This is probably a beginner question...

I'm working my way through Vol1 in the ip routing section.  I have some questions about section 3.3 for longest match routing.

1. The first bullet sounds to me like they want to see a host route because of the "interfaces" key word.

2. The second bullet is pretty clear about that /16 route.

3. The third bullet sounds to me like they want to see a /24 route because of the "networks" key word.

My solution was to put three routes on each router....one host route over the point-to-point link, one network route over the point-to-point link and one /16 route over the frame relay link.  I configured all these routes pointing to the addresses of the next hop as well...everything works.  The solution is two routes on each router...one network route pointing to the point-to-point serial interface and the /16 routing to the address over the frame relay link.  My questions are these:

1.  Am I reading these to hard?  I know that only the two routes in the solution will work but it sounds like they want to see three routes in the configuration.

2.  How do I know to route the /24 in the solution to the serial interface and not the next hop IP?  Why not route the /16 to the frame relay serial interface as well?

Maybe I'm missing something technical here but if I were to come across a question like this in the lab and configured it the way I did and everything worked...would I get it wrong because I put three routes and routed them to the next hop addresses and not out the interface?  Thanks for your response in advance.  I just want to get this portion of my confusion out of the way before I start getting into more heavy stuff.

Thanks, Mike

Comments

  • Hello,

    there are 2 concepts of routing that they want to teach in this lab:

    • Longest match routing: this is the first criteria that the router looks for when routing; does it have a longest match to the destination. The analogy I use when teaching a class is flying from New York to San Francisco. I love straight flight. I do not have to change planes in between and there are less chances for my luggages to get lost. Routers do almost the same thing. In this case, on R5, a packet destined for R4'sloopback (150.1.4.4) is directed toward the serial interface because 150.1.4.0/24 is longer than 150.1.0.0/16. Longer prefixes are always preferred over shorter ones when forwarding a packet. So the /24 route to the loopback via the serial will be chosen over the /16 route via the frame relay. Both routes are installed in the routing table but the route to the frame relay will be used as a backup in case the route to the point-to-point is not longer available. 
    • Nex hop processing: I would highly recommend to watch the COD that Brian McGahan did in the Advanced Troubleshooting class about IP routing; he explains it very well. When routing to a next-hop value the router performs layer 2 to layer 3 resolution on the next hop ex: ip route 150.1.0.0 255.255.0.0 155.1.0.5 . When routing to an interface, the router performs layer 2 to layer 3 resolution on the final destination. For point-to-point interfaces, recursing to the interface is not a problem because there can only be one host at the end of the link. This is not the case for broacast and non-broadcast interface as they can have many hosts at the end other end of the link. Personally, I like to specify the next hop when inserting static routes in the routing table.

    HTH

  • Thanks for the response.  I've seen the COD and it is very good...all of INE's material is good.  I think it's the wording of the question and the way I'm configuring it that brings me to ask these.  Technically I understand longest match routing but would the three route solution I configured be counted wrong in Vol2 or the lab if I was asked the same three questions? 

    I'm really just trying to understand the wording of the questions and for the most part I am.  This one though really got me configuring something that was way off from the solution provided but it worked. If were supposed to be matching our answers to the solution provided would I count this as being incorrect?

    Thanks again for your response, Mike

  • Mike,

     

    remember, in the real lab, what's not forbidden is allowed. As long as the task does not restrict you, you can use whatever tools in have in your toolbox to accomplish the task.

    One thing I can tell you, Cisco will always make sure that the tasks are very specific and clear and that you can only do it one way .

  • Thanks Malick,

    Thats the kind of answer I was looking for. 

    Thanks for your help,

    Mike

     

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