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Fast-reroute: failure detection on ATM


I have read in Designing MPLS TE Networks book that in essence fast-reroute feature failure detection of under 50ms is possible on pure POS interfaces and not possible on ATM interfaces which might also be runnig over POS.

I have noticed a command that had been available since 12.1T - "oam ais-rdi". According to this command description, it is possible to bring down ATM PVC after receiving one AIS indicating cell.

Isn't that exactly how it works on POS?

Was this Ciscopress book so much outdated or am I missing something?




Re: Fast-reroute: failure detection on ATM


there is a major difference (on a ms time scale) in POS and in ATM AIS. With POS interfaces one utilizes the error indication in the SONET/SDH frame. This means the first frame after failure detection of end OR intermediate systems (muxer) will contain the error bits needed to bring down the interface and trigger fast reroute.

If you look into an ATM solution, then only ATM equipment will be able to insert ATM cells for failure indication. This means a mux will not be able to indicate anything. Only ATM switches or end devices might "help" out, but for them failure detection is maybe bound to keepalives as well.

So you MIGHT get very low recovery times, but you could also have failure conditions only detectable by keepalive mechanisms. The latter are slower than in POS.

Once you are relying on keepalives you could also use subsecond keepalives with OSPF or ISIS to achieve pretty fast recovery times.

Hope this helps! Please rate all posts.

Regards, Martin

Cisco Employee

Re: Fast-reroute: failure detection on ATM

Hi David,

I believe the book is referring to other interfaces as whole, where software driver is unable to provide 50ms failure detection, Ethernet is a biggest challenge in the past which was a main drive to BFD;

Refer ri this draft about if you need to know what is BFD

FYI, Our 4xport,OC-12 atm is able to do the TE,FRR etc.

Please let me know if I have any further questions



New Member

Re: Fast-reroute: failure detection on ATM


As far as I understand Sonet/SDH header bits indicating link problem, do not have a "power" to bring down ATM interface/sub-interface/VC, eventhough ATM might be running over Sonet/SDH network. That is why that book stated that 50ms switchover is possible on native POS interfaces only.

I was just wandering why would you be able to configure AIS under ATM PVC. Is that a different AIS than one used in Sonet/SDH networks, because in Sonet/SDH networks AIS is what allows fast failure detection ?


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