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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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OSPF-LDP Association

OSPF-LDP Association


In the networking that uses primary and backup links, when the faulty primary link recovers, traffic is switched from the backup link back to the primary link.

IGP route convergence completes before an LDP session is established. Consequently, the old LSP is deleted before the new LSP is established and LSP traffic is interrupted.


As shown in Figure 5-20, the primary link adopts the path PE1→P1→P2→P3→PE2, and the backup link adopts the path PE1→P1→P4→P3→PE2.

When the primary link is faulty, traffic is switched to the backup link. After the primary link recovers, traffic is switched back to the primary link. During this process, traffic is interrupted for a long period of time.

Figure 5-20 OSPF-LDP association

Synchronizing Label Distribution Protocol(LDP) and IGP on P1 and P2 can shorten traffic interruption caused by traffic switchover from the backup link to the primary link.

Table 5-16 OSPF-LDP association
Enabling Status of OSPF-LDP Association Traffic Interruption Time
Not enabled. Seconds level
Enabled. Milliseconds level


The principle of LDP-IGP synchronization is to delay route switchback by suppressing the establishment of IGP neighbor relationships until LDP convergence is complete. That is, before an LSP on the primary link is established, the backup link continues to forward traffic. Then the link is deleted after the LSP is established.

Synchronization of LDP and IGP involves three timers:

  • Hold-down

  • Hold-max-cost

  • Delay

After the primary link recovers, a router responds as follows:

  1. Starts the hold-down timer. The IGP interface does not establish IGP neighbors but waits for establishment of an LDP session. The Hold-down timer specifies the period that the IGP interface waits.

  2. Starts the hold-max-cost timer after the hold-down timer expires. The hold-max-cost timer specifies the interval for advertising the maximum link metric of the interface in the Link State Advertisement (LSA) to the primary link.

  3. Starts the Delay timer to allow time for establishment of an LSP after an LDP session is re-established for the faulty link.

  4. After the Delay timer expires, LDP notifies IGP that synchronization is complete regardless of the status of IGP.

Updated: 2019-12-27

Document ID: EDOC1000174069

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