Skip to content

Interface Management

Day-to-day interface administration beyond the basic shut/no shut/VLAN assignment already covered in the Cisco IOS CLI Reference — descriptions, speed/duplex, error counters, and neighbor discovery.

Interface Descriptions

Label every port that connects to something specific — future-you (or whoever inherits the config) will thank you:

Cisco IOS — global configuration mode (conf t):

interface Gi0/48
 description UPLINK-TO-CORE-SW02
interface Gi0/1
 description Ruckus-AP-1

Descriptions show up in show interfaces description and show running-config — cheap to add, and the fastest way to answer "what's plugged into this port?" without physically tracing a cable.

Speed & Duplex

Modern gear auto-negotiates correctly almost always — but mismatches still happen, especially with older equipment or long/marginal cable runs:

Cisco IOS — global configuration mode (conf t):

interface Gi0/1
 speed auto
 duplex auto

! Force specific settings only when auto-negotiation is known to be unreliable with the connected device
interface Gi0/1
 speed 100
 duplex full

Duplex mismatch is a classic hard-to-spot fault: the link comes up, but performance is terrible with late collisions on one side. If one end is forced and the other is left on auto, they can each guess differently. Match both ends explicitly, or leave both on auto — never mix.

Checking Interface Health

Cisco IOS — privileged EXEC:

! Full detail: status, speed/duplex, counters, errors
show interfaces Gi0/1

! Quick table of all ports + their description + status
show interfaces description

! Port status, speed, VLAN — see Cisco IOS CLI Reference
show interfaces status

! Filter output down to error-related lines
show interfaces Gi0/1 | include error

! Error counters for every port in one table — the quickest full-switch sweep
show interfaces counters errors

Key counters to watch in show interfaces output:

Counter What it suggests
CRC errors Bad cable, connector, or interference — physical layer problem
Input errors / runts / giants Duplex mismatch or a failing NIC on the connected device
Output drops Congestion — more traffic queued than the port can send
Late collisions Duplex mismatch (half-duplex on one end)

Reset counters to isolate a fault to a specific time window (e.g. after a suspected fix):

clear counters Gi0/1

Neighbor Discovery (CDP / LLDP)

See what's actually connected to each port without physically tracing cables:

Cisco IOS — privileged EXEC:

! Cisco Discovery Protocol — Cisco-to-Cisco devices
show cdp neighbors

! Includes IP address, platform, port ID
show cdp neighbors detail

! Vendor-neutral equivalent — works with non-Cisco gear that speaks LLDP
! (some APs; the Netgate only after installing the 'lldp' package on pfSense —
! System > Packages > Available Packages, then enable it under Services > LLDP)
show lldp neighbors
show lldp neighbors detail

CDP is enabled by default on Cisco switches. LLDP is disabled by default on Catalyst IOS — enable it first from global config mode with lldp run (then give it ~30 seconds to learn neighbors). Per-interface: CDP uses cdp enable; LLDP uses lldp transmit + lldp receive, both on by default once LLDP is running globally.