Config Register

Cisco Aug 30, 2009

The other week I can across a very useful command after we had a failure to one of our Sup720 cards is a 6509’s. The card was replaced but during the boot-up it would keep on dropping back to ROMMON, however when you type ‘boot’ it would load as expected. When I was asked to have a look my immediate thought was ‘config register’ settings.

I carried out the usual configuration checks to ensure the config reg was set at 0x2102 (tried from both ROMMON and from IOS). When reloaded the same problem occurred with the output of the various IOS ‘sh’ (sh boot, sh ver) commands as well as commands form ROMMON showing things as OK. I then started to think we had a problem with the IOS image (even though I knew it was booting when I try manually), or the new card was faulty.

After a bit of digging and head scratching I discovered that I was getting misleading information from the ‘sh  boot’ command as this did not give the answer for the Sup720 card, this was confirmed after issuing ‘remote command switch show boot‘ it showed the config register was et to 0x0. Issuing a config-register 0x2102 and rebooting resolved the issue.

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Rob Edwards

Northern (UK) chap focusing on platforms, automation, cloud and cloud native applications. Recovering network engineer, although it turns out networking is as important, if not more, now than before!

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