Figure 58: LDP Tunneled Through an RSVP-TE Core
In the network topology illustrated in Figure 58 on page 247, 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 on page 247, you
include the address of LSR 5 in the list configured on LSR 2.
Related Topics â– MPLS Label Switching and Packet Forwarding on page 209
â– MPLS Label Distribution Methodology on page 218
â– Mapping IP Data Packets onto MPLS LSPs on page 220
â– MPLS Label Distribution Protocols on page 229
LDP Graceful Restart
The graceful restart mechanism minimizes the negative effect on MPLS forwarding
across an LSR restart. You can configure the neighbors of the LSR to wait for the LSR
to restart (helper mode). When the LSR restarts, the neighbors mark their current
label mapping entries from the LSR as stale. If the LSR recovers within the proper
interval, the entries are no longer marked as stale and are used as they were before
the LDP connection failed. If the LSR does not recover in time, the entries are deleted.
LDP graceful restart supports only the downstream-unsolicited mode of label
distribution. Successful operation of LDP graceful restart requires that stateful SRP
switchover (high availability) be configured on the router. Although you can configure
LDP Graceful Restart â– 247
Chapter 2: MPLS Overview
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