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CDP neighbor table

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

A CDP neighbor table is a tabular representation of directly connected devices and the local and remote interfaces discovered by Cisco Discovery Protocol.

A CDP neighbor table records directly connected devices discovered through Cisco Discovery Protocol advertisements. Each entry identifies the neighbor and maps its port to the local interface. section 1.1 section 1.1.1

Read the standard show cdp neighbors columns as follows:

  • Device ID gives the neighbor hostname; Local Intrfce gives the local device interface; Port ID gives the neighbor’s outgoing interface. section 1.1.1
  • Holdtme shows the remaining time before the entry is removed; receiving another advertisement resets the default 180-second timer. section 1.1.1
  • Capability identifies device functions, while Platform identifies the neighbor’s hardware model. section 1.1.1
  • Use show cdp neighbors detail for additional information such as software version, native VLAN, duplex, and advertisement version. section 1.1.1
R1# show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge       #1
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,    #1
                  D - Remote, C - CVTA, M - Two-port Mac Relay                #1
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW1              Gig 0/0           158             R S I  WS-C2960C Gig 0/1   #2
#1 Codes used in the Capability column
#2 SW1 is R1’s CDP neighbor.

Annotated output demonstrates how a neighbor entry maps SW1 to R1’s GigabitEthernet0/0 and SW1’s GigabitEthernet0/1. section 1.1.1

Mapping an undocumented network

Collect show cdp neighbors output from several remotely accessible devices and connect each Local Intrfce to the corresponding Port ID. This reconstructs device relationships without physical access or an updated diagram. section 1.1.2

R1# show cdp neighbors
. . .
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW1              Gig 0/2           163             R S I            Gig 0/1
SW2              Gig 0/1           151             R S I            Gig 0/2
R2               Gig 0/0           132              R B             Gig 0/1

R2# show cdp neighbors
. . .
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW3              Gig 0/0           160             R S I            Gig 0/1
R1               Gig 0/1           150              R B             Gig 0/0

SW1# show cdp neighbors
. . .
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW2              Gig 0/2           168             R S I            Gig 0/1
R1               Gig 0/1           178              R B             Gig 0/2

SW2# show cdp neighbors
. . .
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW1              Gig 0/1           124             R S I            Gig 0/2
R1               Gig 0/2           157              R B             Gig 0/1

SW3# show cdp neighbors
. . .
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
R2               Gig 0/1           157              R B             Gig 0/0

Multiple device outputs provide the data needed to practice reconstructing a topology from reciprocal local and remote interface mappings. section 1.1.2

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes
Explains table columns, expiration behavior, detailed output, and topology mapping, while deferring CDPv1 and CDPv2 differences.
Examples
Uses annotated single-neighbor output, detailed neighbor output, and multi-device listings for reconstructing a network topology.
What it assumes
Assumes familiarity with Cisco IOS interfaces and basic networking, while teaching neighbor discovery and table-column interpretation explicitly.
What you can do after
Diagnose topology and interface relationships by interpreting show cdp neighbors output and correlating local interfaces with neighbor ports.
How it connects
Connects CDP neighbor tables with CDP advertisements, CDPv2, LLDP, network diagrams, and Cisco IOS show commands.
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This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 2 Advanced Networking and Security. The section links point to the parts of the book it draws on.