  In this paper we address the problem of discovering network topology in
proprietary networks. Namely, we investigate topology discovery in Cisco-based
networks. Cisco devices run Cisco Discovery Protocol (CDP) which holds
information about these devices. We first compare properties of topologies that
can be obtained from networks deploying CDP versus Spanning Tree Protocol (STP)
and Management Information Base (MIB) Forwarding Database (FDB). Then we
describe a method of discovering topology of CDP-based networks. Our
experiments show that the physical topology of the network including links that
are in Forwarding Block state can be discovered.
