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InterfacesPrivileged EXEC

show controllers

Displays detailed hardware and interface controller status information, used to diagnose physical layer issues such as cable faults, clocking problems, or interface errors.

Syntax·Privileged EXEC
show controllers

When to Use This Command

  • Troubleshooting a serial interface that is up but experiencing CRC errors or packet drops.
  • Verifying DCE/DTE cable termination and clock rate settings on a serial link.
  • Checking for buffer overruns or framing errors on a T1/E1 controller.
  • Diagnosing why a FastEthernet interface is not coming up despite a good cable.

Command Examples

Serial Interface Controller Status

show controllers serial 0/0/0
Interface Serial0/0/0
Hardware is PowerQUICC MPC860
DTE V.35 cable detected

cable type : V.35 DTE cable, received clockrate 2000000

idb at 0x80A4B6C0, driver data structure at 0x80A4B7E0

icb at 0x80A4B8A0

HD unit 0, idb = 0x80A4B6C0, ds = 0x80A4B7E0, regs = 0x80A4B8A0

HD unit 0, idle = 0, rxbusy = 0, txbusy = 0


buffer size 1524

cpb = 0x20, eda = 0x4000, cda = 0x4000

RX ring entries:
 0: 0x4000: status 0x0000, size 0, data 0x0
 1: 0x4008: status 0x0000, size 0, data 0x0
 2: 0x4010: status 0x0000, size 0, data 0x0
 3: 0x4018: status 0x0000, size 0, data 0x0
 4: 0x4020: status 0x0000, size 0, data 0x0
 5: 0x4028: status 0x0000, size 0, data 0x0
 6: 0x4030: status 0x0000, size 0, data 0x0
 7: 0x4038: status 0x0000, size 0, data 0x0

TX ring entries:
 0: 0x4040: status 0x0000, size 0, data 0x0
 1: 0x4048: status 0x0000, size 0, data 0x0
 2: 0x4050: status 0x0000, size 0, data 0x0
 3: 0x4058: status 0x0000, size 0, data 0x0
 4: 0x4060: status 0x0000, size 0, data 0x0
 5: 0x4068: status 0x0000, size 0, data 0x0
 6: 0x4070: status 0x0000, size 0, data 0x0
 7: 0x4078: status 0x0000, size 0, data 0x0

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out
0 carrier transitions

DCD=up  DSR=up  DTR=up  RTS=up  CTS=up

Line 1: Interface identifier. Line 2: Hardware type (PowerQUICC MPC860). Line 3: Cable type detected (DTE V.35). Line 4: Received clock rate from DCE. Lines 6-8: Internal data structures. Lines 10-12: HD unit status (idle, rxbusy, txbusy). Lines 14-23: RX ring entries show buffer descriptors; status 0x0000 means no errors. Lines 25-34: TX ring entries. Lines 36-40: Error counters – all zeros indicate no errors. Lines 42-46: Serial line signals (DCD, DSR, DTR, RTS, CTS) – all 'up' is good.

T1 Controller Status

show controllers t1 0/0/0
T1 0/0/0 is up.
  Hardware is DSX1
  Receiver has no alarms.
  Framing is ESF, Line Code is B8ZS, Clock Source is Line.
  Data in current interval (0 seconds elapsed):
     0 Line Code Violations, 0 Path Code Violations
     0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
     0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs
  Total Data (last 24 hours):
     0 Line Code Violations, 0 Path Code Violations,
     0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins,
     0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Line 1: T1 controller status (up). Line 2: Hardware type. Line 3: Alarm status – 'no alarms' is good. Line 4: Framing (ESF), line code (B8ZS), clock source (Line). Lines 5-8: Current interval error counters – all zeros indicate clean link. Lines 9-12: Last 24-hour totals – zeros are ideal. Key metrics: Line Code Violations (LCV) indicate bipolar violations; Path Code Violations (PCV) indicate framing errors; Slip Secs indicate clock slips; Fr Loss Secs indicate loss of frame; Unavail Secs indicate link unavailable.

Understanding the Output

The 'show controllers' command provides low-level hardware diagnostics for physical interfaces. For serial interfaces, the output shows cable type (DTE/DCE), clock rate, ring buffer descriptors, and error counters. Key fields: 'cable type' indicates whether the interface is DTE (expects clock from DCE) or DCE (provides clock); 'received clockrate' shows the clock speed from the DCE; 'input errors' includes CRC, frame, overrun, ignored, and abort errors – any non-zero value indicates physical layer issues; 'carrier transitions' counts link flaps. For T1 controllers, look for 'Receiver has no alarms' (good) or alarms like 'Loss of Signal' (LOS), 'Alarm Indication Signal' (AIS), or 'Yellow Alarm' (RAI). Error counters like Line Code Violations and Path Code Violations should be zero; increasing values indicate line impairments. The output also shows framing (ESF or SF) and line code (B8ZS or AMI) – mismatches cause errors. In production, use this command to isolate cabling, clocking, or line card problems before escalating to the service provider.

CCNA Exam Tips

1.

CCNA exam tip 1: 'show controllers' is used to verify DTE/DCE cable type and clock rate on serial interfaces; remember that DCE provides clock, DTE receives it.

2.

CCNA exam tip 2: For T1/E1 controllers, know that 'show controllers t1' displays framing (ESF/SF) and line code (B8ZS/AMI); mismatches cause errors.

3.

CCNA exam tip 3: The exam may ask which command to use to check for CRC errors on a serial link – answer is 'show controllers serial'.

4.

CCNA exam tip 4: 'show controllers' is a Layer 1 troubleshooting command; do not confuse with 'show interfaces' which shows Layer 2/3 status.

Common Mistakes

Mistake 1: Using 'show controllers' on Ethernet interfaces expecting detailed output – most Ethernet controllers show minimal info; use 'show interfaces' instead.

Mistake 2: Misinterpreting 'cable type : V.35 DTE cable' as the interface being DCE – DTE means it expects clock from the remote DCE.

Mistake 3: Ignoring 'carrier transitions' counter – a high number indicates frequent link flaps, often due to loose cables or faulty hardware.

Related Commands

show interfaces

Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.

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