Thank you for your reply Smallpond.
I found the solution for the problem. Instead of creating the external bridge on the external network of the network node i created the bridge on the tunnel network. This conflict was the reason why the ARP-resolution was acting so strange.
It had to be like this: (X is the ext-bridge)
Compute node <--> Network node X<--> External network
But it was like this:
Compute node <-->X Network node <--> External network
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