Configuring for Network Management Applications

CDP

4.If a CDP switch does not detect an IP address on the connecting port of a CDP neighbor, then the loopback IP address is used (127.0.0.1).

For example, in figure 13-20,port A1 on CDP switch “X” is connected to port C5 on CDP neighbor switch “Y”, with the indicated VLAN configuration on port C5:

VLAN Membership in Port C5 of Switch "Y"

VID

IP Address?

DEFAULT_VLAN (Primary VLAN)

1

No

Blue_VLAN

200

10.28.227.103

Red VLAN

300

10.28.227.88

 

 

 

Switch "X"

 

Port A1

 

 

 

Switch "Y"

CDP Enabled on Port A1

 

 

 

 

 

CDP Enabled on Port C5

CDP Neighbor Table

 

 

 

 

 

CDP Neighbor Table

Port Data

 

 

 

 

Port Data

 

 

 

Port C5

 

------ ------------------

 

 

 

 

 

------------------------

A1 10.28.227.103

 

 

 

 

 

C5 Switch "X" data

 

 

 

 

 

 

 

Thus, CDP switch "X" detects CDP switch "Y" on port A1 and shows 10.28.227.103 in its CDP table entry because in CDP switch "Y" the Primary VLAN does not have an IP address and the Blue_VLAN has a lower VID than the Red_VLAN.

Figure 13-20. Example of IP Address Selection when a CDP Neighbor Has Multiple VLANs with IP Addresses

CDP Neighbor Data and MIB Objects

The switch places the data received from inbound CDP packets into its MIB (Management Information Base). This data is available in three ways:

Using the switch’s show cdp neighbors command to display a subset of Neighbor data

Using the walkmib command to display a listing of the CDP MIB objects

Electronically, using an SNMP utility designed to search the MIB for CDP data

As shown under “Viewing the Switch’s Current CDP Neighbors Table” on page 13-32,you can list a subset of data for each CDP device currently found in the switch’s CDP Neighbors table. Table 13-4,“CDP Neighbors Data”, describes the CDP Neighbor data set available in the switch.

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