Tuesday, April 29, 2014

CCNA And CCNP Success! Command Discussion:

Dr. Strangeroute

(Or, "How I Learned To Stop Worrying And Love The ip route Command")

If you have a particular command or Cisco routing / switching feature that's giving you a little trouble during your studies, don't sweat it.   Same thing happened to me when I first went after my CCNA.

The ip route command used to kick my butt.  If it wasn't the almost-impossible-to-understand explanations of the syntax that were around at the time, it was the multiple choice questions with choices like the following....

ip route ethernet0

ip route ethernet0

ip route ethernet0

.... followed by about five more choices that looked like those.

Put six to eight choices like that next to each other, and it occurs to you that you REALLY need to master the syntax of that particular command.

That's what this CCNA / CCNP Command Reference is all about. Knowing the command inside and out can add serious points to your exam score....

.... and when you take the command one step at a time, it's actually pretty darn easy.  Let's use a Cisco router to go through the command one step at a time.

You may see additional choices for this command on live equipment, depending on the IOS you're using, but the ones you'll see here will always be there -- and they're the ones to have down stone cold (STONE COLD!  STONE COLD!) for your exams.

The first value to know 

R3(config)#ip route ?
  A.B.C.D  Destination prefix

We'd have a hard time routing if we didn't have a destination to route packets to, and that's the first value in the ip route command.  In this lab, we'll set the network /24 as the destination.

R3(config)#ip route ?
  A.B.C.D  Destination prefix mask

R3(config)#ip route

Watch that on your exams -- the mask you put behind the network number is a network mask, not a wildcard mask.

ip route = Good

ip route  = Bad

Simple enough.  So what's next?

A lot of options, that's what!

R3(config)#ip route ?
  A.B.C.D            Forwarding router's address
  Async              Async interface
  BRI                ISDN Basic Rate Interface
  BVI                Bridge-Group Virtual Interface
  CTunnel            CTunnel interface
  Dialer             Dialer interface
  Ethernet           IEEE 802.3
  Lex                Lex interface
  Loopback           Loopback interface
  Multilink          Multilink-group interface
  Null               Null interface
  Serial             Serial
  Tunnel             Tunnel interface
  Vif                PGM Multicast Host interface

  Virtual-TokenRing  Virtual TokenRing

Holy crap.  That's a lot of options -- or at least it looks like a lot of options.   Really, that entire list is made up of two options:

1.  The next-hop IP address of the router to which you want to send the packets.

2.  The exit interface on the local router through which you want to send the packets.

I bolded "local" for a reason. Let's take a look at our lab:

In real-world networking, there's generally not a right or wrong choice to make when it comes to deciding whether to send packets to a specific next-hop IP address or just naming the local router's exit interface.   I personally like to use the next-hop address option, since it's easier to troubleshoot IMHO, but that's up to the individual.

When it comes to your CCENT, CCNA, and CCNP exams, it's an extraordinarily good idea to be very fluent with both options.

We'll proceed with the labs right after this brief message -- and getting your mitts on these is another extraordinarily good idea!

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Say we've decided to use the IP address option for our next ip route command entry.  That's always going to be the next-hop IP address, and since we're on R3, the next-hop will be

A Cisco router will mention "forwarding router's address" in the IOS Help readout, but it's unlikely your exam will mention that. A word to the wise is sufficient. : ) 

R3(config)#ip route ?
  A.B.C.D            Forwarding router's address

R3(config)#ip route

There are options still available, but once you see the <CR> at the bottom of the list, you have a nice, legal command!

R3(config)#ip route ?
  <1-255>    Distance metric for this route
  name       Specify name of the next hop
  permanent  permanent route
  tag        Set tag for this route


Now, about those 213 interface types shown earlier...

If you haven't even heard of some of them, don't worry about it. The important thing to know is that when you specify an interface at this point in the command, you're specifying the local router's exit interface.

With this lab, it would be easy for you to be presented with a question about the correct ip route command to use here, and it could just include the following options....

ip route Serial0

ip route Serial1

... and you gotta know which one is right for this situation.   

In the lab drawing, R3's Serial0 interface is on the link to R1, and that's the interface we'll be sending packets through.  R1's remote interface is Serial1, and that should not be used in the ip route command.

I'll have a new Command Reference here for you on Tuesday. Right now, take advantage of this opportunity to earn your certifications and help the hungry at the same time.

We're contributing 20 meals to the Central Virginia Food Bank for every paid signup in April to any single-exam course, and a whopping 100 meals for every CCNP All-In-One Video Boot Camp.

Ignore the prices you see below -- every single one of my single-exam courses is just $44 when you use these links, and my CCNP All-In-One course is only $99.

Thanks for reading, and I'll see you in class!

Chris B.

From all of us at TBA and everyone at the Central Virginia Food Bank -- thank you!

1 comment:

Robert McCormack said...

Hello Chris, I remember the route sequence by reciting "Too & Through"

The Network you are going "Too" & the Interface or next hop address you go "Through"

Hope this helps someone.


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