http://en.wikipedia.org/wiki/Link_Layer_Discovery_Protocol
The Link Layer Discovery Protocol or LLDP is a vendor-neutral Layer 2 protocol that allows a network device to advertise its identity and capabilities on the local network. The protocol was formally ratified as IEEE standard 802.1AB-2005 in May 2005. It supersedes proprietary protocols like Cisco Discovery Protocol, Extreme Discovery Protocol and Nortel Discovery Protocol (also known as SONMP).
Information gathered with LLDP are stored in the device and can be queried using Simple Network Management Protocol. The topology of a LLDP-enabled network can be discovered by crawling the hosts and querying this database. Information that can be retrieved include:
system name and description
port name and description
VLAN name
IP management address
system capabilities (switching, routing, etc.)
MAC/PHY information
MDI power
link aggregation
Support
LLDP is still young but is supported on various types of equipment:
HP ProCurve switches and routers
Extreme switches and routers
Nortel 55x0 and 425 series
Allied Telesis switches and routers
Cisco switchs and routers with recent IOS
Alcatel-Lucent switches with AOS > 6.3.1
Juniper EX series switches
AASTRA 9480i (35i) and 675i series VoIP phones
Enterasys Secure Networks
如果所有设备都支持LLDP，那拓扑发现程序就很容易写了。