Juniper JUNOSE 11.2.X BGP AND MPLS, Configuration Manual

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timer value. If the neighbor liveness timer expires, the neighbor deletes all the stale
mappings from the LSR. The configurable value represents the maximum time that the
neighbor waits for the restarting LSR to reestablish the LDP session. This enables the
neighbor to avoid having to wait an unreasonably long time set by the reconnect timeout
value from the restarting LSR.
If the recovery time value in the FT session TLV is zero when a neighbor receives the new
LDP initialization message, the neighbor deletes all the stale mappings from the LSR.
If the recovery time value is nonzero, the neighbor starts a neighbor recovery timer set to
the lesser of the two values, the recovery time value and its own configurable maximum
recovery timeout value. The neighbor also cancels its neighbor liveness timer because
the LDP session has been reestablished; it is now waiting on the successful completion
of the restart.
The restarting LSR and its neighbors then exchange label mapping information. When a
neighbor receives a label–to–FEC binding that matches a stale entry, it removes the
staleness marker from the entry. If instead the neighbor receives a new label for the same
FEC that is in a stale entry, the neighbor updates the entry with the new label and removes
the staleness marker from the entry.
The neighbor deletes any stale entries that remain when the neighbor recovery timer
expires.
Dynamic exchange of the graceful restart capability is not supported. In some
circumstances, such as when a standby SRP module is removed, an LSR that has
communicated to neighbors that it supports graceful restart might subsequently be
unable to do so. In such cases, the neighbors receive no indication of that change in
support unless you bounce the LDP sessions, for example by issuing the
clear mpls ldp
neighbor command.
Related Topics Configuring LDP Graceful Restart on page 289
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LDP-IGP Synchronization Overview
LDP is often used to establish MPLS LSPs throughout a complete network domain using
an IGP such as OSPFv2 or IS-IS. In such a network, all links in the domain have IGP
adjacencies as well as LDP adjacencies. LDP establishes the LSPs on the shortest path
to a destination as determined by IP forwarding.
MPLS data packets can be discarded in these networks when the network IGP is
operational on a link for which LDP is not fully operational, because there is no coupling
between the LDP operational state and the IGP. When LDP is not fully operational, LDP
is considered to not be synchronized with the IGP.
This issue is especially significant for applications such as a core network that does not
employ BGP. Another example is an MPLS VPN where each given PE router depends on
the availability of a complete MPLS forwarding path to the other PE routers for each VPN
that it serves. This means that along the shortest path between the PE routers, each link
Copyright © 2010, Juniper Networks, Inc. 258
JunosE 11.2.x BGP and MPLS Configuration Guide