If you are running RSVP-TE in the core, LDP can tunnel through the core by stacking an LDP LSP over an RSVP-TE LSP, as shown in Figure 58. With LDP over RSVP-TE, LDP establishes targeted sessions among the LDP routers at the edge of the RSVP core. From the perspective of the LDP LSP, the RSVP-TE core is a single hop.
Figure 58: LDP Tunneled Through an RSVP-TE Core
In the network topology illustrated in Figure 58, the RSVP-TE LSP consists of LSR 2, LSR 3, LSR 4, and LSR 5. The LDP LSP consists of LER 1, LSR 2, LSR 5, and LER 6. The RSVP-TE tunnel appears to LDP as a single hop.
The initial LDP label 19 is switched with label 24 at LSR 2. Because this is the entrance to the RSVP-TE tunnel, label 21 is pushed onto the stack. Label 21 is switched with label 43 at LSR 3. Label 43 is switched with label 56 at LSR 4. LSR 5 pops both labels, pushes label 17, and forwards the packet to LER 6.
On the LDP routers that are on the edge of the core, you must configure a list of peer addresses. The LDP router sends targeted hello messages to those addresses in order to establish targeted sessions across the RSVP-TE domain. The list includes other LDP routers on the edge of the core; for example, in Figure 58, you include the address of LSR 5 in the list configured on LSR 2.
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