show interfaces
Comprehensive interface health check — diagnose physical connectivity, duplex mismatches, CRC errors, input drops, and queue drops.
Definition: show interfaces is a Cisco IOS privileged exec command. Displays detailed Layer 1 and Layer 2 statistics for all interfaces or a specific interface: line protocol state, hardware type, MAC address, MTU, bandwidth, duplex, speed, input/output rates, error counters, and queue statistics.
Overview
The 'show interfaces' command is one of the most fundamental and frequently used commands in Cisco IOS. It provides a detailed snapshot of the operational status, configuration, and performance statistics for every interface on the device (or a specific interface if specified). This command is essential for network engineers because it reveals whether an interface is up or down, how much traffic it has processed, whether errors are occurring, and many other critical metrics. The command operates at Layer 1 (physical) and Layer 2 (data link) of the OSI model, giving insight into the health of the physical connection and the data link protocol (e.g., Ethernet, PPP, HDLC).
Networking concept: Interfaces are the points of connection between a network device and the network medium. Each interface has a hardware (physical) state and a line protocol (data link) state. The 'show interfaces' command displays both: 'up/up' means the interface is operational and the line protocol is active; 'down/down' indicates a physical problem; 'up/down' suggests a Layer 2 issue (e.g., no keepalives, mismatched encapsulation). The command also shows counters for packets input/output, errors (CRC, runts, giants, collisions), and interface resets. These counters help diagnose issues like faulty cables, duplex mismatches, or excessive collisions.
When to use: You reach for 'show interfaces' as a first step in troubleshooting connectivity issues. For example, if a user cannot reach a server, you check if the interface connecting to that user is up/up. If it's down/down, you inspect the cable or port. If it's up/down, you check encapsulation or keepalives. The command is also used for capacity planning (monitoring traffic loads) and for verifying configuration changes (e.g., after setting speed/duplex). Alternatives include 'show interfaces status' (brief summary), 'show interfaces description' (only descriptions), and 'show interfaces counters' (error counters only). However, 'show interfaces' remains the most comprehensive.
Workflow: In a typical troubleshooting workflow, you start with 'show interfaces' to get an overall health check. If you see errors, you might drill down with 'show interfaces [interface] counters errors' or 'show interfaces [interface] accounting'. For performance, you use 'show interfaces [interface] | include rate' to see input/output rates. The command does not affect the running configuration; it is a read-only diagnostic tool. It requires privileged EXEC mode (enable). Output can be lengthy, so it is often piped to 'include' or 'begin' to filter. On some platforms, output is buffered; you can use 'terminal length 0' to disable paging.
IOS behaviour: The command output is generated from live counters and state information. It does not impact device performance significantly, but on very busy routers, parsing the output can be CPU-intensive if done repeatedly. The command is available in all IOS versions and modes (user EXEC shows limited output, privileged EXEC shows full). For specific interfaces, use 'show interfaces gigabitethernet 0/0' or 'show interfaces serial 0/0/0'. The output format is consistent across IOS versions, though newer versions may include additional fields like 'input queue drops' or 'output buffer failures'.
show interfaces [interface-id]When to Use This Command
- Identify input errors and CRC errors indicating duplex mismatch or bad cable
- Confirm interface speed and duplex negotiation for a new link
- Check output drops and queue depth for congestion troubleshooting
- Verify interface is line protocol up (Layer 2 keepalives passing)
Parameters
| Parameter | Syntax | Description |
|---|---|---|
| interface-type interface-number | e.g., GigabitEthernet0/0, Serial0/0/0, FastEthernet0/1 | Specifies a particular interface to display. If omitted, all interfaces are shown. The interface type must be a valid type on the platform (e.g., GigabitEthernet, Serial, Loopback). Common mistake: using the wrong slot/port number or omitting the interface type entirely. |
Command Examples
Show a specific interface
Router# show interfaces GigabitEthernet0/0 GigabitEthernet0/0 is up, line protocol is up Hardware is CN Gigabit Ethernet, address is c8b3.73a2.1234 (bia c8b3.73a2.1234) Internet address is 192.168.1.1/24 MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec, Full-duplex, 1000Mb/s, media type is RJ45 5 minute input rate 1000 bits/sec, 2 packets/sec 5 minute output rate 500 bits/sec, 1 packets/sec 12345 packets input, 1234567 bytes, 0 no buffer Received 100 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 12000 packets output, 1200000 bytes, 0 underruns 0 output errors, 0 collisions, 1 interface resets
Understanding the Output
Line 1 status meanings: 'up/up' = working. 'up/down' = Layer 1 OK but keepalives failing (encapsulation mismatch or far end down). 'down/down' = no physical signal (cable, SFP, speed mismatch).
