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The "%C4K_HWPORTMAN-4-BLOCKEDTXQUEUE" error message on Cisco Catalyst 4500 series switches connected to a device

Core issue

The %C4K_HWPORTMAN-4-BLOCKEDTXQUEUE:Blocked transmit queue HwTxQId[dec]on[char], count=[dec] error message is generated on a Cisco Catalyst 4500 series switch when connected to a device. This error is a rate-limited message that indicates a transmit queue blockage on a port.

In this case, the traffic on the affected port is limited and blocked for reasons other than being paused. This blockage can occur if the supervisor is not able to send packets to the line card because a busybit message is received. In this situation, blocked transmit queue messages are seen. A bad hardware or speed and duplex mismatch can cause this problem.


Perform these procedures as a workaround for this issue:

  1. Issue the shut /no shut commands to recover the port and configure both ends to operate at the same speed and duplex, as per Cisco bug ID CSCsb62330.

  2. If the problem persists, move the connected device to another port and see if the problem also happens there. 

  3. Issue the hw-module reset command to reboot the switch or reset the line card, as a final attempt to unblock the Transmit (Tx) queue.

Alternatively, upgrade the the Cisco IOS  version to the 12.2(25)EWA2 and 12.2(25)SG releases, which have the fix for this problem, as per Cisco bug ID CSCsb01311.

Community Member

Looks like this was a flow control problem. I was seeing this message on the console log of a 4500 switch. No incrementing counters, everything looked good but was getting a log message about every 8 minutes. While looking at the switchport config I noticed that flowcontrol was on both send and receive. I disabled flow control both ways and the message stopped immediately.Evidently when the port negotiated with the host, something went wrong and the switch thought it was receiving pause bits from the endpoint. So check your flow control settings on the port in question.