Hello David
Hmm that is strange. The first thing I’d like to address is the fact that in the Wireshark capture, your multiplier is 5, and your Tx and Rx intervals are set to 1000 ms, and the Echo Interval is 9000 ms. Those values are different from what you set up in the two routers.
I don’t have an answer for your concerning why these are different, however, I do have some information bout how BFD operates that may shed some light on the discrepancies seen. First of all, looking at RFC 5880 that describes BFD, we see that:
- Timers are negotiated - BFD will negotiate timers based on what is supported. Now you put in a value of 9999 for the interval, and this appears as 9000 in the capture. It could be that the platform may accept such a value in the command, but may default to a maximum value that the platform supports. This may be IOS specific.
- The control packets are the ones that would adhere to your interval and min_rx settings. In Echo Mode, however, BFD will send echo packets at a more frequent rate (usually on the order of milliseconds). The intention here is for these echo packets to be “echoed” back by the recipient. The missed echo packets don’t count against the multiplier, but it’s a means to verify the forwarding path without putting the additional load of control mode packet processing on the control plane.
- When viewing the actual frequency of packets sent, the RFC says that it will differ from the set values by up to 25% based on jitter values and on a level of randomness introduced into the control packets to prevent self-syncrhonization.
- The RFC also states that some of the timers must be initialized to 1 second (i.e. 1000 ms) before being changed and negotiated.
I suggest you take a look at the configuration of your routers using the show bfd neighbors details command to see what values are actually negotiated and used. Once that’s done, take a look at the steam of BFD packets several dozen seconds into the exchange to see if the values of these timers have changed, and to see if the frequency of their transmission has steadied. Please take a look at these and let us know what you find.
I hope this has been helpful!
Laz