when to use "bfd slow-timer"?

[BenBen]

It is a little confusing in the Nexus 7000 configuration guide on when "bfd slow-timer" should be used. After consult RFC2880 (Bidirectional Forwarding Detection (BFD)), I have a better understanding that slow-timer can be used when Echo function is used. Apparently, BFD is a standard and leaded by Juniper Networks.

The following is for your reference:

"In BFD Asynchronous mode, the systems periodically send BFD Control packets to one another, and if a number of those packets in a row are not received by the other system, the session is declared to be down."

"When Echo funciton is active, a stream of BFD Echo packets is transmitted in such a way as to have the other system loop them back through its forwarding path. If a number of packets of the echoed data stream are not received, the session is declared to be down."

You can use Echo function with Asynchronous mode. When Echo function is used, the echo packets (not the control packets) are used for fail detection. As a result, the Control packets can be sent in a much slower interval.

The quoted parts are from RFC5880.

HTH.

-Frank

[Speltz.Matthew]

So I've been digging into this myself, and here is what I've been able to dig up.  If someone else sees an opportunity for improvement, please chip in.

In BFD Echo mode, you have Router A sending BFD Echo packets to Router B, which are then returned to Router A.  Given that these can be sent and returned quickly and reliably (via forwarding hardware instead of via CPU), Router A will use the BFD Echo packets to detect the neighbor being up or down instead of the BFD Control packets.  That allows BFD to send Control packets much less frequently, reducing CPU load on both routers.  The BFD slow timer is configuring how often BFD Control packets should be sent once the BFD session has been established.