Problem:
ldp establishes mpls lsps path that determined by the IGP best route ,if the lsp not formed between the PE routers, a blackhole exists and services that depending on mpls forwading will fail,this feature will help to avoid packet lost in this situations:
1.when the IGP adjacency is up and the LDP sessions are not establishes or the lable-bindings not excheanged with the neighbor.
2.when someone remove the ldp under the interface and the LDP session not form (Configuration error)
Solution:
the solution is to discourage a link from being used as long as the LDP sessions are not established.
so when you enable the feature of IGP-LDP sync and the LDP is not fully operational on a given link,
the IGP will advertise the cost of the link to be maximum in order to avoid any traffic to flow through it,
The actual value advertised in OSPF (65535).
The actual value advertised in IS-IS regular metric (63) and in IS-IS wide-metric is (16777214).
in Cisco :
enter the command mpls ldp igp sync under the IGP (ospf or isis) the IGP waits indefinitely for LDP session to be fully operational ,To limit the length of time the IGP session must wait, enter the no mpls ldp igp sync holddown command
in ALU :
enter the command ldp-sync-timer seconds .
source:
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