Just wanted to post some information about a configuration I had some trouble with, but managed to find a solution to. I was trying to setup an aggregated link on an EX 4200 switch to a bonded interface on a Centos 5.x server. The only way I could get it to work was using the following configuration:
On the EX 4200 switch I created an aggregated link (ae0) and assigned 2 ports to the link. I then set the link to use lacp in passive mode. I also added the switches to be a part of the management VLAN of which the switch is assigned ip address 192.168.1.254.
On the linux machine I setup 2 NICs to be bonded (BOND0) then in /etc/modprobe.conf I added the following:
alias bond0 bonding options bond0 miimon=100 arp_ip_target=192.168.1.254 mode=4 lacp_rate=1
I loaded the bonding interface with modprobe bond.