Throughput not calculated/displayed between Layer 2 nodes

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What steps will reproduce the problem?
1. Create and link 2 switch nodes
2. Create and link a single Layer 3 node for each switch (router tested)
3. Generate traffic of some sort (RJ45 connected to router i.e. real network
traffic tested)
4. Enable the throughput widget

What is the expected output?
All links should display current throughput.

What do you see instead?
Throughput is not displayed on the Switch to Switch link

What version of the product are you using? On what operating system?
SVN snapshot (JAN 2015) on Fedora 21

Please provide any additional information below.
I suspect that this is not a common use case. I am using this topology to avoid a qdisc clash with netem on custom router nodes.

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Original issue reported on code.google.com by `shasenbo...@gmail.com` on 30 Jan 2015 at 6:17