Switch Interface Error Counts

---

• interface status commands will report whether any collisions are being generated
• collisions might occur if:
  • the duplex setting on the switch port and host is mismatch
  • legacy hub device or host NIC is connected to a switch
• other interface errors might indicate:
  • misconfiguration problem at Data Link layer
  • interference at Physical layer

Increasing Interface Counters

• interface might change rapidly between up and down states
  • called flapping
• interface counters record the number of events over time
  • allows you to diagnose an interface that is up but is unreliable or performing poorly
  • link state
    • measures whether an interface is working (up) or not (down)
    • configure an alert if an interface goes down
    • track the uptime or downtime percentage so that you can assess links reliability over time
  • resets
    • number of times an interface has restarted over the counter
    • interfaces can be reset manually or restart automatically if traffic volumes are very high or large number of errors
    • frequent resets should be closely monitored and investigated
  • discards/drops
    • interface may discard incoming or outgoing frames for several reasons:
      • checksum errors, mismatched MTUs, packets too small (runts) or too large (giants), high load, permissions, sender not in ACL, or VLAN config problem
    • each interface classes the type of discard or drop separately to help find precise cause

Cyclic Redundancy Check Errors

• a cyclic redundancy check (CRC) is calculated by an interface when it sends a frame
  • CRC value is calculated from the frame contents to derive a 32-bit value
  • added to header as the frame check sequence
  • receiving frame uses the same calculation:
    • if derives different value, frame is rejected
• number of CRC errors can be monitored per interface

Runt Frame Errors

A runt is a frame that is smaller than the minimum size (64 bytes for Ethernet).

• usually caused by a collision
• in switched environment, collisions should only be on an interface connected to a legacy hub device and there is a duplex mismatch
• if runts are generated in other conditions
  • suspect driver issue on transmitting host

Giant Frame Errors

a giant is a frame that is larger than the max permissible size (1518 bytes).

• two likely causes:
  • jumbo frames
    • host might be configured to use jumbo frames, but switch interface is not
    • MTU value in the show interface output will indicate whether jumbo frames are accepted on a port
  • Ethernet trunks
    • trunk link carries traffic between switches or between a switch and router
    • often use 802.1Q framing to carry VLAN information
    • if one switch interface is configured for 802.1Q but other is not:
      • the frames will appear too large to the receiver
        • 802.1Q adds 4 bytes to the header, making max frame size 1522 bytes

Info

Ethernet frame that is slightly larger (up to 1600 bytes) is called a baby giant.