Every once in a while, you just have to reload a Cisco router or a router interface to make a change take effect. Learn about three such occasions from Chris Bryant, CCIE #12933.
Passing the CCNA certification exam means that you know how to configure and troubleshoot a Cisco router instead of using what I call the "hope method" - you know, "Let's reload the router and hope that takes care of it." The majority of Cisco router configurations take effect without the need for a reload, but every once in a while you just have to reload a router or shut and reopen an interface. Let's take a look at three such scenarios.
The first is when you change an OSPF Router ID from its default. For the new RID to take effect, you must either reload the router or clear the OSPF process, which means that all existing adjacencies will come down. Cisco routers are kind enough to tell you this with the following message after you configure a new RID: "Reload or use "clear ip ospf process" command, for this to take effect".
In a previous tutorial, I showed you how to configure an Etherchannel. You have to place each port into the Etherchannel with the channel-group command, and if you do so individually, some of the ports may go into error-disabled state, or "err-disable". This can also happen as a result of port security enforcement. You can see this with the show interface command:
sw1#show int fast 0/1
FastEthernet0/1 is down, line protocol is down (err-disabled)
A syslog message putting that port into err-disabled state will look like this:
04:10:23: %PM-4-ERR_DISABLE: channel-misconfig error detected on Po1, putting Fa0/1 in err-disable state
If this happens during an Etherchannel configuration, just finish the config and then shut and reopen the ports in err-disabled state. They'll come back up and be placed into the Etherchannel.
Finally, our old friend the SPID often makes us shut and reopen the BRI interface. If the BRI interface is open and you configure SPIDs on it, the SPID can be absolutely correct and you'll still see this in the output of show isdn status:
spid1 configured, spid1 NOT sent, spid1 NOT valid
At least the Cisco router puts "NOT" in caps, right? It's easier to see that way! With SPIDs, before you call the service provider or check the SPID you entered about 40 times, just shut and reopen the interface. That usually does the trick.
When you earn your CCNA certification, that means that you know what you're doing instead of hoping that you do - and part of that is knowing when a simple reload or open/shut will take care of the issue.
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