[FROG] Zebra FIB update/withdrawal looping issue

[Templin (US), Fred L]

I have a simple BGP configuration with one "hub" ASBR and two "spoke" ASBRs.
The spoke ASBRs report IPv6 BGP routes to the hub, while the hub originates
"default" to the spokes. So, the hub ASBR receives all advertised routes while
the spokes receive only "default".

When I run my network, BGP routes indeed show up at the hub ASBR and Zebra
installs them in the FIB. However, after about 2-5 seconds, Zebra withdraws the
routes and then about 2-5 seconds later it reinstalls them in the FIB again. This
process recurs indefinitely even though there are no dynamic BGP protocol
updates coming from the spokes.

Any idea what could cause Zebra on the hub ASBR to loop and repeatedly
install and withdraw the BGP routes?

Thanks - Fred

PS I am running the latest FRR release on Ubuntu 16.0.4.