Configuring for Network Management Applications

CDP

Using the example in figure 10-7,the CDP Neighbor table for switches “A” and “B” would appear similar to these:

Switch A:Switch B:

(Note that no CDP devices appear on port B5, which is connected to a device on which CDP is present, but disabled.)

Figure 10-8. Example of Viewable CDP Neighbor Table for Switches “A” and “B in Figure 10-7

Thus, based on the CDP packets it receives, each CDP device maintains a per- port data entry for each of its neighbors that are running CDP, but not for other CDP devices that are accessible only through a CDP neighbor. (See the relationship between switches A, B, and C in figure 10-7.) In other words, a CDP device will have data on its immediate CDP neighbors (including those reached through a device that is transparent to CDP), but not to other CDP devices in the network.

Table 10-2. How Devices Handle Incoming CDP Packets

Status of Device Receiving

Action of Receiving Device

a CDP Packet

 

 

 

Running CDP

CDP Disabled

No CDP Capability

Router Running CDP

Router with CDP (1) Disabled or (2) Not CDP-Capable

Stores neighbor data in CDP Neighbor table. Does not forward CDP packet.

Drops CDP packet. There is no CDP Neighbor table and no CDP neighbor data is stored. Forwards CDP packet out all ports except the port on which the packet was received. Stores neighbor data in CDP Neighbor table. Does not forward CDP packet.

Drops CDP packet.

Non-CDP devices (that is, devices that are not capable of running CDP) are transparent to CDP operation. However, an intervening CDP-aware device that is CDP-disabled is not transparent. For example, in figure 10-7(page

10-17), “B”, “D”, and “E” are not CDP neighbors because “D” (the intervening

10-18