Cabletron Systems SEHI-32/34 Runt Frames, Giant Frames, The SEHI Error Priority Scheme, 2-18

Using the SEHI Hub View

802.3specifications, or a node on the net is transmitting without first listening for carrier sense (and beginning its illegal transmission more than 51.2 ∝s after the first station began transmitting). Note that in both cases, the occurrence of the errors can be intermittent: in the case of excessive network length, OOW collisions will only occur when the farthest stations transmit at the same time; in the case of the node which is transmitting without listening, the malfunctioning node may

only fail to listen occasionally, and not all of its failures to listen will result in OOW collisions — some may simply result in collisions (if the 51.2 ∝s window has not yet closed), and some will get through fine (if no one else happens to be transmitting).

Runt Frames

The number of received packets smaller than the minimum Ethernet frame size of 64 bytes (excluding preamble). This minimum size is tied to the maximum propagation time of an Ethernet network segment — the maximum propagation time is 51.2 ∝s, and it takes approximately 51.2 ∝s to transmit 64 bytes of data; therefore, every node on the segment should be aware that another node is transmitting before the transmission is complete, providing for more accurate collision detection. Runts can sometimes result from collisions, and, as such, may be the natural by-product of a busy network; however, they can also indicate a hardware (packet formation), transmission (corrupted data), or network design (more than four cascaded repeaters) problem.

Giant Frames

The number of received packets that are longer than the maximum Ethernet size of 1518 bytes (excluding preamble). Giant packets typically occur when you have a jabbering node on your network — one that is continuously transmitting, or transmitting improperly for short bursts — probably due to a bad transmitter on the network interface card. Giant packets can also result from packets being corrupted as they are transmitted, either by the addition of garbage signal, or by the corruption of the bits that indicate frame size.

The SEHI Error Priority Scheme

Each Cabletron device employs an error priority scheme which determines how packets with multiple errors will be counted, and ensures that no error packet is counted more than once. The priority scheme for the SEHI counts errors in the following order:

1.	OOW Collisions
2.	Runts
3.	Giants
4.	Alignment Errors
5.	CRC Errors

Knowing the priority scheme employed by the SEHI can tell you a lot about the error counts you are seeing. For example, you know that the number of packets

2-18 Monitoring Hub Performance