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[QFX] BGP flap on QFX5100 with BFD configured when running multiple 'request support information' commands at same time

  [KB34061] Show Article Properties


Summary:

QFX5100 has a hardware limitation that might cause BFD hello packets to be lost / dropped when BFD timers are set below the supported settings. The minimum supported BFD timer is 1000ms. This issue was reproduced with BFD timers set to 300 and 350.

 

Symptoms:

When running a RSI, the PPM drops BFD hello packets. Then the BGP session will drop.

The following conditions need to be in place for this issue to occur:
1)  BFD is set to 300ms (an unsupported timer).
2)  More than one RSI is run at the same time, i.e., an automated script running na RSI and a user logged into a switch running an RSI not knowing the script is running at that same time.
3)  Some event that causes the PFE micro kernel to be busy and miss a BFD hello packet because the timer is set to 300ms.

 

Cause:

The microkernel polls the PFE looking for packets. The PFE does NOT interrupt and send to the microkernel. This is the reason for the recommended 1000ms (1sec)  BFD timers.

root@lab-switch-1> start shell
root@lab-switch:RE:0% vty fpc0
FPC0(lab-switch vty)# sh ppm adjacencies

PPM distributed adjacencies

Protocol   Holdtime (msec) PPM handle  Inline

BFD        3000            7           No
BFD        3000            8           No
LACP       3000            17          No
LACP       3000            21          No
LACP       3000            22          No
LACP       3000            18          No
LACP       3000            19          No
LACP       3000            20          No

Total adjacencies: 8    <-- at risk if this number is 20 or higher and BFD is set to 350 or less
Solution:

This is a product/hardware limitation.  You can either lower the PPM adjacencies, or raise the BFD timer.  You can also deactivate BFD, then collect the RSI.

Modification History:
2019-08-23: Updated the symptoms and cause with more details.
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