE2
|-------LDP LSP------|
|-RSVP LSP-|
PE1 still does a double push: the VPN label is still the inner label,
and the LDP-assigned label is pushed on top of it. But now, at Px, an
RSVP-assigned label is pushed on and becomes the outer label, laking the
LDP-assigned label the middle label.
Py pops the RSVP label. Pz pops the LDP label. PE2 pops the VPN label
and forwards the packet to CE2.
This scenario is useful when you want to run LDP at the PE routers and
still achieve some traffic engineering in the core. LDP does not do any
traffic engineering, so you need RSVP to accomplish that.
More scenarios can be constructed when you start looking at L2 VPNs and
inter-carrier/carrier-of-carrier VPNs.
As Jesper stated, if you're not doing VPNs and just tunneling LDP over
RSVP, you'll get a label stack depth of two.
There are some good white papers at:
https://www.juniper.net/techcenter/techpapers/
HTH,
/sfr
Jesper Skriver wrote:
>
> On Sun, Jan 13, 2002 at 04:42:02PM -0500, Mark M. Forest wrote:
>
> > I have a question concerning the MPLS label stack.
> >
> > I have read we can stack 3 labels deep...last on first off. I
> > have also read where this information is used in multiple push
> > scenarios. Can someone provide a decent explanation of how this is
> > used and where the most benefit is seen.
>
> RSVP signalled LSP's with LDP tunneling will give you 2 labels.
>
> Add L3 VPN's and you have 3 labels.
>
> /Jesper
>
> --
> Jesper Skriver, jesper(at)skriver(dot)dk - CCIE #5456
> Work: Network manager @ AS3292 (Tele Danmark DataNetworks)
> Private: FreeBSD committer @ AS2109 (A much smaller network ;-)
>
> One Unix to rule them all, One Resolver to find them,
> One IP to bring them all and in the zone to bind them.
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