'administratively down/down' = shutdown applied. CRC errors + input errors = duplex mismatch or bad cable. Output drops = congestion.
Interface resets = keepalive failures.
Configuration Scenarios
Verify interface status after enabling an interface
A network engineer has just configured a new GigabitEthernet interface on a router and wants to confirm it is operational. The interface was previously shut down and now needs to be enabled.
Topology
R1(Gi0/0)---10.0.12.0/30---(Gi0/0)R2Steps
- 1.Step 1: Enter privileged EXEC mode: enable
- 2.Step 2: Enter global configuration mode: configure terminal
- 3.Step 3: Select the interface: interface GigabitEthernet0/0
- 4.Step 4: Remove the shutdown: no shutdown
- 5.Step 5: Exit configuration mode: end
- 6.Step 6: Verify the interface status: show interfaces GigabitEthernet0/0
! interface GigabitEthernet0/0 no shutdown end
Verify: show interfaces GigabitEthernet0/0 Expected output: 'GigabitEthernet0/0 is up, line protocol is up'
Watch out: Forgetting to also configure an IP address or encapsulation may leave the line protocol down even if the interface is physically up.
Troubleshoot duplex mismatch between switch and router
A router interface is experiencing high error rates and poor performance. The connected switch port is set to full duplex, but the router interface may be set to half duplex, causing a duplex mismatch.
Topology
Router(Gi0/0)---(Gi0/1)SwitchSteps
- 1.Step 1: Check current interface status and errors: show interfaces GigabitEthernet0/0
- 2.Step 2: Look for 'runts', 'giants', 'CRC', 'frame', 'collisions' errors. High values indicate duplex mismatch.
- 3.Step 3: Check the duplex setting: show interfaces GigabitEthernet0/0 | include duplex
- 4.Step 4: If half-duplex, change to full duplex: configure terminal, interface GigabitEthernet0/0, duplex full
- 5.Step 5: Verify the change: show interfaces GigabitEthernet0/0 | include duplex
! interface GigabitEthernet0/0 duplex full speed 1000 end
Verify: show interfaces GigabitEthernet0/0 Expected: 'Full-duplex' and error counters should stop incrementing.
Watch out: If the switch port is set to auto-negotiate, setting the router to full duplex may cause a mismatch if the switch negotiates to half duplex. Best practice is to set both sides to the same manual setting or both to auto.
Troubleshooting with This Command
When troubleshooting network issues, 'show interfaces' is your first line of defense. A healthy interface displays 'up/up' for the line status and protocol. The counters show minimal errors: CRC errors should be zero or very low, runts and giants should be zero, and input/output errors should be near zero. High input errors (CRC, runts, giants) often point to physical layer problems: faulty cables, bad connectors, electromagnetic interference, or a duplex mismatch. For example, a duplex mismatch typically causes a high number of CRC errors and collisions on the half-duplex side. Output errors (underruns, overruns, ignored) may indicate a speed mismatch or a saturated interface. Interface resets can occur due to hardware issues or if the interface is flapping.
Step-by-step diagnostic flow: 1. Check the line status: if it's 'administratively down', the interface is shut down; use 'no shutdown' to enable. If it's 'down/down', the physical connection is broken (cable, transceiver, or far-end device powered off). If it's 'up/down', the line protocol is down; check Layer 2 issues like keepalives, encapsulation mismatch, or no clocking on serial links. 2. Examine the 'Last input' and 'Last output' timestamps. If 'Last input' is old (e.g., '00:00:00' means recent activity; if it shows 'never', the interface has not received any packets). This helps identify if traffic is flowing. 3. Look at the 'Input queue' and 'Output queue' drops. High input queue drops indicate the router cannot process packets fast enough (CPU overload). High output queue drops indicate the interface is congested (traffic exceeds bandwidth). 4. Check the '5 minute input rate' and '5 minute output rate' to see current utilization. If utilization is consistently high (e.g., >80%), consider upgrading bandwidth or implementing QoS. 5. Correlate with other commands: 'show interfaces counters errors' gives a quick error summary; 'show interfaces [interface] accounting' shows protocol-level statistics; 'show logging' may reveal interface flaps; 'debug interface' can show real-time events but use with caution in production.
