Figure 19: Ldp Over Rsvp Application; Ldp Over Rsvp Tunnels - Alcatel-Lucent 7950 SR User Configuration Manual

Label Distribution Protocol

LDP over RSVP Tunnels

LDP over RSVP-TE provides end-to-end tunnels that have two important properties, fast reroute
and traffic engineering which are not available in LDP. LDP over RSVP-TE is focused at large
networks (over 100 nodes in the network). Simply using end-to-end RSVP-TE tunnels will not
scale. While an LER may not have that many tunnels, any transit node will potentially have
thousands of LSPs, and if each transit node also has to deal with detours or bypass tunnels, this
number can make the LSR overly burdened.
LDP over RSVP-TE allows tunneling of user packets using an LDP LSP inside an RSVP LSP.The
main application of this feature is for deployment of MPLS based services, for example, VPRN,
VLL, and VPLS services, in large scale networks across multiple IGP areas without requiring full
mesh of RSVP LSPs between PE routers.
7950 XRS
IP/MPLS
Metro Network
(Area 1)
The network displayed in
core area, Area 3. Each area makes use of TE LSPs to provide connectivity between the edge
routers. In order to enable services between PE1 and PE2 across the three areas, LSP1, LSP2, and
LSP3 are set up using RSVP-TE. There are in fact 6 LSPs required for bidirectional operation but
we will refer to each bi-directional LSP with a single name, for example, LSP1. A targeted LDP
(T-LDP) session is associated with each of these bidirectional LSP tunnels. That is, a T-LDP
adjacency is created between PE1 and ABR1 and is associated with LSP1 at each end. The same is
done for the LSP tunnel between ABR1 and ABR2, and finally between ABR2 and PE2. The
loopback address of each of these routers is advertised using T-LDP. Similarly, backup
bidirectional LDP over RSVP tunnels, LSP1a and LSP2a, are configured by way of ABR3.
This setup effectively creates an end-to-end LDP connectivity which can be used by all PEs to
provision services. The RSVP LSPs are used as a transport vehicle to carry the LDP packets from
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LSP1
7950 XRS
PE 1
ABR 1
7950 XRS
LSP1a
LSP2a
ABR 3

Figure 19: LDP over RSVP Application

Figure 19
7950 XRS
LSP2
ABR 2
IP/MPLS
7950 XRS
Core Network
(Area 3)
ABR 4
consists of two metro areas, Area 1 and 2 respectively, and a
7950 SR OS MPLS Configuration Guide
7950 XRS
LSP3
PE 2
IP/MPLS
Metro Network
(Area 2)