This KB will explain a change in behaviour with inline BFD.

• A device can either support hardware-assisted or software-assisted inline BFD.
• After 1819988, there is a way to change inline BFD from hardware-assisted (Default) to software-assisted

For QFX5120-48Y & QFX5120-32C “Inline BFD” was supported from 21.2R1 and was Hardware-assisted BFD.

Below are the limitations of “Hardware-assisted inline BFD”:

• Cannot be used with ECMP on QFX5120 Series devices.
• Cannot be used with VXLAN encapsulation of BFD packets.

Hence, you would see ECMP BFD neighbours stuck in an INIT state.

root@qfx5120# run show bfd session
                                                  Detect   Transmit
Address                  State     Interface      Time     Interval  Multiplier
10.26.232.2              Init                     6.000     2.000        3 ---> BFD down
10.26.232.10             Init                     6.000     2.000        3 ---> BFD down
192.168.254.129          Up        et-0/0/1.0     1.050     0.350        3  
192.168.254.133          Up        et-0/0/31.0    1.050     0.350        3  
 
4 sessions, 4 clients
Cumulative transmit rate 6.7 pps, cumulative receive rate 6.7 pps.
root@qfx5120#

After the PR1819988, a knob was added to switch from hardware-assisted and software-assisted BFD. Customers can continue to use Inline BFD with a sw-assisted knob in place and have subsecond intervals configured on the QFX5120.

Before adding the knob:

root@qfx5120# run show bfd session
                                                  Detect   Transmit
Address                  State     Interface      Time     Interval  Multiplier
10.26.232.2              Init                     6.000     2.000        3 ---> BFD down
10.26.232.10             Init                     6.000     2.000        3 ---> BFD down
192.168.254.129          Up        et-0/0/1.0     1.050     0.350        3  
192.168.254.133          Up        et-0/0/31.0    1.050     0.350        3  
 
4 sessions, 4 clients
Cumulative transmit rate 6.7 pps, cumulative receive rate 6.7 pps.
root@qfx5120#

# set forwarding-options inline-bfd-sw-based ---> The device will use "software-assisted inline BFD".
 

root@qfx5120# show routing-options ppm     
inline-processing-enable;

root@qfx5120# set forwarding-options inline-bfd-sw-based
root@qfx5120# commit
root@qfx5120# run clear bfd session all 

After adding the knob:

root@qfx5120# run show bfd session
                                                  Detect   Transmit
Address                  State     Interface      Time     Interval  Multiplier
10.26.232.2              Up                       3.000     1.000        3 ---> BFD up
10.26.232.10             Up                       3.000     1.000        3 ---> BFD up 
192.168.254.129          Up        et-0/0/1.0     1.050     0.350        3  
192.168.254.133          Up        et-0/0/31.0    1.050     0.350        3  
 
4 sessions, 4 clients 
Cumulative transmit rate 6.7 pps, cumulative receive rate 6.7 pps.
root@qfx5120#