Ok got It working.
Setting static ports made it easy to configure the loadbalancer.
Any reasons why it's dynamic in the default exchange 2010 configuration ?
And to share the configuration (foundry serveriron) :
==========
server real exchangenode01
port 25
port 80
port http healthck exchangenode01
port 443
port 135
port 55000
port 55001
server real exchangenode02
port 25
port 80
port http healthck exchangenode02
port 443
port 135
port 55000
port 55001
server virtual exchange
sym-priority 50
dyn-sym-pri-factor 30
advertise-vip-route
vip-route-subnet-mask-length 32
port smtp
port http sticky
port ssl sticky
port 135 sticky
port 55000 sticky
port 55001 sticky
track-group http ssl 135 55000 55001
bind smtp exchangenode01 smtp exchangenode02 smtp
bind http exchangenode01 http exchangenode02 http
bind ssl exchangenode01 ssl exchangenode02 ssl
bind 135 exchangenode01 135 exchangenode02 135
bind 55000 exchangenode01 55000 exchangenode02 55000
bind 55001 exchangenode01 55001 exchangenode02 55001
==========
