Disable BFD in multiple Router Sub interfaces that participates in OSPF

[edgegwapo]

Hi team,

Please help me on this. Here is the scenario:

1. We are on an enterprise set up and running on 100+ routers.
2. We have 200 to 300+ sub interfaces for virtual circuits
3. Our protocol is OSPF over MPLS
4. One of our provider in LA encountered link flaps on SONET causing our LA router that is directly connected to that link to recalculate multiple times.
5. Recalculation of OSPF routes caused disconnection of users in LA VM's.
6. We were advised by our provider in LA to disable BFD so minor link flaps will no affect recalculation of routes.
7. We are now tasked by our design team to Disable BFD in multiple Router Sub interfaces that participates in OSPF.

My questions are:

1. What is the implication in disabling all BFD in routers' interface and sub interface?
2. Will this improve recalculation of OSPF routes in cause of link flaps or it will totally ignore the link flaps?
3. Will the routers only recognize a "full down" status of the interface?
4. How can we Disable BFD in multiple Router Sub interfaces that participates in OSPF in a faster way? Or do we have to do this one by one?

Please advise before we present this to the CAB and implementation. Thank you.

[Mohamed Sobair]

My questions are:

1. What is the implication in disabling all BFD in routers' interface and sub interface?

Answer:  the implication would be eliminating sub-second millisecond convergence.

BFD detect failure at the link layer very fast , once detected it informs the upper layer protocol about the failure causing it to converge immediately. 

1. Will this improve recalculation of OSPF routes in cause of link flaps or it will totally ignore the link flaps?

Answer: if your Provider experiencing intermittent flaps, then yes it will be advisable to turn BFD off. this however doesn't totally ignore the link flaps, once the upper protocol detect the failure based on the dead interval parameter on OSPF, it will recalculate OSPF routes again.  Keep in mind, if you have redundant or more links to your provider , then I wouldn't recommend disabling BFD , as it should improve Convergence and you shouldn't notice the failure. 

1. Will the routers only recognize a "full down" status of the interface?

Answer: disabling BFD allows the router recognize a full down status once the upper protocol dead interval occurs or full down status of interface. which ever occurs the earliest.

1. How can we Disable BFD in multiple Router Sub interfaces that participates in OSPF in a faster way? Or do we have to do this one by one?

You can disable it one by one. or if you have configuration management software, it allows you to do it for all nodes at a time. but this depends if you have it or not.

Please consider not to disable BFD if you have multiple OSPF links towards your provider from any branch, as it shouldn't impact your VMs, it should rather improve Convergence at milliseconds which is absolutely not noticeable.

BR,

Mohamed 

[edgegwapo]

Thank you for the answer. We will have this reviewed.

[paul driver]

Hello

1.What is the implication in disabling all BFD in routers' interface and sub interface? - Once disabled a BFD session to its peers should be seen in a admin down state, meaning bfd is awaiting to negotiate via a control packet - So its as if BFD isnt applied at the other end.

2.Will this improve recalculation of OSPF routes in cause of link flaps or it will totally ignore the link flaps? - It could - as my understanding is ospf will determine if it needs  to terminate its peering session based on bfd, Once bfd is disabled  ospf convergence will be as per hello+ dead timers unless you've applied other fast convergence methods and if your link flaps are still occurring and then you could possibly have another issue with you connections.

3.Will the routers only recognize a "full down" status of the interface? - Not sure what your asking here - do you mean if BFD is disabled will opsf take that as a full down status if so - My understanding is No , otherwise answer 2 would be applicable

4.How can we Disable BFD in multiple Router Sub interfaces that participates in OSPF in a faster way? Or do we have to do this one by one? - Not sure there is any other way but manually one by one on subinterface unless the the global bfd all interface disabled command is applicable to bfd configured subinterfaces?

res
Paul

 

[edgegwapo]

Thank you for the answer. We will have this reviewed.