OSPF-LDP Association
Definition
In a network 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 original LSP is deleted before a new LSP is established and LSP traffic is interrupted.
Purpose
As shown in Figure 5-22, 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.
Associating Label Distribution Protocol (LDP) and IGP on P1 and P2 can shorten traffic interruption caused by traffic switchback from the backup link to the primary link.
OSPF-LDP Association |
Traffic Interruption Period |
|---|---|
No |
Seconds |
Yes |
Milliseconds |
Implementation
IGP-LDP association delays 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. After an LSP is established on the primary link, traffic is switched back to the primary link.
LDP-IGP association involves three timers:
Hold-down
Hold-max-cost
Delay
After the primary link recovers on a router, the router responds as follows:
Starts the Hold-down timer. The IGP interface does not establish IGP neighbor relationships but waits for the establishment of an LDP session. The Hold-down timer specifies the period that the IGP interface waits.
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 cost of the interface in an LSA to the primary link.
Starts the Delay timer to wait for the establishment of an LSP after an LDP session is re-established for the primary link.
Enables LDP to notify IGP that synchronization is complete regardless of the IGP status after the Delay timer expires.