Alcatel-Lucent 7210 SAS M User Manual page 234

Label Distribution Protocol
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 via 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
one area to another. Note that only the user packets are tunneled over the RSVP LSPs. The T-LDP
control messages are still sent unlabeled using the IGP shortest path.
Note that in this application, the bi-directional RSVP LSP tunnels are not treated as IP interfaces
and are not advertised back into the IGP. A PE must always rely on the IGP to look up the next hop
for a service packet. LDP-over-RSVP introduces a new tunnel type, tunnel-in-tunnel, in addition
to the existing LDP tunnel and RSVP tunnel types. If multiple tunnels types match the destination
PE FEC lookup, LDP will prefer an LDP tunnel over an LDP-over-RSVP tunnel by default.
The design in
without requiring a full mesh of RSVP LSPs between PEs across the three areas.
In order to participate in a VPRN service, PE1 and PE2 perform the autobind to LDP. The LDP
label which represents the target PE loopback address is used below the RSVP LSP label.
Therefore a 3 label stack is required.
In order to provide a VLL service, PE1 and PE2 are still required to set up a targeted LDP session
directly between them. Again a 3 label stack is required, the RSVP LSP label, followed by the
LDP label for the loopback address of the destination PE, and finally the pseudowire label (VC
label).
This implementation supports a variation of the application in
LDP area. In that case, PE1 will push a two label stack while ABR1 will swap the LDP label and
push the RSVP label as illustrated in
Page 234
ration Guide
Figure 25 consists of two metro areas, Area 1 and 2 respectively, and a
Figure 25 allows a service provider to build and expand each area independently
Figure
Figure
26.
7210 SAS M, T, X, R6, Mxp MPLS Configu-
25, in which area 1 is an