Igp-Ldp And Static Route-Ldp Synchronization - Alcatel-Lucent 7705 Configuration Manual

Configuring IP Router Parameters

IGP-LDP and Static Route-LDP Synchronization

With LDP, FECs learned from an interface do not necessarily link to that interface state. As
long as the router that advertised the label(s) is reachable, the learned labels are stored in the
incoming label map (ILM) table.
Although this feature gives LDP a lot of flexibility, it can also cause problems. For
example, when an interface comes back up from a failure or from a shutdown state, the static
routes bound to that interface are installed immediately. However, the LDP adjacency to the
next hop might not be up, which means that the LDP SDP remains down. In this case, the
MPLS traffic will be blackholed until the LDP adjacency comes up.
The same issue is also applicable to dynamic routes (OSPF and IS-IS).
To resolve this issue, the LDP synchronization timer enables synchronization of IGP or static
routes to the LDP state.
With IGP, when a link is restored after a failure, IGP sets the link cost to infinity and
advertises it. The value advertised in OSPF is 0xFFFF (65535). The value advertised in
IS-IS regular metric is 0x3F (63) and in IS-IS wide-metric is 0xFFFFFE (16777214).
After IGP advertises the link cost, the LDP hello adjacency is brought up with the neighbor.
The LDP synchronization timer is started by IGP from the time the LDP session to the
neighbor is up over the interface. This synchronization timer allows time for the label-FEC
bindings to be exchanged.
When the LDP synchronization timer expires, the link cost is restored and is readvertised.
IGP will announce a new best next-hop and LDP will use it if the label binding for the
neighbor's FEC is available.
The above behavior is similar for static routes. If the static route is enabled for ldp-sync,
the route is not enabled immediately after the interface to the next hop comes up. Routes are
suppressed until the LDP adjacency with the neighbor comes up and the synchronization
timer expires. The timer does not start until the LDP adjacency with the neighbor node is fully
established. For static routes, the ldp-sync-timer function requires LDP to use the
interface address, not the system address, as its transport address.
40 7705 SAR OS Router Configuration Guide