Ldp Graceful Restart; Control Plane Failure - Cisco CRS Configuration Manual

Implementing MPLS Label Distribution Protocol

LDP Graceful Restart

LDP (Label Distribution Protocol) graceful restart provides a control plane mechanism to ensure high
availability and allows detection and recovery from failure conditions while preserving Nonstop Forwarding
(NSF) services. Graceful restart is a way to recover from signaling and control plane failures without impacting
forwarding.
Without LDP graceful restart, when an established session fails, the corresponding forwarding states are
cleaned immediately from the restarting and peer nodes. In this case LDP forwarding restarts from the
beginning, causing a potential loss of data and connectivity.
The LDP graceful restart capability is negotiated between two peers during session initialization time, in FT
SESSION TLV. In this typed length value (TLV), each peer advertises the following information to its peers:
Reconnect time
Recovery time
FT flag
Once the graceful restart session parameters are conveyed and the session is up and running, graceful restart
procedures are activated.
When configuring the LDP graceful restart process in a network with multiple links, targeted LDP hello
adjacencies with the same neighbor, or both, make sure that graceful restart is activated on the session before
any hello adjacency times out in case of neighbor control plane failures. One way of achieving this is by
configuring a lower session hold time between neighbors such that session timeout occurs before hello
adjacency timeout. It is recommended to set LDP session hold time using the following formula:
Session Holdtime <= (Hello holdtime - Hello interval) * 3
This means that for default values of 15 seconds and 5 seconds for link Hello holdtime and interval respectively,
session hold time should be set to 30 seconds at most.
For more information about LDP commands, see MPLS Label Distribution Protocol Commands module of
the Cisco IOS XR MPLS Command Reference for the Cisco CRS Router.
Related Topics
Setting Up LDP NSF Using Graceful Restart, on page 43
Configuring LDP Nonstop Forwarding with Graceful Restart: Example, on page 82

Control Plane Failure

When a control plane failure occurs, connectivity can be affected. The forwarding states installed by the router
control planes are lost, and the in-transit packets could be dropped, thus breaking NSF.
Advertises the maximum time that other peer will wait for this LSR to reconnect after control channel
failure.
Advertises the maximum time that the other peer has on its side to reinstate or refresh its states with
this LSR. This time is used only during session reestablishment after earlier session failure.
Specifies whether a restart could restore the preserved (local) node state for this flag.
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
LDP Graceful Restart
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