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We also looked at various topologies and their applicability to LFA’s. We looked at Microloops and how LFA and Remote LFA tries to mitigate them. So as you can see that LFA/RLFA doesn’t completely eliminate MicroLoops. Ordered FIB is one solution which can solve this problem and I will leave that research to the reader. But WASH FIB is still pointing to NYC for DEN traffic until WASH receives the LSP update, runs the SPF and programs the new route. Till then we will have a MicroLoop between NYC-WASH.

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Let’s assume LSP update reaches NYC first, it runs the SPF and programs the new route (Which will be through WASH) and start sending the traffic towards WASH. So what other events will occur as a result of KYC to STL link failure? STL will send an LSP update to the other nodes about the failure of STL-KYC. Please recall that ATL isn’t a valid LFA as ATL’s best path goes through STL for DEN (Inequality#1 is false 180 < 60 + 120). HSTN in our case and tunnels the traffic towards him. Now assume that we have IP FRR enabled in the network and we have a link failure between STL and KYC. So let’s look at a POP with a ring topology like below. As you can see in Fig.15, R3 doesn’t meet inequality#1 (3 CHI->STL->KYC-DEN path as shown in the diagram. This kind of multi-hop repair paths are more complicated than single hop repair paths as computations are needed to determine if a path exists (to begin with) and then a mechanism to send the packet to that hop. This problem can be solved if we can find a router which is more than one hop away from the calculating router, from which traffic will be forwarded to the destination without traversing the failed link and somehow we tunnel the packet to that router. Reason is simple i.e.in many cases backup next hop best path goes through the router calculating the backup next hop.

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This is a continuation from Part 1 Remote LFAĪt this point we already know that simple LFA doesn’t always provide full coverage and its very topology dependent.

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