Currently only aggregation can be performed only on one target field. This has to be changed to allow aggregation across more target fields. Two nodes should be agregated only when they equals in every specified target fields.
For examle:
topo1:
node 1A
-ip:192.168.1.1
-manufacturer:some manufacturer
node 1B
-ip:192.168.1.2
-manufacturer:some manufacturer

topo2:
node 2A
-ip:192.168.1.1
-manufacturer:some manufacturer
node 2B
-ip:192.168.1.2
-manufacturer:nicira

created overlay topology (in unification case):
overlayTopo:
node O1
node 1A
node 2A
node O2
node 1B
node O3
node 2B