Understanding Port Errors

Typically, at the port level, you are interested in Layer 1 and Layer 2 errors. Some of the common errors are bad cable, alignment issues, frame check sequence (FCS), and collisions seen on the port. Example 12-9 shows the output from the show port command.

Example 12-9 shows excessive collisions seen on port 10/3. Recall from Chapter 1, "LAN Switching Foundation Technologies," excessive collisions are associated with too much traffic on the segment or too many hosts contenting to access the segment.

Example 12-9. Examining Port Fields

Switch1 (enable) show port 10/3

* = Configured MAC Address

Port  Send FlowControl  Receive FlowControl   RxPause    TxPause

      admin    oper     admin     oper

----- -------- -------- --------- ---------   ---------- ----------

10/3  off      off      off       off         0          0

Port  Status     Channel              Admin Ch

                 Mode                 Group Id

----- ---------- -------------------- ----- -----

10/3  connected  auto silent            620     0

Port  Status      ErrDisable Reason    Port ErrDisableTimeout  Action on Timeout

----  ----------  -------------------  ----------------------  -----------------

10/3  connected                     -  Enable                  No Change

Port  Align-Err  FCS-Err    Xmit-Err   Rcv-Err    UnderSize

----- ---------- ---------- ---------- ---------- ---------

10/3          11          0          0          0         6

Port  Single-Col Multi-Coll Late-Coll  Excess-Col Carri-Sen Runts     Giants

----- ---------- ---------- ---------- ---------- --------- --------- ---------

10/3      218713     197092          0     401867         0        27         0

The following define the more important fields associated with the show port command:

  • Alignment Errors? Frames received that do not end with an even number of octets. Frame also has a bad cyclic redundancy check (CRC). Misconfiguration is generally the culprit. Ensure that adjacent connected devices have the same duplex setting configuration.

  • FCS Errors? Bad CRC on the frame packet transmitted/received. These packets are dropped by the switch at the port level.

  • Xmit Errors? Internal transmit buff is full.

  • RCV Errors? Internal receive buff is full.

  • Single Collisions? The number of times the transmitting port had one collision before successfully transmitting the frame on the wire. Single collisions are part of normal operation in a half-duplex environment because of carrier sense multiple access collision detect (CSMA/CD) rule.

  • Multiple Collisions? The number of times the transmitting port had more than one collision before successfully transmitting the frame on the wire. Multiple collisions do occur if the medium is busy enough.

  • Late Collisions? Usually indicative of the cable exceeding IEEE specifications. Cascading hubs can also cause the length of the collision domain to increase above specification. A Time Delay Reflectometer (TDR) can be used to detect cable fault and whether the cable is within the IEEE standard. Other factors that cause late collisions include mismatched duplex settings and bad transceivers.

  • Excessive Collisions? The number of frames that are dropped because the transmitting port saw 16 collisions in a row. This should not occur and can be attributed to possibly a design flaw that is exacerbating an already congested link.

  • Runts? Frames smaller than 64 bytes with a bad frame check sequence (FCS). Bad cabling or inconsistent duplex setting usually cause runts.

  • Giants? Frames greater than 1518 with a bad FCS. Investigate NIC issues on the port.