Cisco N5K-C5020P-BF Troubleshooting Guide - Page 54

MTU violation, Handling CRC errors, MAC Statistics

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Page 54 highlights

Registers and Counters Chapter 3 Troubleshooting Layer 2 Switching Issues Send document comments to [email protected]. Total Multicast crossbar statistics: Mcast pkts received from the cross-bar : 0 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. • 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 show queuing interface ex/y 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. 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. • 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. MAC Statistics During normal operation, a Nexus 5000 encounters frames that cannot be forwarded. Frames are characterized as good frames or bad frames. • 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 All counters include MAC Control frames where applicable. 3-12 Cisco Nexus 5000 Series Troubleshooting Guide OL-25300-01

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Send document comments to [email protected].
3-12
Cisco Nexus 5000 Series Troubleshooting Guide
OL-25300-01
Chapter 3
Troubleshooting Layer 2 Switching Issues
Registers and Counters
Total Multicast crossbar statistics:
Mcast pkts received from the cross-bar
: 0
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.
•
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.
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.
•
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.
MAC Statistics
During normal operation, a Nexus 5000 encounters frames that cannot be forwarded.
Frames are characterized as good frames or bad frames.
•
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
All counters include MAC Control frames where applicable.