Configuring for Network Management Applications

CDP

Switch "B"

CDP-Aware

Switch with

CDP Running

 

 

Switch "C"

 

 

CDP-Aware

 

 

Switch with

 

 

CDP Disabled

Switch "A"

 

 

Switch with CDP

 

 

 

Router "X"

Running and

 

 

With CDP

Forwarding CDP

 

Packets to Down-

 

Running

stream Devices

 

 

 

 

 

 

 

 

 

 

Router "Y"

 

 

 

With CDP

 

 

Disabled or

 

 

Nonexistent

 

 

 

 

 

 

 

 

Device "Z"

 

 

Hub or Other

 

 

Non-CDP

 

 

Device

 

 

 

Accepts, but does not forward CDP packets describing Switch "A". Also transmits CDP packets describing itself (Switch "B") out all ports.

Drops CDP packets describing Switch "A". Also, does not transmit any CDP

packets describing itself (Switch "C").

Accepts, but does not forward CDP packets from Switch "A". Also transmits CDP packets describing itself (Router "X") out all ports.

Drops CDP packets describing Switch "A". Also does not transmit any CDP packets

describing itself (Router "Y").

Forwards CDP packets from Switch "A" out all ports (except the port receiving the packets from "A") without any awareness of CDP operation.

Figure 13-11. Example of Outgoing CDP Packet Operation

Incoming CDP Packets

When a CDP-enabled switch receives a CDP packet from another CDP device, it enters that device’s data in the CDP Neighbors table, along with the port number where the data was received (and does not forward the packet). The switch also periodically purges the table of any entries that have expired. (The hold time for any data entry in the switch’s CDP Neighbors table is configured in the device transmitting the CDP packet, and cannot be controlled in the switch receiving the packet.) A switch reviews the list of CDP neighbor entries every three seconds, and purges any expired entries.

Non-CDP devices such as some hubs and other devices that do not have CDP capability are transparent to CDP operation. (Other hubs are CDP-aware, but still forward CDP packets as if they were transparent to CDP operation. See “CDP-Capable Hubs” on page 13-41.)However, an intervening CDP-aware device that is CDP-disabled is not transparent. For example, in figure 13-12,the CDP neighbor pairs are as follows: A/1, A/2, A/3, A/B, B/C. Note that “C”

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