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Feature Description - MPLS 01

NE05E and NE08E V300R003C10SPC500

This is NE05E and NE08E V300R003C10SPC500 Feature Description - MPLS
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LDP-IGP Synchronization

LDP-IGP Synchronization


LDP-IGP synchronization enables the LDP status and the IGP status to go Up simultaneously, which helps minimize traffic interruption time if a fault occurs.

On a network with both primary and backup LDP LSPs, if the primary LDP LSP fails, both IGP and LSP traffic switches to the standby link. When the primary LSP recovers, IGP traffic is switched back to the primary LSP earlier than LDP traffic because IGP route convergence is faster than LDP convergence. As a result, LSP traffic is lost because the primary LSP is not reestablished.When the LDP session between nodes on the active fails, the LSP on the link is deleted, but the IGP continues to use the active link. As a result, LSP traffic cannot be switched to the standby link, and packets are lost continuously.


LDP-IGP synchronization supports OSPFv2 and IS-IS IPv4.

On a network enabled with LDP-IGP synchronization, an IGP keeps advertising the maximum cost of an IGP route over the new active link to delay IGP route convergence until LDP converges. Traffic keeps switching back and forth between the standby and active links. The backup LSP is torn down after the LSP on the active link is established.

LDP-IGP synchronization involves the following timers:

  • Hold-max-cost timer

  • Delay timer


Figure 3-10 Switchback in LDP-IGP synchronization

  • In Figure 3-10, a network has both an active and standby link. When the active link recovers from any fault, traffic is switched from the standby link to the active link. During the traffic switchback, the backup LSP cannot be used, and a new LSP cannot be set up over the active link once IGP route convergence is complete. This causes a traffic interruption for a short period of time. To help prevent this problem, LDP-IGP synchronization can be configured to delay the IGP route switchback until LDP converges. The backup LSP is not deleted and continues forwarding traffic until an LSP over the active link is established. The process of LDP-IGP synchronization is as follows:
    1. A link recovers from a fault.

    2. An LDP session is set up between LSR2 and LSR3. The IGP advertises the maximum cost of the active link to delay the IGP route switchback.

    3. Traffic is still forwarded along the backup LSP.

    4. Once set up, the LDP session transmits Label Mapping messages and advertises the IGP to start synchronization.

    5. The IGP advertises the normal cost of the active link, and its routes converge on the original forwarding path. The LSP is reestablished and delivers entries to the forwarding table.The whole process usually takes milliseconds.

  • If an LDP session between two nodes on an active link fails, the LSP on the active link is torn down, but the IGP route for the active link is reachable. In this case, traffic fails to switch from the primary LSP to a backup LSP and is discarded. To prevent this problem, LDP-IGP synchronization can be configured so that after an LDP session fails, LDP notifies an IGP of the failure. The IGP advertises the maximum cost of the failed link, which enables the route to switch from the active link to the standby link. In addition to the LSP switchover from the primary LSP to the backup LSP, LDP-IGP synchronization is implemented. The process of LDP-IGP synchronization is as follows:
    1. An LDP session between two nodes on an active link fails.

    2. LDP notifies an IGP of failure in the LDP session over which the primary LSP is established. The IGP then advertises the maximum cost along the active link.

    3. The IGP route switches to the standby link.

    4. A backup LSP is set up over the standby link and then forwarding entries are delivered.

    To prevent an LDP session from failing to be established, you can set the value of a Hold-max-cost timer to always advertise the maximum cost, which enables traffic to be transmitted along the backup link before the LDP session is reestablished on the active link.
  • LDP-IGP synchronization state machine

    After LDP-IGP synchronization is enabled on an interface, the LDP-IGP synchronization state machine operates based on the flowchart shown in Figure 3-11.
    Figure 3-11 LDP-IGP synchronization status transition

    Note differences when different IGP protocols are used:
    • When OSPF is used, the status transits based on the flowchart shown in Figure 3-11.
    • When IS-IS is used, the Hold-normal-cost state does not exist. After the Hold-max-cost timer expires, IS-IS advertises the actual link cost, but the Hold-max-cost state is displayed even though this state is nonexistent.

Usage Scenario

Figure 3-12 shows an LDP-IGP synchronization scenario.

On the network shown in Figure 3-12, an active link and a standby link are established. LDP-IGP synchronization and LDP FRR are deployed.
Figure 3-12 LDP-IGP synchronization scenario


Packet loss is reduced during an active/standby link switchover, improving network reliability.

Updated: 2019-01-14

Document ID: EDOC1100058933

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