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The router was broken down and has changed config register itself, why? What is the reason of router fault?

Hello everybody! :)

One of our client reported problem with a router - router is break down (I don't know what it does mean for this associate).  When router was delivered to my office I make a test and what happened next: when I powered on a router and connect console cable (exactly in that sequence) terminal show me rommon mode. Next I entered a confreg command and in that moment I noticed that configuration register have a 0x2101 value. I have changed config register to 0x2041 value and powered off the device. After powering on router started normally. Then I have changed config register to 0x2102 value.

Now I have some doubt:

  1. Is it possible that 0x2101 was configured some one for special purposes it is normal for router working?
  2. Is it possible that router has changed value of config register itself? If it possible, why router did that? Why router can change a configuration of config register in general?
  3. What could be the most likely reason that router loaded ROMMON during normal functionality? 

    Thank you for your answers.


It's a second accident with Cisco routers. Before that we had case where router changed conf register to 0x2142 and how do you think what was happened? :-D
 

P.S. I'm so sorry for my English, if something is not clear I could explain again.

 

1 REPLY
Hall of Fame Super Gold

When it comes to config

When it comes to config-registry, the default config should be 0x2102.   Now here comes the "rub".  If you are talking about routers (and ONLY routers), 0x2102 is just fine.  Basically this means that boot using one of two methods: 

  1. If there is an absence of a (valid) boot variable statement, the router will boot the FIRST IOS file it'll see. 
  2. If there is a boot variable statement, it'll follow the boot variable statement.  If the boot variable statement is incorrect, see #1.  :)

Catalyst switches, however, is a different basket.  The non-chassis-based models have a default value of 0xF.  And this is fine.  However, when you're dealing with supervisor cards, it's a whole new ballgame.  

 

For supervisor cards, 0x2102 is a two-edged sword that one should be very careful.  Unlike routers, 0x2102 depends entirely on a boot variable statement.  If you don't have one or if your boot variable statement is incorrect, the supervisor card will boot into ROMmon.  It will even boot into ROMmon even if there is a valid IOS file.  

 

I've seen supervisor cards straight out of the factory sporting a configuration register of 0x2101.  The value, for a supervisor card, means boot the first available IOS.

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