Consider the following topology (all links assumed lossless), where we're trying to route from A to S:
B---C / \\ / \\A S \ / \ / B'---*C' (C' has just a single interface)
In Babel, even with just Z1,the diversity information is encoded in the metric, and so B announces a smaller metric than B'; this causes A to prefer the ABCS route to the AB'C'S route. Unless I'm misunderstanding something, in BATMAN no information is propagated to A -- the information about the extra diversity in the upper route is purely local to C.
Hi Juliusz
I'm no expert here, but if Simon happens to fall on his head.....
The metric is simply based on TQ. C=S will probably have a better local TQ than C'-S because of the load balancing over the two links, meaning there are less packets in the air, and so probably less collisions, etc.. A gets to know the complete link path TQ going both ways around the loop, so does get some idea that C=S is better than C'-S, assuming it is actually better.
Andrew