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Layer 2 Layout: Layer 2 Discovery Digs Deep

But don't blame network-management vendors. The reason it's hard to manage Layer 2 networks is because it's so easy to build them: Switches can connect transparently, so it's really just a matter of plugging in cables to create a LAN. Trouble is, this connection ease also makes LANs transparent to network-management applications.

That doesn't mean you can't do Layer 2 discovery. Key infrastructure vendors have developed proprietary discovery protocols, storing the data in enterprise extensions of SNMP. Cisco Systems has CDP (Cisco Discovery Protocol); Extreme Networks, EDP (Extreme Discovery Protocol); Enterasys Networks, CDP (Cabletron Discovery Protocol); and Nortel Networks, NDP (Nortel Discovery Protocol).

The obvious problem here is that these protocols don't work in mixed-vendor networks. One way to get around this is to have a single-vendor network, using that vendor's management application. But that's not realistic or desirable. A better approach is to map Layer 2 connections.

Mapping Options

But mapping Layer 2 connections in heterogeneous networks is difficult for several reasons. There is no single standard, and the often-used SNMP Bridge MIB typically is poorly implemented. Many networks run older SNMP implementations or don't even have SNMP turned on. And network devices don't always behave, failing to flush cached but aged MAC (Media Access Control) address entries, for example, so defunct equipment appears to be still attached to the network.

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