20 May

Cannot write configuration on Nexus

One of my customers told me that he can’t save configuration on Nexus 7000 switch. It has been working for over 6 months with no problems.

switch1# copy running-config startup-config
Configuration update aborted: another request for config change is already in progress

It looks like a CSCtj44206 bug. In this case software switchover is not a solution as this request will be refused by system due to configuration lock. Solution, except full chassis reload, is performing hardware switchover by removing active supervisor from chassis. This will unlock configuration and let you perform ISSU to latest NX-OS where this bug is solved.

14 May

No ISSU on Nexus 7000 with vPC between VDCs on same chassis

If you are using multiple VDCs on your Nexus 7000 switch and you create vPC between two of them you can forget about upgrading NX-OS via ISSU. This scenario is not supported, but there is no information about that in documentation. If you try to perform upgrade you will see following error message:

Notifying services about the switchover.            [#                   ]   0% -- FAIL. Return code 0x401E007B (request was aborted by service).
Please issue "show install all failure-reason" to find the cause of the failure.

Failure recovery action::
"Standby will be rebooted to force netboot and image download".
Install has failed. Return code 0x401E007B (request was aborted by service).

Please identify the cause of the failure, and try 'install all' again.

n7k-switch# show install all failure-reason
Service "vpc" in vdc: 2 returned error: configuration lock can not be acquired for peer switch (x41B7004B)
Service "vpc" in vdc: 3 returned error: configuration lock can not be acquired for peer switch (x41B7004B)

The only way to upgrade is to download new software, replace bootvar and reboot whole chassis.

09 May

Detecting Unidirectional Link Failure on STP

UDLD is a nice mechanism detecting unidirectional transmission over fiber or copper link. But it tests only physical layer. Therefore if there is a problem with transmitting BPDUs over link UDLD won’t detect this problem which may cause loops in network.

IEEE 802.1D-2004 Rapid STP standard defines dispute mechanism that works similar to UDLD but on layer 2 of OSI model. Root bridge is sending Superiors BPDUs over it’s links and is waiting for replies from the neighbors. If an adjacent switch respond in a way suggesting it didn’t receive SBPDU, root bridge block its port, thus preventing the bridging loop.

Dispute mechanism works only for RSTP and MST BPDUs because those ones includes the role and state of the sending port.

Dispute can be found on Catalyst 6500 with IOS 12.2(33) SXI and above, Catalyst 4500 and 4900 since 12.2(52)SG release as well as on Nexus switches.