Whether you're preparing for the CCNA certification exam or not, you must be prepared for the following question:

™

"Hey, I reloaded this router and it wants an enable password. Do you know it?"

Because if you don't, and there's no one available who does, you need to perform a password recovery technique on the router - without erasing the current configuration. This involves manipulating the config register, and a misstep here can be fatal to the router's chances of recovery!

Obviously, that can make you pretty nervous about changing the config register, CCNA or not. Different Cisco routers and switches have different techniques for password recovery, so the following discussion is limited to the 2500 series. If you need to do this for another model, do a quick search engine check for "password recovery cisco" and you should quickly find a document for the Cisco router you're working with.

For the 2500 series, you start by reloading the router and sending a BREAK signal during the first 60 seconds of the reload. Depending on the terminal program you're using, this can be the hardest part of the entire process! For most, just press CTRL-BREAK during this one-minute period. If this doesn't work, you may need to check Help in your terminal program to find out how to send this break signal.

As a result of the break sequence, the router will go into ROM Monitor mode. The commands here are totally different than the ones you're used to working Click for more info with in the router's IOS. Use the command o/r 0x2142 to change the config register setting, and reload the router by entering the letter "i".

This config register setting doesn't erase anything, but it does make the router ignore the contents of NVRAM. This means that you'll be prompted to go into the dreaded Setup Mode, which you do NOT want to do. Simply press "N" and type "enable" when you're at the router prompt. (If you do go into Setup Mode, you can always get out with CTRL-C, a handy command to know for the CCNA exam and for real life, as you can see!)

Be very careful with the next step. You want to enter the command "configure memory" or "copy start run" at this point - don't enter "write memory" or "copy run start". Success on the CCNA exam and in working with real-world networks is all about the details, and this is a very important detail.

At this point, you can look at the running configuration and see the passwords, and change them if you wish. However, we're not done. The config register needs to be set back to its default of 0x2102, and you do so with the global command "config-register 0x2102". Now you want to save your config with "write memory" or "copy run start", and reload the router. The router will now boot as it normally would.

Knowing how to recover from a lost password is a vital skill for both the Cisco CCNA certification exam and for success in real-world networks. It's not something we have to do every day, but when the time calls for it, we have to do it correctly and completely - and that includes that final config-register change!

OSPF is a major topic on your CCNA exam, as well it should be. OSPF is a widely-used WAN protocol, and you need to learn the fundamentals before moving on to more complicated configurations. One such detail is the OSPF Router ID, or RID.

The RID is the dotted decimal value by which other OSPF routers will identify a given OSPF router. There are some interesting defaults for this value, and a command you should know to hardcode the RID. You had also better know what has to happen for this command to take effect, so let's take a more detailed look at the OSPF RID.

In this example, R1 has an adjacency with R2 and R3 over the 172.12.123.0/24 frame network. R1 is the hub, with R2 and R3 as the spokes. No other interfaces are OSPF-enabled on any of the routers. Running show ip ospf neighbor on R1, we see some unusual values under "Neighbor ID", which is another name for the OSPF RID.

R1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

3.3.3.3 0 FULL/DROTHER 00:01:57 172.12.123.3 Serial0

2.2.2.2 0 FULL/DROTHER 00:01:57 172.12.123.2 Serial0

Notice the Neighbor ID of each remote address is the loopback address. How can that be if they're not OSPF-enabled?

When determining the Router ID (RID) of an OSPF-enabled router, OSPF will always use the numerically highest IP address on the router's loopback interfaces, regardless of whether that loopback is OSPF-enabled.

What if there is no loopback? OSPF will then use the numerically highest IP address of the physical interfaces, regardless of whether that interface is OSPF-enabled.

BOTTOM LINE: An interface does not have to be running OSPF to have its IP address used as the OSPF RID.

The OSPF RID can be changed, but it requires a restart or to reinitialize the OSPF routing process. Use the router-id command to change the default RID of each router as shown, and clear the OSPF process to do so.

R1#conf t

Enter configuration commands, one per line. End with CNTL/Z.

R1(config)#router ospf 1

R1(config-router)#router-id 11.11.11.11

Reload or use "clear ip ospf process" command, for this to take effect

R1#clear ip ospf process

Reset ALL OSPF processes? [no]: yes

1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0 from 2WAY to

DOWN, Neighbor Down: Interface down or detached

1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0 from 2WAY to

DOWN, Neighbor Down: Interface down or detached

After entering the router-id command, the router console informed you that you have to reload the router or reset the OSPF processes for this to take effect. You enter the clear ip ospf process command to do this. Notice that when you're asked if you really want to do this, the prompt is "no"? That's because all the OSPF adjacencies on this router will be lost and will have to begin the process again. That's OK on a practice rack, not good in a production network. Don't use that one at work.

The OSPF RID is not a complicated concept, but the fact that an interface doesn't have to be OSPF-enabled in order to have its IP address act as the RID takes some getting used to. And remember - when the router or switch asks you a question and the prompted answer is "no", take one step back and make sure you really want to do what you're about to do!