Last minute questions

I have the ICND1 exam scheduled for Wednesday, but had some last minute questions.

1. The first one is syntax. On the Boson exams the command to configure an interface command is "interface s0/0" but in the NetSim by Boson they use "interface serial 0/0/0". Which is correct? I'm assuming that I should do the format listed in the question. Also, I've just used configure instead of configure terminal but the NetSim requires "configure terminal" to be used on switches.

2. What's up with the zero subnet? I believe the "no ip zero subnet" means that I cannot use the zero subnet or broadcast subnet. Does this also mean that I should subtract 2 when I calculate the number of subnets. (Similar to calculating the number of hosts.)

3. Finally, can someone explain the config register in easy to understand terms? The Odom book did a horrible job explaining it IMO.

I'm probably overthinking the first 2, but the configuration register stumps me. Hopefully, I don't get hit with a bunch of question about it.

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NetworkVeteran

Boxer77 wrote: »

I have the ICND1 exam scheduled for Wednesday, but had some last minute questions.

1a. "interface s0/0" vs. "interface serial 0/0/0".

Both are correct. That's like asking, "Is United States of America correct or is USA correct?" In most day-to-day use, I would use the shortest form you can get away with, as less typing equals getting done faster.

1b. "configure" vs "configure terminal"

Both are valid IOS commands, but "configure terminal" or a shortcut thereof is almost always what you want.

2. What's up with the zero subnet?

Not much.

I believe the "no ip zero subnet" means that I cannot use the zero subnet or broadcast subnet. Does this also mean that I should subtract 2 when I calculate the number of subnets. (Similar to calculating the number of hosts.)

It only prevents using the zero subnet. There is no command to disable the all-ones subnet. If a question specially mentions applying the "no ip subnet-zero" command, then I would subtract one from the number of subnets.

R1(config)#no ip subnet-zero
R1(config)#int fa0/1
R1(config-if)#ip add 172.16.255.1 255.255.255.0 ! all-ones subnet
R1(config-if)#ip add 172.16.0.1 255.255.255.0 ! all-zeroes subnet
Bad mask /24 for address 172.16.0.1

3. Finally, can someone explain the config register in easy to understand terms?

If Odom isn't working for you, there's always Lammle, or Cisco's online documentation.

theodoxa

Boxer77 wrote: »

1. The first one is syntax. On the Boson exams the command to configure an interface command is "interface s0/0" but in the NetSim by Boson they use "interface serial 0/0/0". Which is correct?

Both. It just depends on the router and whether or not you chose to shorten the command. As best I can tell, fixed configuration routers like certain 2500 series use a single digit format - serial0, se0, ser0, and s0 would all work just fine [as would any shortening of the word serial - s, se, ser, seri, seria]. Modular routers seem to use either a 2 or 3-digit format - serial0/0, ser0/0, or s0/0 if the router uses 2-digits or serial0/0/0, ser0/0/0, or s0/0/0 if it uses 3-digits. On an odd side-note, I seem to remember the 1750 (modular router) actually using a 1-digit format.

The quickest way to find out which it is using is to issue the "show ip interfaces brief" command. Just use whatever it says and feel free to shorten the port type (F/Fa are common for FastEthernet, G/Gi/Gig for GigabitEthernet, and S/Se/Ser for Serial). If there's a diagram, many times it will indicate the correct format (1, 2, or 3-digit)

Boxer77 wrote: »

I'm assuming that I should do the format listed in the question. Also, I've just used configure instead of configure terminal but the NetSim requires "configure terminal" to be used on switches.

I personally prefer "config t", since that was how I learned it back in the NetAcademy. But, you can use any shortening that isn't ambiguous (could match multiple commands).

Boxer77 wrote: »

2. What's up with the zero subnet? I believe the "no ip zero subnet" means that I cannot use the zero subnet or broadcast subnet. Does this also mean that I should subtract 2 when I calculate the number of subnets. (Similar to calculating the number of hosts.)

Basically, WAY, WAY BACK IN THE DAY (Prior to IOS 12.0) you were not able to use the all zeroes or all ones subnet by default since the "no ip subnet zero" command was configured by default. At one time [even further back] routers could not use these subnets at all and in some cases, such as configuring the "no ip subnet zero" command or when using a clasfull routing protocol (RIPv1 or IGRP) they may not be supported even on a current IOS. 12 years ago we were told that the current IOS supported these subnets by default, but that for the test we should assume they were not supported unless told otherwise. Sometime since then, Cisco has reversed that. Now, you are supposed to assume that these subnets are usable unless indicated otherwise ("no ip subnet zero" command or RIPv1 used).

If this is the case, you cannot use the first or last subnet derived from a classfull network. If you were asked how many subnets I would assume you should subtract 2. But, what is more likely is that they will give you some addresses and ask which is valid or something along those lines. If the "no ip subnet zero command" is configured watch out for any addresses in the first or last subnet.

Boxer77 wrote: »

3. Finally, can someone explain the config register in easy to understand terms? The Odom book did a horrible job explaining it IMO.

There are alot of variations. I posted a chart in another topic recently. But, you'll probably be fine knowing 0x2102 (Default) and 0x2142 (Ignore Startup-Config/NVRAM). The latter is used for password recovery. You might also remember that if the last digit is 0, that usually means it will boot to ROMMON.