BFD for long haul circuit

Thu Jul 16 17:34:57 UTC 2020

On 16/Jul/20 05:51, Harivishnu Abhilash wrote:
>
> *Classification:**Public*
>
>  
>
> Guys, I’m looking for recommendation regarding BFD timers that we can
> use for long haul circuit. RTT is roughly around 110 ms. In fact this
> is a l2vpn ckt provided by a telco.
>
> Can you please advise the factors we can consider when setting the BFD
> timers (or any recommended values)? I have set 10 ms dead time but
> this is causing BFD to go down occasionally.
>

We run different intervals and multipliers depending on whether the
connection is LAN or WAN.

For LAN (so within the same data centre), intervals are set to 150ms and
multipliers are set to 3.

For WAN (any backbone regardless of latency), intervals are set to 250ms
and multipliers are set to 5.

Since our network spans multiple countries and continents, we wanted a
uniform value for the WAN side of things, so we don't have too many
customized configurations. We found these settings to work well in mixed
environments where implementations vary between CPU and line card
processing, and also to strike a balance between accuracy and false
positives.

We've been running this on IOS XE, IOS XR and Junos platforms since
2014. The only issues we found were:

  * BFD on LAG's on IOS XR platforms in a LAN environment don't work. A
    point-to-point mechanism is required, so we disabled it there. Junos
    and IOS XE have no problems running BFD on LAG's in LAN's, so we
    have it on there. This is for within the data centre.

  * BFDv6 on the MX does not run in hardware. Since IS-IS (for us) ties
    in BFD for link state event detection, a transient lack of CPU
    resources to service BFDv6 traffic will result in not only BFDv6
    going down, but also the entire IS-IS protocol flapping on the
    assumption that a link event has occurred. So if you run BFDv6
    alongside BFDv4, recommend that you disable BFDv6 until Juniper
    introduce hardware support for it on the MX (and I'm guessing all
    other Junos platforms). We have an ER out for this since 2019, and
    we are told it should be appearing sometime between Q4'20 - 1H'21.

  * Syntax for BFD in Junos has changed to incorporate address families.
    So while the old syntax will commit, it will leave an annotation in
    the configuration about not being supported anymore. Recommend you
    convert your Junos BFD configurations to IPv4 and IPv6 specificity,
    if you haven't already done so. I can't remember when this came into
    effect, but it likely was Junos 16. We are on Junos 17 now.

Our longest circuit point-to-point is 140ms (Cape Town - London). These
settings have been running fine on there since Day 1 (IOS XR-to-IOS XR),
and overall detection and re-convergence of IS-IS + LFA leaves us happy
and sleeping well at night.

Mark.

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