Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual page 295

Copyright © 2010, Juniper Networks, Inc.
must have an operational hello adjacency and an operational LDP session, and MPLS
label bindings must have been exchanged over each session.
When LDP has not completed exchanging label bindings with an IGP next hop, traffic is
discarded if the head end of the LSP forwards traffic because the LSP is assumed to be
in place. The following are some examples of when this can happen.
When an LDP hello adjacency or an LDP session with a peer is lost due to some error
while the IGP still points to that peer. IP forwarding of traffic continues on the IGP link
associated with the LDP peer rather than being shifted to another IGP link with which
LDP is synchronized.
When a new IGP link comes up, causing the next hop to a certain destination to change
in the IGP's SPF calculations. Although the IGP might be up on the new link, LDP might
not have completed label exchange for all the routes. This condition might be transient
or due to a misconfiguration.
The LDP protocol is unable to indicate to a dependent service the availability of an
uninterrupted LSP to the desired destination. LDP-IGP synchronization minimizes this
disruption due to LDP not being operational on a link for which the IGP is operational.
When synchronization is in effect, the IGP advertises the maximum possible cost or metric
for that link. If an alternative next hop exists for traffic, the IGP can choose that next hop
for forwarding. If LDP is operational for that next hop, then no traffic is discarded.
The IGP does not advertise the original cost or metric for the link until either LDP label
exchange has been completed with the peer on the link or a configured amount of time
has passed (the holddown period).
With synchronization configured, LDP notifies the IGP to advertise the maximum cost
for the link when one of the following triggering events takes place:
The LDP hello adjacency goes down.
The LDP session goes down.
LDP is configured on an interface.
If the holddown timer has been configured, the timer starts when the triggering event
takes place. When the timer expires, LDP notifies the IGP to resume advertising the original
cost.
If the holddown timer has not been configured, the IGP waits (endlessly) until bindings
have been received from downstream routers for all the FECs that have a next hop on
that interface. Only after that takes place does LDP notify the IGP to bring down the cost
on the interface.
LDP-IGP synchronization is supported only for directly connected peers and links with
the platform label space.
Chapter 3: MPLS Overview
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