CCNA 200-301 v1.1 reference — Network Access

CDP and LLDP

Layer 2 discovery protocols: enable and scope CDP and LLDP, read neighbor tables, and use detail output safely.

Objective 2.3v1.1 exam target

CDP and LLDP compared

Layer 2 discovery protocols let neighbors exchange identity, capabilities and addressing directly over the link. CDP is Cisco proprietary; LLDP (802.1AB) is the vendor-neutral standard. They run on the wire regardless of IP configuration — useful for mapping, dangerous as an information leak on untrusted ports.

CDP versus LLDP

Property	CDP	LLDP
Standard	Cisco proprietary	IEEE 802.1AB (open)
Default on Cisco	Enabled globally, per-interface on	Disabled until `lldp run`
Timer / holdtime	60 s / 180 s	30 s / 120 s
TLVs	Device ID, port ID, platform, IP, native VLAN, duplex, VTP domain	Chassis/port ID, system name/desc, management address, capabilities
Use	Cisco-only estates	Mixed-vendor networks, IP phone voice-VLAN delivery

Discovery defaults worth memorizing

Values are platform defaults — verify against your device documentation.
Setting	CDP default	LLDP default
Advertisement timer	60 seconds	30 seconds
Holdtime	180 seconds	120 seconds
Global state on Cisco IOS	Enabled	Disabled
Per-interface control	`cdp enable` / `no cdp enable`	`lldp transmit` / `lldp receive`

Values are platform defaults — verify against your device documentation.

Configuration

Enable and scope LLDP

Enable LLDP globally and tune a port — Privileged EXEC → Global + interface configuration

Cisco IOS / IOS XE

configure terminal
lldp run
interface gigabitethernet1/0/24
 lldp transmit
 lldp receive
 end

Verify

show lldp neighbors
show lldp neighbors detail
show lldp

Neighbor table shows chassis ID, port ID, TTL and system name.
Detail output reveals management IP, capabilities and VLAN info.

Disable transmit on untrusted/user-facing ports — LLDP leaks topology details.

Changes device configuration.

Reading neighbor output

$ show cdp neighbors (interpreted)

Capability Codes: R - Router, T - Trans Bridge, B - Source Route BridgeLetters reveal device function.

S - Switch, H - Host, I - IGMP, r - Repeater, P - PhoneP marks IP phones — voice VLAN verification.

Device ID Local Intrfce Holdtme Capability Platform Port IDLocal and remote port pair on each row.

SW2 Gig 1/0/24 158 S C9300 Gig 1/0/24SW2 reachable via its Gi1/0/24 from your Gi1/0/24.

SEP001A2B3C4D5E Gig 1/0/1 143 P IP Phone Port 1A phone — confirm it is in the voice VLAN.

Map an undocumented port

Task: Identify what is plugged into Gi1/0/7

show cdp neighbors detail on Gi1/0/7 returns a device ID, platform and IP address.
Capability letter shows whether it is a switch (S), router (R) or phone (P).
The remote port ID tells you the far end's exact interface for the network diagram.

Discovery turns an unknown cable into a documented link in one command.

Common mistakes

Expecting CDP on non-Cisco gear — use LLDP in mixed environments.
Leaving discovery running on untrusted edge ports — it advertises management IPs and platform details.
Confusing the holdtime counter with link health — an expired holdtime only means no recent advertisement.
Assuming discovery proves VLAN/config correctness — it proves link-layer adjacency only.

A switch appears with no CDP entry

Observation: Two directly-connected Cisco switches show no CDP entry for each other.

Hypotheses

CDP disabled globally or on the interface
The link is not actually direct — an intermediate active device (hub switch or in-line device) sits between them
Link down between them

Check	Indicates
`show cdp`	Global state and timers.
`show run interface` → `cdp enable`	Per-interface disable overrides the global.
Physical path and `show interfaces status`	If the link isn't L2-adjacent, discovery cannot cross it.

Correction: Re-enable CDP/LLDP where appropriate, or correct the assumed topology.

Neighbor entries appear within one advertisement interval.

On this page

CDP and LLDP compared
Configuration
Reading neighbor output

Related resources

Practise this topic Source topic 36: CDP & LLDP (Layer 2 Discovery Protocols)Practise topic 36 deck

Sources

Cisco CCNA 200-301 v1.1 Exam Topics (official PDF) — reviewed 2026-11-02
IEEE 802.1 Working Group (802.1AB LLDP) — reviewed 2026-11-02
Cisco — Configuration Fundamentals Command Reference — reviewed 2026-11-02

Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.