Common symptoms and what they mean: - 'runts' (frames smaller than minimum size): often caused by collisions or faulty NIC. - 'giants' (frames larger than maximum): usually due to misconfigured MTU or faulty hardware. - 'CRC errors': checksum failures, typically physical layer issues. - 'collisions': normal on half-duplex, but excessive collisions indicate a duplex mismatch or too many devices on a collision domain. - 'interface resets': the interface was reset (e.g., due to a hardware error or 'clear counters').
By systematically analyzing these fields, you can isolate the root cause of many connectivity and performance problems.
CCNA Exam Tips
CCNA exam: memorise the status line combinations. 'administratively down' = shutdown command applied
For duplex mismatch diagnosis: high CRC and input errors on one side, high collisions on the other (half-duplex) side
BW and DLY values affect routing protocol metric calculations (EIGRP, OSPF cost)
Common Mistakes
Confusing 'up/down' (keepalive issue) with 'down/down' (physical issue)
Ignoring CRC errors as minor — high CRC rate indicates a real cabling or duplex problem
Not checking both ends of a link for errors (one side may show clean while the other has all the errors)
show interfaces vs show ip interface brief
These two commands are commonly confused because both display interface status, but they differ significantly in detail and output format. While 'show ip interface brief' provides a concise summary of IP addresses and operational states, 'show interfaces' offers comprehensive Layer 1 and Layer 2 statistics.
| Aspect | show interfaces | show ip interface brief |
|---|---|---|
| Scope | All interfaces with detailed L1/L2 stats | All interfaces with IP and status summary |
| Detail Level | High (error counters, queue, hardware) | Low (IP, status, protocol) |
| Output Format | Verbose per interface, multi-line | Compact one-line per interface |
| Typical Use | Troubleshooting physical/link issues | Quick inventory or health check |
| Speed of Execution | Slower on devices with many interfaces | Fast, lightweight |
| Information Provided | MAC, MTU, bandwidth, duplex, speed, errors | IP address, status, protocol |
Use show interfaces when you need to diagnose Layer 1 or Layer 2 problems, such as excessive errors, duplex mismatches, or line protocol flapping.
Use show ip interface brief when you need a rapid, high-level view of interface IP addresses and operational status for initial troubleshooting or documentation.
Platform Notes
In IOS-XE (e.g., Catalyst 9000 switches), the 'show interfaces' command syntax and output are very similar to classic IOS. However, IOS-XE may include additional fields like 'Input queue: 0/2000/0/0 (size/max/drops/flushes)' or 'Output queue: 0/40/0/0 (size/max/drops/flushes)'. The command is fully supported.
For NX-OS (e.g., Nexus switches), the equivalent command is 'show interface' (singular) or 'show interfaces'. The output format differs: NX-OS uses a more structured display with fields like 'admin state', 'link state', 'hardware: Ethernet, address: xxxx.xxxx.xxxx'. NX-OS also supports 'show interface brief' for a summary.
On ASA firewalls, the command is 'show interface' (singular) and output includes firewall-specific counters like 'packets input/output' and 'drops'. ASA also has 'show interface ip brief' for IP addresses. In IOS-XR (e.g., ASR 9000), the command is 'show interfaces' but the output is quite different: it uses a hierarchical format with extensive detail.
IOS-XR also requires the 'detail' keyword for full statistics. For example, 'show interfaces GigabitEthernet0/0/0/0 detail'. In older IOS versions (12.x), the output may not include 'input rate' and 'output rate' lines; these were introduced in later versions.
Always check the specific platform documentation for exact syntax.
Related Commands
clear counters
Resets all interface packet and error counters to zero without affecting the interface operational state, allowing a clean baseline for traffic monitoring or error rate measurement.
show interfaces trunk
Displays all trunk ports including the trunking encapsulation (802.1Q or ISL), the native VLAN, VLANs allowed on the trunk, VLANs in the spanning tree forwarding state, and VLANs active in the management domain.
show ip interface brief
Displays a one-line summary of all interfaces showing the IP address assigned, operational status (up/down), and line protocol status. The fastest way to get a device health overview.
Practice for the CCNA 200-301
Test your knowledge with practice questions covering all CCNA 200-301 exam domains.
Practice CCNA 200-301 Questions