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LDP sync state in 'show ospf interface extensive' may show 'LDP session up' or 'LDP adjacency up'

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Article ID: KB37373 KB Last Updated: 16 Sep 2021Version: 1.0
Summary:

When 'ldp-synchronization' is enabled for OSPF, ensure that LDP is fully established before the IGP path is used for forwarding traffic.
In the output of '
show ospf interface extensive', the 'LDP sync state' should appear under each interface. 

In most scenarios, when everything is up and stable, you should see 'LDP session up' as the reason for 'LDP sync state'. However, in certain scenarios, you may see 'LDP adjacency up' for 'LDP sync state'.

This article will explain a scenario when 'LDP adjacency up' is seen instead of 'LDP session up' as the expected behavior.

Solution:
In a scenario, where there are 2 interfaces (layer 3 interfaces, not one AE bundle) between 2 routers, OSPF is established in each interface. LDP session is established between the loopback of those 2 routers by default, so there will be 1 LDP session and 2 OSPF interfaces. When those sessions established post reboot or configured, the output displays all reasons to be 'LDP session up'.
regress@PE1_re> show ospf interface extensive    
Jun 03 06:29:23
Interface           State   Area            DR ID           BDR ID          Nbrs
ge-0/0/3.0          PtToPt  0.0.0.0         0.0.0.0         0.0.0.0            1
  Type: P2P, Address: 10.0.35.170, Mask: 255.255.255.252, MTU: 9178, Cost: 100
  Adj count: 1
  Hello: 10, Dead: 40, ReXmit: 5, Not Stub
  Auth type: None
  Protection type: Link
  Topology default (ID 0) -> Cost: 100
  LDP sync state: in sync, for: 00:00:02, reason: LDP session up
           config holdtime: infinity
ge-1/0/3.0          PtToPt  0.0.0.0         0.0.0.0         0.0.0.0            1
  Type: P2P, Address: 10.0.35.174, Mask: 255.255.255.252, MTU: 9178, Cost: 100
  Adj count: 1
  Hello: 10, Dead: 40, ReXmit: 5, Not Stub
  Auth type: None
  Protection type: Link
  Topology default (ID 0) -> Cost: 100
  LDP sync state: in sync, for: 00:00:02, reason: LDP session up
           config holdtime: infinity
lo0.0               Waiting 0.0.0.0         0.0.0.0         0.0.0.0            0
  Type: LAN, Address: 128.51.138.74, Mask: 255.255.255.255, MTU: 65535, Cost: 0
  Priority: 128
  Adj count: 0
  Hello: 10, Dead: 40, ReXmit: 5, Not Stub
  Auth type: None
  Protection type: None
  Topology default (ID 0) -> Cost: 0


If one of the OSPF interface flaps, the OSPF session on that interface turns down. However, the LDP session and the other OSPF session remains up. When it comes back, the OSPF session on that interface establishes again. However, the LDP session never flaps. Hence, the LDP can only tell OSPF is 'Adjacency up' instead of 'Session up'.

The output will appear as below. This is an expected behavior and has no affect to any of the protocols or services.
regress@PE1_re> show ospf interface extensive    
Jun 03 06:30:22
Interface           State   Area            DR ID           BDR ID          Nbrs
ge-0/0/3.0          PtToPt  0.0.0.0         0.0.0.0         0.0.0.0            1
  Type: P2P, Address: 10.0.35.170, Mask: 255.255.255.252, MTU: 9178, Cost: 100
  Adj count: 1
  Hello: 10, Dead: 40, ReXmit: 5, Not Stub
  Auth type: None
  Protection type: Link
  Topology default (ID 0) -> Cost: 100
  LDP sync state: in sync, for: 00:01:00, reason: LDP session up
           config holdtime: infinity
ge-1/0/3.0          PtToPt  0.0.0.0         0.0.0.0         0.0.0.0            1
  Type: P2P, Address: 10.0.35.174, Mask: 255.255.255.252, MTU: 9178, Cost: 100
  Adj count: 1
  Hello: 10, Dead: 40, ReXmit: 5, Not Stub
  Auth type: None
  Protection type: Link
  Topology default (ID 0) -> Cost: 100
  LDP sync state: in sync, for: 00:00:07, reason: LDP adjacency up  
           config holdtime: infinity
lo0.0               DR      0.0.0.0         128.51.138.74   0.0.0.0            0
  Type: LAN, Address: 128.51.138.74, Mask: 255.255.255.255, MTU: 65535, Cost: 0
  DR addr: 128.51.138.74, Priority: 128
  Adj count: 0
  Hello: 10, Dead: 40, ReXmit: 5, Not Stub
  Auth type: None
  Protection type: None
  Topology default (ID 0) -> Cost: 0
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