HP HSR6800 Configuration Manual page 30

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GR restarter—Router that gracefully restarts due to a manually configured command or a fault.
It must be GR-capable.
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GR helper—Neighbor of the GR restarter. A GR helper maintains the neighbor relationship with
the GR restarter and helps the GR restarter restore its LFIB information. A GR helper must be
GR-capable.
Figure 12 LDP GR
GR helper
Two LDP peers perform GR negotiation when establishing an LDP session. The LDP session
established is GR capable only when both peers support LDP GR.
LDP GR works in the following procedure:
1. Whenever restarting, the GR restarter preserves all MPLS forwarding entries, marks them as
stale, and starts the MPLS forwarding state holding timer for them.
2. After a GR helper detects that the LDP session with the GR restarter is down, it marks the
FEC-label bindings learned from the session as stale and keeps these FEC-label bindings for a
period of time defined by the fault tolerant (FT) reconnect time argument. The FT reconnect
time is the smaller one between the reconnect time advertised from the peer GR restarter and
the neighbor liveness time configured locally.
3. During the FT reconnect time, if the LDP session fails to be re-established, the GR helper
deletes the FEC-label bindings marked stale.
4. If the session is re-established successfully, during the LDP recovery time, the GR helper and
the GR restarter uses the new LDP session to exchange the label mapping information, update
the LFIB, and delete the stale marks of the corresponding forwarding entries. The LDP recovery
time is the smaller one between the recovery time configured locally and that configured on the
peer GR restarter.
5. After the recovery time elapses, the GR helper deletes the FEC-label bindings that are still
marked stale.
6. When the MPLS forwarding state holding timer expires, the GR restarter deletes the label
forwarding entries that are still marked stale.
Configuration prerequisites
Configure MPLS LDP capability on each device acting as the GR restarter or a GR helper. (The
device can act as a GR restarter or a GR helper as needed in the LDP GR process.)
GR helper
GR restarter
GR helper
LDP session with GR capability
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