# Topology-Driven LSPs Overview

Topology-driven LSPs are implemented for best-effort, hop-by-hop routing. In topology-driven LSP mode, LDP automatically sets up LSPs for IGP, direct, and static routes, subject to filtering by access-lists. JunosE supports downstream-unsolicited LDP using the platform label space.

If you use the topology-driven LSP mode to forward plain IP packets, use the ldp ip-forwarding command to place LSPs into the IP routing table for forwarding plain IP traffic.

You can use the mpls ldp advertise-labels command to limit the number of routes for which labels are advertised. In most cases, you can issue mpls ldp advertise-labels host-only.

## LDP over RSVP-TE

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.

#### Related Documentation

- MPLS Label Distribution Methodology
- MPLS Label Switching and Packet Forwarding Overview
- MPLS Label Distribution Protocols Overview
- IP Data Packet Mapping onto MPLS LSPs Overview