I have couple of QFX-10Ks which connects two QFX5K in mclag.
on the 5K's there are servers in 4 Vlans - two of them are active on QFX10K1 and two are active on QFX10K2 (VRRP).
it seems that traffic that gets from servers to the 5Ks layer is balanced, but, as both 10K's are connected to two wan interfaces (2x100G) with isis and iBGP, traffic arrives to 10K-2, will pass on to 10K-1 via the ICCP link, and will exit the wan links in 10K-1.
10K1.RE0> show vrrp brief Interface State Group VR state VR Mode Timer Type Address irb.10 up 10 master Active A 0.036 lcl x.x.x.2 vip x.x.x.1 irb.20 up 20 backup Active D 0.273 lcl x.x.x.34 vip x.x.x.33 mas x.x.x.35 irb.30 up 30 master Active A 0.006 lcl x.x.x.66 vip x.x.x.65 irb.40 up 40 backup Active D 0.318 lcl x.x.x.98 vip x.x.x.99 mas x.x.x.97
10K2.RE0> show vrrp brief Interface State Group VR state VR Mode Timer Type Address irb.10 up 10 backup Active D 0.326 lcl x.x.x.3 vip x.x.x.1 mas x.x.x.2 irb.20 up 20 master Active A 0.034 lcl x.x.x.35 vip x.x.x.33 irb.30 up 30 backup Active D 0.281 lcl x.x.x.67 vip x.x.x.65 mas x.x.x.66 irb.40 up 40 master Active A 0.063 lcl x.x.x.97 vip x.x.x.99
regarding the 2nd questions, all 4 switches are configured as MAC aging interval of 300, and LE aging time/LE VLAN aging time of 1200
i do not run server-to-server database replication
the servers doesn't speak with one another, all traffic is served for a lot of end hosts, just like a cdn would.
i do not have any special aging time out for arp configured on any interface nor on system level, so i guess they're all in default configurations.
regarding the server configs, i can't tell, those are closed boxes i don't have access to.
i guess it has something to do with switches configs, same servers with same vrrp solution (active/standby) with other switches vendors equally load balance, so i don't see how servers will age out their arp entries and perform this amound of flood & learn.