The Cisco Discovery Protocol is of use in most networked environments however it is probably most useful in a Cisco switched environment. This is because one the difficulties in troubleshooting an environment with mostly switches is that they usually provide a lot less information than routers. It can also pose further difficulties because a switched environment is sometimes more confusing because these networks are not always clearly segmented and can be confusing. CDP provides a useful tool for identifying and detecting Cisco switches and routers and to build up a picture of the complete network topology of all the Cisco hardware.

The Cisco Discovery Protocol is a data link multicast protocol, which uses a standard multicast MAC address. These broadcasts are formed as SNAP type 200 packets and as such they have no layer 3 component. CDP must be enabled on a Cisco router or switch before it can be used for detecting other Cisco devices which can be seen on all it’s interfaces. It should be noted that CDP is not like extended network management protocols like SNMP it can only detect directly attached devices. Even if two routers were plugged into a single Cisco switch, CDP will allow each router to see the switch but it would not allow them to see the other router. The routers will appear to the switches as CDP neighbours, the primary reason for this is that the protocol is not designed to forward frames between devices.

Although this sounds of limited use, the information that CDP provides about directly attached devices is extensive and extremely useful for troubleshooting things like network problems or anonymous torrenting taking place on the network. You can for example use the protocol for completely defining whole wiring closers, and detailing the exact models and versions of routers and switches. The information is extensive and you’ll soon know even the versions of Catalyst switches that are connected and which port/interface. Other protocols will normally go little further than identifying whether the hardware is a router or switch.

There are a host of commands which can be used to provide extensive information on all Cisco hardware. The most used ones are the SHow and Clear commands which can be used to bring up general information on the devices plus details of the software versions, modules installed, port configuration and any error messages. The command can also be used to display the full routing and switching tables plus much more information.

The show config command is used to display the configuration information which is stored in NVRAM of the device. For example in Catalyst 5000 series of routers, the entire configuration is stored automatically in this memory, the command write terminal can also be used to display the same information. To clear the configuration information the clear config command will erase the information on a specified module. The displayed information can be categorized into the following broad categories:

  • switch management parameters
  • IP configuration
  • Virtual LANS
  • Bridging Parameters

There is further information which can be accessed including extensive information on module specific information. You can also use the show cam command which refers to the special memory on the switches which has a low access time. THe information stored here is normally bridging and switching tables, it needs a fast response time because it can be updated very quickly in fast dynamic environments.

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