MPLS Protection and Restoration – Part 8 (Actions After the Failure) ⋆ IpCisco

What Happens After the Failure Recovered By Fast Reroute (FRR) ? After detecting the failure in an MPLS network, in Fast Reroute (FRR) mechanism, PLR switches the traffic to the protected tunnel. At the same time it sends a PATH ERR message to the Head-End. In this message, it is said that, the Primary LSP is active but the traffic is flowing through protection path. So in Head-End, the Primary LSP is up and active.

The certain guarantees of the traffic is also provided in backup link. This parameters send from the Head-End to the PLR by Fast Reroute Object.

After getting the PATH ERR message, Head-End starts the “Global Revertive” procedure. It starts a “retry-timer” and after this time expire, it searches a better Primary LSP and starts a CSPF calculation. This “retry-timer” is 30 seconds by default. If it achieve to find a better LSP, switches the traffic to that LSP. If it can not find a better LSP, then it searches after new retry-timer expires.

If a better LSP found and the traffic swicthes to it, a PATH TEAR Message is sent to the old Primary LSP by Head-End. And the old LSP, finishes the detour sessions. This is done for One-to-One Backup only, because at Facility Backup, the Bypass tunnel can protect another LSP too.

At the new LSP, the new detours are also built, because in the PATH Message that is sent on new LSP, still Fast Reroute (FRR) request exist.

For every LSP, there is “retry-timer” and “retry-limit”. The default “retry-timer” is 30 seconds. To configure it use the following command on Alcatel-Lucent 7750 Service Routers:

configure router mpls lsp "to_RouterD" retry-timer [1...600]

The default “retry-limit” is 0. This means that retry until a better path is found. To configure it use the following command on Alcatel-Lucent 7750 Service Routers:

configure router mpls lsp "to_RouterD" retry-limit [1...10000]

By default the calculated LSP can be active forever. But this is configurable by “resignal-timer”. This is disabled by default. The minimum value for resignal-timer is 30 minutes. To configure “resignal-timer” use the below command:

configure router mpls lsp "to_RouterD" resignal-timer [30...10080]

After expire of this resignal-timer, Head-End make a new CSPF calculation. If it finds a better LSP, it switches the traffic to that LSP. If not, the the trafic remains in the same LSP.

If the first Primary LSP come online, then Head-End still needs to wait till resignal-timer expire. After that it make a new CSPF calculation and see the Primary again, and switches the traffic to the previous Primary LSP. Head-End does this optimization, whenever the resignal-timer expires.

If the alternate path can not found by HEad-End, LSP can be torn down or it let it stay on the Backup path. But to keep the traffic forever in the Backup path needs some RSVP messaging. Because without some periodic RSVP path and Resv messages, LSP will time-out. So these messages needs to be forwarded over the Backup link.

You can check the other parts of these MPLS Protection and Restoration Article series below.