In order to make XenServer 5.5 work with LACP, we need to modify the start up scripts

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1#vi /opt/xensource/libexec/interface-reconfigure +863

We will see the following:

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1# The bond option defaults
2 bond_options = {
3 "mode": "balance-slb",
4 "miimon": "100",
5 "downdelay": "200",
6 "updelay": "31000",
7 "use_carrier": "1",
8 }

We will change the mode from "balance-slb" to "802.3ad"

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1# The bond option defaults
2 bond_options = {
3 "mode": "802.3ad",
4 "miimon": "100",
5 "downdelay": "200",
6 "updelay": "31000",
7 "use_carrier": "1",
8 }

Don't forget to configure your switch for LACP.