Runts and Giants Appearing on Interface — What They Mean
Presenting Symptom
The network engineer observes a high number of runts and giants in the interface counters of a Cisco router or switch, along with possible CRC errors and input errors.
Network Context
This issue occurs in a small to medium-sized enterprise network where a Cisco 2960 switch connects to a Cisco 4321 router via a copper Ethernet link. The switch is configured with default duplex and speed settings, and the router is set to auto-negotiate. The network experiences intermittent connectivity and performance degradation.
Diagnostic Steps
Check Interface Counters for Runts and Giants
show interfaces GigabitEthernet0/1GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 1000 bits/sec, 2 packets/sec
12345 packets input, 1234567 bytes
Received 100 broadcasts (0 multicast)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
12345 packets output, 1234567 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped outIf runts and giants are non-zero, it indicates frames that are smaller than 64 bytes (runts) or larger than 1518 bytes (giants) are being received. This often points to a duplex mismatch or faulty cabling. Normal output should show 0 runts and 0 giants.
Check Duplex and Speed Settings on Both Ends
show interfaces GigabitEthernet0/1 | include duplexFull-duplex, 1000Mb/s
If one end shows Full-duplex and the other shows Half-duplex, a duplex mismatch exists. This is a common cause of runts and giants. Also verify speed matches on both ends.
Check for CRC and Frame Errors
show interfaces GigabitEthernet0/1 | include CRC0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
CRC errors often accompany runts and giants in a duplex mismatch scenario. If CRC errors are present, it confirms physical layer issues.
Check the Connected Device's Interface Counters
show interfaces GigabitEthernet0/0 (on the router)GigabitEthernet0/0 is up, line protocol is up
Hardware is Gigabit Ethernet, address is xxxx.yyyy.zzzz (bia xxxx.yyyy.zzzz)
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Half-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
...
12345 packets input, 1234567 bytes
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
...If one device shows Full-duplex and the other shows Half-duplex, a duplex mismatch is confirmed. The half-duplex side will often see late collisions and runts.
Root Cause
A duplex mismatch between the switch and router. The switch is set to auto-negotiate and successfully negotiates to Full-duplex, while the router is manually configured to Half-duplex (or vice versa). This causes collisions and frame errors on the half-duplex side, resulting in runts and giants.
Resolution
Verification
After applying the fix, run 'show interfaces GigabitEthernet0/1' on both devices. Verify that both show 'Full-duplex' and that runts, giants, CRC errors are now 0. Also check that the interface counters are incrementing normally without errors.
Prevention
1. Always use the same duplex and speed settings on both ends of a link. 2. Prefer auto-negotiation for copper Ethernet links to avoid misconfiguration. 3. Regularly monitor interface counters for errors as part of proactive network management.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify the cause of runts and giants from show interface output. The exam tests your understanding of duplex mismatch symptoms and the ability to interpret interface counters. Key fact: Runts and giants often indicate a duplex mismatch or physical layer issue.
Exam Tips
Memorize the normal frame size range: 64-1518 bytes. Runts are <64 bytes, giants are >1518 bytes.
In a duplex mismatch, the half-duplex side will see late collisions and runts; the full-duplex side may see giants.
Always check both ends of the link for duplex and speed settings when troubleshooting interface errors.
Commands Used in This Scenario
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