Configuring for Network Management Applications

CDP

Switch "A"

 

 

 

 

 

 

CDP Packets from Switch "A" to Switch "B"

 

 

 

 

Switch "B"

 

Port A3

 

CDP Enabled

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CDP Enabled

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CDP Neighbor Table

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Port B1

 

 

Port C5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

STP Root Device

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CDP Packets from

 

 

Port Data

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch "B" to Switch "A"

 

 

 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CDP Neighbor Table

A3 Switch "B"data

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Port Data

C5 Switch "B" data

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

B1 Switch "A" data

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Port D8

STP block on port C5 of switch "A" blocks outbound transmission of CDP packets from switch "A", but does not block inbound packets to switch "A" from switch "B".

Figure 13-19. Example of STP Effect on CDP Packet Transmission

Switch "A" sends outbound CDP packets on the forwarding link, and the switch "B" CDP Neighbors table shows switch "A" on only one port.

Switch "B" sends outbound CDP packets on both links, and the switch "A" CDP Neighbors table shows switch "B" on both ports.

To summarize, in a CDP neighbor pair running STP with redundant links, if one of the switches is the STP root, it transmits CDP packets out all ports connecting the two switches, while the other switch transmits CDP packets out only the unblocked port. Thus, the STP root switch will appear on multiple ports in the non-root switch’s CDP Neighbors table, while the non-root switch will appear on only one port in the root switch’s CDP Neighbors table.

How the Switch Selects the IP Address To Include in

Outbound CDP Packets

A switch with CDP enabled uses the following prioritized criteria to determine which IP address to include in its outbound CDP packets:

1.If only one VLAN on the port has an IP address, the switch uses that IP address.

2.If the Primary VLAN on the port has an IP address, the switch uses the Primary VLAN IP address.

3.If 1 and 2 do not apply, then the switch determines which VLANs on the port have IP addresses and uses the IP address of the VLAN with the lowest VID (VLAN Identification number) in this group.

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