1.2.11 Discover Devices Using CDP
Consider the lack of documentation in the topology shown in the figure. The network administrator only knows that R1 is connected to another device.
With CDP enabled on the network, the show cdp neighbors command can be used to determine the network layout, as shown in the output.
R1# show cdp neighbors Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone, D - Remote, C - CVTA, M - Two-port Mac Relay Device ID Local Intrfce Holdtme Capability Platform Port ID S1 Gig 0/0/1 179 S I WS-C3560- Fas 0/5
No information is available regarding the rest of the network. The show cdp neighbors command provides helpful information about each CDP neighbor device, including the following:
- Device identifiers - This is the host name of the neighbor device (S1).
- Port identifier - This is the name of the local and remote port (G0/0/1 and F0/5, respectively).
- Capabilities list - This shows whether the device is a router or a switch (S for switch; I for IGMP is beyond scope for this course)
- Platform - This is the hardware platform of the device (WS-C3560 for Cisco Catalyst 3560 switch).
The output shows that there is another Cisco device, S1, connected to the G0/0/1 interface on R1. Furthermore, S1 is connected through its F0/5 port, as shown in the updated topology.
The network administrator uses show cdp neighbors detail to discover the IP address for S1. As displayed in the output, the address for S1 is 192.168.1.2.
R1# show cdp neighbors ------------------------- Device ID: S1 Entry address(es): IP address: 192.168.1.2 Platform: cisco WS-C3560-24TS, Capabilities: Switch IGMP Interface: GigabitEthernet0/0/1, Port ID (outgoing port): FastEthernet0/5 Holdtime : 136 sec Version : Cisco IOS Software, C3560 Software (C3560-LANBASEK9-M), Version 15.0(2)SE7, R RELEASE SOFTWARE (fc1) Technical Support: http://www.cisco.com/techsupport Copyright (c) 1986-2014 by Cisco Systems, Inc. Compiled Thu 23-Oct-14 14:49 by prod_rel_team advertisement version: 2 Protocol Hello: OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=00000000FFFFFFFF010221FF000000000000002291210380FF0000 VTP Management Domain: '' Native VLAN: 1 Duplex: full Management address(es): IP address: 192.168.1.2 Total cdp entries displayed : 1
By accessing S1 either remotely through SSH, or physically through the console port, the network administrator can determine what other devices are connected to S1, as displayed in the output of the show cdp neighbors in the figure.
S1# show cdp neighbors Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone, D - Remote, C - CVTA, M - Two-port Mac Relay Device ID Local Intrfce Holdtme Capability Platform Port ID S2 Fas 0/1 150 S I WS-C2960- Fas 0/1 R1 Fas 0/5 179 R S I ISR4331/K Gig 0/0/1
The output displays the specifics of the connection with R1. It also displays S2, another switch that S1 is connected to. Specifically, the output specifies that S2 is a Catalyst 2960 switch using F0/1 to connect to the F0/1 interface on S1.
Again, the network administrator can use show cdp neighbors detail to discover the IP address for S2, and then remotely access it. After a successful login, the network administrator uses the show cdp neighbors command to discover if there are more devices.
S2# show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
D - Remote, C - CVTA, M - Two-port Mac Relay
Device ID Local Intrfce Holdtme Capability Platform Port ID
S1 Fas 0/1 141 S I WS-C3560- Fas 0/1