| Abstract | Many enterprise, campus, and data-center networks have complexlayer-2 virtual LANs (“VLANs”) underlying the layer-3 network. The interac-
tion between layer-2 and layer-3 topologies can introduce dependencies that are
not visible when analyzed solely from layer 3; this paper studies the extent and
impact of these dependencies. We first present EtherTrace, a tool that infers the
layer-2 topology using data passively collected from Ethernet switches. Using
this tool, we infer the layer-2 topology for a large campus network and compare
the two topologies. Our comparison yields some striking initial findings: almost
70% of layer-2 edges are shared by 10 or more IP edges, and a single layer-2 edge
may be shared by as many as 34 different IP edges. This sharing has significant
implications for both robustness and for network tomography. Applying network
tomography to the IP topology to diagnose failures caused by layer-2 devices
results in only 54% accuracy, compared to 100% accuracy when tomography is
performed at layer 2.
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