12-24
Configuring for Network Management Applications
CDP
Monitoring and Managing
the Switch
■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 theport 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 Identificatio n number) in this group.
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 12-12-15, port A1 on CDP switch "X" is connected to
port C5 on CDP neighbor switch "Y", with the indicated VLAN configuration
on port C5: