From Mexico.purplecow.org
The casual documentation provided by Canonical et al for link aggregation ("bonding") on Linux doesn't make much mention of trunk keys, etc.
Number the bond interface to match the trunk key to make it work.
For example, this extract from /etc/network/interfaces:
auto eth0 iface eth0 inet manual bond-master bond7 bond-primary eth0 auto eth4 iface eth4 inet manual bond-master bond7 auto bond7 address 192.168.10.10 netmask 255.255.255.0 network 192.168.10.0 broadcast 192.168.10.255 gateway 192.168.1.1 bond-mode 802.3ad bond-lacp-rate 1 bond-miimon 100 bond-slaves eth0 eth4 bond-xmit-hash-policy layer3+4