Switches learn network topoogy (L2, L3) via standardized message exchange, e.g., Spanning Tree or LDP at L2 and RIP, BGP or ISIS at L3. Those messages can be passed by the OpenFlow enabled switch to the OpenFlow controller and therefore the controller can build L2 forwarding tables and L3 routing tables, just as "normal" switches do.
Regards, Dave David B. Nelson Director of Technology Elbrys Networks, Inc. www.elbrys.com +1.603.570.2636
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