What are runts and giants, and what usually causes them?
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A runt is a frame shorter than the Ethernet minimum of 64 bytes. A giant is longer than the port accepts: 1518 bytes untagged, or 1522 with one 802.1Q tag, on a standard 1500-byte port. Runts usually point at collisions, a duplex mismatch or a failing NIC. Giants usually mean an MTU mismatch: a neighbor sends jumbo frames or extra tags to a port that is not configured for them, and those frames are dropped.
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Runts
- The minimum frame is 64 bytes including the FCS, which is why a minimum payload is 46 bytes.
- Causes: collision fragments on half-duplex links, a duplex mismatch where the half-duplex side aborts frames mid-transmission, or a failing NIC or transceiver.
Giants
- The standard maximum is 1518 bytes: 1500 MTU, 14-byte header, 4-byte FCS. 1522 with an 802.1Q tag.
- Causes: a server or neighbor sending 9000-byte jumbo frames to a 1500 port, extra encapsulation such as QinQ or MPLS labels without raising the MTU, or a faulty NIC.
- Fix: make the MTU consistent across the whole layer 2 path. The command is platform-specific: a global
system mtuon many Catalyst models, a per-interfacemtuon NX-OS, EOS and Junos.
Reading them with other counters
| Pattern | Likely cause |
|---|---|
| Runts and CRC on one side, late collisions on the other | duplex mismatch |
| Giants climbing on a port facing a server or a provider | MTU mismatch, jumbo frames or extra tags |
| Runts on a full-duplex link with no collisions anywhere | failing NIC or optic |
They will ask next
Why is 64 bytes the minimum Ethernet frame?
Half-duplex Ethernet needed a sender to still be transmitting when a collision from the far end of a maximum-length segment came back, so it could detect it. At 10 Mb/s that slot time is 512 bit times, which is 64 bytes. The minimum stayed for compatibility.
Is a 1522-byte frame on a trunk a giant?
No. A switch expects the 4-byte tag on trunks and accepts 1522. Giants are frames larger than the port is set to take, and they are dropped.
Traps
- Saying the maximum Ethernet frame is 1500 bytes. 1500 is the MTU, the payload; the frame is 1518 untagged.