Juniper JUNOSE, Configuration Manual

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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.
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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:
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The LDP hello adjacency goes down.
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The LDP session goes down.
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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.
Synchronization Behavior During Graceful Restart
LDP-IGP synchronization does not take place while the IGP is in the process of a
graceful restart. When the graceful restart completes, links for which synchronization
has been configured are advertised with maximum metrics in either of the following
cases:
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LDP is not yet operational on the link and no holddown timer has been
configured.
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The configured holddown timer has not expired.
LDP-IGP Synchronization
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Chapter 2: MPLS Overview