Juniper JUNOSE, Configuration Manual

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During LDP graceful restart, no synchronization operations are done. If the LDP
graceful restart is terminated, LDP notifies the IGPs to advertise the links with the
maximum metric.
Synchronization Behavior on LAN Interfaces
LDP-IGP synchronization does not take place on LAN interfaces unless the IGP has
a point-to-point connection over the LAN configured on the interface. The reason for
this is that multiple LDP peers might be connected on such an interface unless a
point-to-point connection to a single peer has been configured. Because
synchronization raises the cost on the interface high enough to prevent traffic being
forwarded to that link, if multiple peers are connected, the cost is raised on all the
peers even though LDP might be unsynchronized with only one of the peers.
Consequently, traffic is diverted away from all the peers, an undesirable situation.
Synchronization Behavior on IGP Passive Interfaces
On IGP passive interfaces, the link cost is not raised when LDP-IGP synchronization
is configured and a triggering event occurs.
Synchronization and TE Metrics
When traffic engineering is configured for an IGP, LDP-IGP synchronization does not
affect the traffic engineering metric advertised for the link, regardless of whether the
TE metric is explicitly configured or the default value.
Determining Peer Reachability with RSVP-TE Hello Messages
RSVP-TE hello messages enable the router to detect when an RSVP-TE peer is no
longer reachable. When the router makes this determination, all LSPs that traverse
that neighbor are torn down. Hello messages are optional and can be ignored safely
by peers that are not configured to use the feature.
When you enable the hello feature on a virtual router or interface configured with
RSVP-TE, that RSVP-TE node periodically sends a unicast hello message to each
neighbor with which the node has established an LSP. The exchange of hello messages
between the peers establishes a hello adjacency. You can configure the hello interval
to establish how frequently the node sends hello messages. Hello messages are
exchanged when an LSP is set up and are stopped when the last LSP between the
two peers goes away.
You can use the hello feature to reduce the impact of RSVP-TE on system resources.
Because a hello timeout is treated as a link failure, RSVP-TE can use the hello timeout
instead of path and resv timeouts to determine when to bring down an LSP. High
RSVP-TE refresh values reduce the amount of control traffic (and CPU cycles) needed
by RSVP-TE to refresh LSP state across the network, thus reducing the impact of
RSVP-TE on system resources.
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Determining Peer Reachability with RSVP-TE Hello Messages
JUNOSe 11.0.x BGP and MPLS Configuration Guide