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LDP GR
LDP GR overview
LDP Graceful Restart (GR) enables an LSR to retain MPLS forwarding entries during an LDP restart,
ensuring continuous MPLS forwarding.
Figure 12 LDP GR
As shown in Figure 12, GR defines the following roles:
• GR restarter—An LSR that performs GR. It must be GR-capable.
• GR helper—A neighbor LSR that helps the GR restarter to complete GR.
The device can act as a GR restarter or a GR helper.
Figure 13 LDP GR operation
As shown in Figure 13, LDP GR works in the following steps:
1. LSRs establish an LDP session. The L flag of the Fault Tolerance TLV in their Initialization
messages is set to 1 to indicate that they support LDP GR.
2. When LDP restarts, the GR restarter starts the MPLS Forwarding State Holding timer, and
marks the MPLS forwarding entries as stale. When the GR helper detects that the LDP session
with the GR restarter goes down, it performs the following operations:
a. Marks the FEC-label mappings learned from the session as stale.
GR helper
GR helper GR helper
GR restarter
LDP session
LDP session
LDP session
GR restarter
GR helper
Send label mappings
Set up an LDP session, and identify that
they are LDP
GR capable
Protocol
restarts
MPLS
forwarding state
holding time
Reconnect time
LDP recovery time
Re-establish the LDP session
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