Cisco N5K-M1600 - Expansion Module - 6 Ports Troubleshooting Manual page 54

Registers and Counters
S e n d d o c u m e n t c o m m e n t s t o n e x u s 5 k - d o c f e e d b a c k @ c i s c o . c o m .
Total Multicast crossbar statistics:
MTU violation
The Nexus 5000 is a cut-through switch at 10 Gb/s. This means that an MTU can be checked, but the
frame will already be transmitting before the length is known. Therefore, the frame cannot be dropped.
The frame is truncated after the MTU is reached and the CRC value is stomped. The ingress interface
increments an Rx Jumbo and the egress interface will increment a Tx CRC and a Tx Jumbo.
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Handling CRC errors
When a CRC error is seen in the FCS on a cut-through port, the Rx CRC counter of the show interface
command is incremented. However, the frame cannot be dropped because the FCS is at the end of the
Ethernet frame on the wire.
The egress interface increments a Tx CRC error and it propagates through to the next device in the path.
You can use the show hardware internal gatos counters interrupt match stomp command to
determine if the Nexus 5000 is propagating CRCs or generating them.
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MAC Statistics
During normal operation, a Nexus 5000 encounters frames that cannot be forwarded.
Frames are characterized as good frames or bad frames.
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All counters include MAC Control frames where applicable.
Cisco Nexus 5000 Series Troubleshooting Guide
3-12
Mcast pkts received from the cross-bar
If jumbo frames are seen with the show interface or the show hardware internal gatos port
ethernet 1/1 counters rx commands, this is not an indication that the frames are being dropped. A
jumbo frame is just an Ethernet frame, greater than 1500 bytes, that was received or transmitted.
The <b>show queuing interface <i>ex/y</i></b> command shows the current configured MTU
(per class).
A drop due to an MTU violation can be seen with the show hardware internal gatos counters
interrupt match mtu* command.
A counter that matches the Gatos number and fw_instance from the show hardware internal gatos
port ethernet 1/1 | include instance|mac command is the indicator that an MTU violation has taken
place and that the frame has been stomped.
If stomp values exist, they should have matching CRC values on that interface.
If Rx CRC values exist, then you know it entered the switchport with the error already. You can move
on to the connected device to trace it back.
A good frame is a frame that does not have a CRC error or other kind of error
A bad frame is a frame that has a CRC error or other kind of error
Chapter 3 Troubleshooting Layer 2 Switching Issues
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