Can MPLS TE FRR AutoTunnels suffer from temporary loops?

I am currently working through the design studies in the Cisco Press book "Definitive MPLS Network Design"

I have come accross one thing in the USCom FRR AutoTunnel section, that I don't understand.

 

It is the part with Temporary Loop Avoidance. You can read it here http://www.ciscopress.com/articles/article.asp?p=402224&seqNum=7 about half-way down the page.

 

The section sates that in case of reconvergence, when the LSDBs are not yet syncronized, it may lead to temporary loops.

But I don't think this is the case, as the network is using One-Hop tunnels with NHOP backup tunnels.

 

When the traffic hits the St. Louis router, it will not get routed back to Chicago, beacuse St. Louis has a LSP going directly to Washington via Dallas.

Can enyone explain to me, in what condition this would result in a loop?
For sure, it will cause suboptimal routing, but in my opinion never a loop.

 

Regards,

Andreas

Comments

  • I agree with you. I dont see any temporary loop happening. The fact is that the bypass tunnel is pre-signalled. Let's assume it is setup via Chicago from St. louis to Washington. In case of link failure, St. Louis will reroute the traffic towards Chicago. Yes, the LSDB of Chicago might not be updated but it doesn't matter as all it has to do is label-switch towards Washington.

    EDIT: My bad, I didn't check the diagram properly. The backup tunnel is via Dallas. In any case, there will not be any loop as thr traffic is routed over to Washington via Dallas. 

    Cheers,

    AB.

  • This is the first book that I find that talks about Auto-Tunnels! (Not auto-tunnel mesh, but just backup/primary ones). Thank you for pointing it out! 

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