SD WAN : BFD + OSPF

[RS19]

In Cisco SD-WAN want to understand how BFD & OSPF co-work together.

I understand that BFD is used to monitor the underlay. If it find that the remote end point is not responding it brings down the bfd session. Want to understand how this works along with the OSPF. OSPF as routing protocol has it own timers to bring up and bring down the OSPF neighbourship. Wanted to understand how this both works together ?

[RS19]

Also wanted to understand what is the meaning of the below alert.

2024/7/19 21:08 SDWAN 10.1.1.13 bfd-state-change major BFD host-name=SDWAN; src-ip=10.2.255.2; dst-ip=10.3.255.1; proto=ipsec; src-port=12386; dst-port=12386; local-system-ip=10.1.1.13; local-color=metro-ethernet; remote-system-ip=10.1.254.1; remote-color=metro-ethernet; new-state=down; deleted=false; flap-reason=timeout

[MHM Cisco World]

Ospf first send hello and it make established 

Then bfd start send periodically to neighbors when it down "bfd" it inform ospf to down neighbors

The  circle start again. 

MHM

[RS19]

So u mean OSPF timers will not be effective. BFD timers are the one which will be effective.

[MHM Cisco World]

Yes ospf hello is in "S" but the bfd is in "mS"

And for your note in SD-WAN not only ospf use bfd but all peers vmanage vedge vsmart vbond use bfd to mark neighbor UP or down

MHM

[MHM Cisco World]

This error message is for SDWAN bfd  not underlying.

This command to see SDWAN bfd status 

vEdge-2# show bfd sessions 

[RS19]

Thanks, I checked my existing configuration.  For BFD the below are the timings currently configured.
Multiplier - 6
Poll Interval - 120000 milliseconds  (equals to 2 minutes)

Considering the above value, it looks more than the OSPF timers. Is my understanding right ?

[MHM Cisco World]

Friend I think you misunderstanding my previous comment 

There are two bfd 

https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/routing/ios-xe-17/routing-book-xe/m-bfd-for-routing-protocols.html

One for overlaying which run inside tunnel between deivce 

Other legacy for underlying' this must be less than ospf hello otherwise it no need to config it

MHM

[RS19]

ok got it.

In SD-WAN how to check which is the overlay BFD and which is the underlay BFD ?

The above which I shared is it underlay BFD or overlay BFD?

[MHM Cisco World]

It for overlay BFD friend.

MHM

[RS19]

Thanks.
How should I check the underlay BFD ?

[RS19]

cEdge# show sdwan running bfd

bfd default-dscp 48
bfd app-route multiplier 6
bfd app-route poll-interval120000

The above is the output I got. From your reply it is overlay right ?

[MHM Cisco World]

Yes it for overlay sure 

MHM

[RS19]

My understanding is that these overlay BFD is mainly used for application aware routing. 
Not with OSPF. To work with OSPF it uses underlay BFD. Is my understanding right ?

[RS19]

Pls clarify. So BFD runs under IPSec . Is my understanding correct ?

For ex R1 - 100.0.0.1 , R2 - 200.0.0.1 These are transport WAN IPs of R1 & R2 (public IPs). So does BFD also runs between these 2 IPs to detect any underlay fault ?