[c-nsp] BFD timers in SXH3a - can't go below 1s?

[Oliver Boehmer (oboehmer)]

Gert Doering <> wrote on Friday, January 30, 2009 09:08:

> Hi,
> 
> I'm a bit confused.  I have enabled BFD on a few links between SXF*
> and SXH3a machines, and on other links between SXH3a and SXH3a boxes.
> SXF 
> box is 7604+Sup720, SXH3a boxes are 6504+Sup720-10G.
> 
> All interfaces are configured with
> 
>  bfd interval 50 min_rx 50 multiplier 10
> 
> (which is fairly conservative, as BFD goes, but 500ms second error
> detection time is perfectly fine for us, and we're aware that this
> platform is not really happy with faster BFD)
> 
[...]
> 
> On the SXH3a-SXH3a link, I see:
> 
> SXH3a#sh bfd neigh det
> 
> OurAddr       NeighAddr     LD/RD  RH/RS  Holddown(mult)  State    
> Int 195.30.XX.AA  195.30.XX.BB   1/1    Up        0    (10)   Up     
> Gi2/8 
> 
> Session state is UP and using echo function with 50 ms interval.
> Local Diag: 0, Demand mode: 0, Poll bit: 0
> MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 10
> Received MinRxInt: 1000000, Received Multiplier: 10
> Holddown (hits): 0(0), Hello (hits): 1000(780297)
> Rx Count: 780119, Rx Interval (ms) min/max/avg: 624/1128/877 last:
> 188 ms ago Tx Count: 780298, Tx Interval (ms) min/max/avg:
> 756/1000/880 last: 332 ms ago Elapsed time watermarks: -1 0 (last: 0)
> Registered protocols: EIGRP
> Uptime: 1w0d
> Last packet: Version: 1            - Diagnostic: 0
>              State bit: Up         - Demand bit: 0
>              Poll bit: 0           - Final bit: 0
>              Multiplier: 10         - Length: 24
>              My Discr.: 1          - Your Discr.: 1
>              Min tx interval: 1000000    - Min rx interval: 1000000
>              Min Echo interval: 50000
> 
> ... which looks to me like "it's sending packets in 1-second
> intervals" (and not 50ms), so the error detection time would be "10
> seconds" - which 
> is certainly not what we are aiming for.
> 

The SXH3a-SXH3a session is running BFD echo mode (as it's supported by
both ends), as indicated in "using echo function with 50 ms interval".

When you configure BFD in Echo mode, there are two types of packets
traversing across the two ends 
1) BFD Echo packets 2) BFD Control/Async packets.

The BFD Echo packets will be sent/received at the rate configured by
"bfd interval...", and it shows at "using echo function with 50 ms
interval" as well as at "Min Echo interval: 50000", so your failure
detection is just fine (half a second in your config).
However: The BFD Async packets will be sent/received at the rate
configured by "bfd slow-timers <xxx>" (default 1 sec), and this one is
shown by Min rx and tx intervals. As the echo packets test the
forwarding, we can reduce the control/async packet rate to a lower
value.

In BFD Async mode, there are only BFD Async packets traversing across
the two ends, with the rate configured by "bfd interval .."..

you can also disable bfd echo mode on the interface ("no bfd echo") if
you want to use the same BFD mechanism (and hence the same output) on
all your links..

	oli