Runt and Giant Packet

Unanswered Question
mlund Wed, 08/05/2009 - 23:36

Hi Neo

They will be discarded.

Giants:

Description: CatOS sh port and Cisco IOS sh interfaces and sh interfaces counters errors. Frames received that exceed the maximum IEEE 802.3 frame size (1518 bytes for non-jumbo Ethernet) and have a bad Frame Check Sequence (FCS).

Common Causes: In many cases, this is the

result of a bad NIC. Try to find the offending device and remove it from the network.

Platform Exceptions: Catalyst Cat4000

Series that run Cisco IOS Previous to

software Version 12.1(19)EW, the giants

counter incremented for a frame > 1518bytes.

After 12.1(19)EW, a giant in show interfaces increments only when a frame is received >1518bytes with a bad FCS.

Runts:

Description: CatOS sh port and Cisco IOS sh interfaces and sh interfaces counters errors.

The frames received that are smaller than the minimum IEEE 802.3 frame size (64 bytes for Ethernet), and with a bad CRC.

Common Causes: This can be caused by a

duplex mismatch and physical problems, such as a bad cable, port, or NIC on the attached device.

Platform Exceptions: Catalyst 4000 Series

that run Cisco IOS Previous to software

Version 12.1(19)EW, a runt = undersize.

Undersize = frame < 64bytes. The runt counter only incremented when a frame less than 64 bytes was received. After 12.1(19EW, a runt = a fragment. A fragment is a frame < 64 bytes but with a bad CRC. The result is the runt counter now increments in show interfaces, along with the fragments counter in show interfaces counters errors when a frame <64 bytes with a bad CRC is received.

/Mikael

jbrenesj Thu, 08/06/2009 - 11:15

Just wanted to add something..

Most switches could report 801.1q tagged frames as giants and they won't get dropped. This happens if a frames that is coming to the switch is 1518 bytes long + 4 bytes of dot1q = 1522

The switch will strip the tag, report the giant but will still let the frame get in without getting dropped.

If you see a giant, what you do is check the:

Core3560#sh controllers ethernet-controller f0/1

Look for:

- Invalid frames, too large

- Valid frames, too large

Example:

sh int fa 0/1 sees 288 giants but sh controller eth f0/1 sees 288 Valid frames, too large ===== no problem

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