Technical data

Brocade 6910 Ethernet Access Switch Diagnostic Guide 13
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Chapter
3Layer 1 Diagnostics
This chapter describes common Layer 1 diagnostic procedures for the Brocade 6910 series
switches. In general, Layer 1 issues are related to hardware, the most common being the following
physical connectivity problems:
• Faulty ports
• Faulty cables
• Faulty hardware
• Input and output errors
• Cyclic redundancy check (CRC) errors
• Excessive or late collisions
• Overruns
• Output buffer failures
Ethernet diagnostics
The following sections describe how to troubleshoot Layer 1 issues for Ethernet interfaces.
Duplex mismatches
A duplex mismatch can occur between devices in the following situations:
• One device is manually set to half duplex and one device is manually set to full duplex.
• One device is set to autonegotiation and one device is manually set to full duplex.
Duplex mismatches are difficult to diagnose because the network still appears to be working.
Simple tests, such as ping, report a valid connection even though network performance can be
much slower than normal.
When one device operates in full duplex while the other one operates in half duplex, the connection
works at a very low speed if both devices attempt to send frames at the same time. This is because
a full-duplex device may transmit data while it is receiving, but if the other device is working in half
duplex, it cannot receive data while it is sending. The half-duplex device senses a collision and
attempts to resend the frame it was sending. Depending on timing, the half-duplex device may
sense a late collision, which it will interpret as a hard error, and will not attempt to resend the
frame. At the other end, the full-duplex device does not detect a collision and does not resend the
frame, even if the half-duplex device has already discarded it as corrupted by collision.
The packet loss happens when both devices are transmitting at the same time, and may happen
even when the link is used, from the user's perspective, in one direction only. A Transmission
Control Protocol (TCP) stream requires that all packets sent be acknowledged by the receiving
device, even if actual data is sent in one direction only. Packet collisions may occur with
acknowledgement packets traveling in the other direction.