Which route will a packet take?

workfrom925workfrom925 Member Posts: 196
in CCNA & CCENT
I have trouble to solve this question. Can someone tell me the steps to get to the solution? Thanks.


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  • networker050184networker050184 Mod Posts: 11,962 Mod
    What do you think it is and why? We can help you go from there.
    An expert is a man who has made all the mistakes which can be made.
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  • workfrom925workfrom925 Member Posts: 196
    networker050184 wrote: »
    What do you think it is and why? We can help you go from there.

    Cool.

    Since it's IOS 12.3, it runs "ip classless" by default. It can do classful routing, but the question doesn't mention that. So I go by its default.

    A packet to 172.16.1.130 first matches the parent route 172.16.0.0/16 since they both have 172.16.

    The packet then tries to match the child routes. The child route 172.16.0.0/24 means the packet's destination IP needs to match to its first 24 bits. But it doesn't because the packet's third octet is 0000 0001, while the route's third octet is 0000 0000.

    The child route 172.16.1.0/25 says the first 25 bits need to match. The packet's destination IP 172.16.1.130 matches the route's first 24 bits. Its fourth actet is 1000 0010, while the route's 4th octet is 0000 0000. So the 25th bits don't match with each other.

    It doesn't match 172.17.0.0/25 either.

    After that, because it's classless routing, it goes for the default route which is 0.0.0.0/0. So the answer is "the default route".
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    What about the 172.16.0.0/13 route?
    An expert is a man who has made all the mistakes which can be made.
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  • DANMOH009DANMOH009 Member Posts: 241
    networker050184 wrote: »
    What about the 172.16.0.0/13 route?

    :D quit playing with him networker help the guy out
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    Letting someone think through the problem is the best way to help them!
    An expert is a man who has made all the mistakes which can be made.
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  • DANMOH009DANMOH009 Member Posts: 241
    networker050184 wrote: »
    Letting someone think through the problem is the best way to help them!

    :D im only joking, however your making me doubt my answer though now.
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  • goldenlightgoldenlight Member Posts: 378 ■■□□□□□□□□
    man I'm late to the question, but here is a hint pulled from my studies

    HINt: Routers choose the most specific router-the route with the longest prefix length

    I think I know the answer ?icon_confused.gif:
    “The Only way to do great work is to love what you do. If you haven't found it keep looking. Don't settle” - Steve Jobs
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  • chapapachapapa Member Posts: 40 ■■□□□□□□□□
    goldenlight wrote: »
    man I'm late to the question, but here is a hint pulled from my studies

    HINt: Routers choose the most specific router-the route with the longest prefix length

    I think I know the answer ?icon_confused.gif:

    so the answer will be the 172.16.0.0/13 ?

    my first answer was the default route. but with your hint I change my answer to the 172.16.0.0/13 since 172.16.1.130 will have a match at that subnet.
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  • goldenlightgoldenlight Member Posts: 378 ■■□□□□□□□□
    I'm not an expert. Still learning like you. The Hint comes straight from my notes Hopefully networker050184 will chime in here
    Here is my thinking so far..

    172.16.1.130
    >10101100000100000000000110000010
    172.16.1.0/25
    10101100000100000000000100000000
    172.16.0.0/16
    >10101100000100000000000000000000 HOpefully my Binary is right
    172.16.0.0/13
    >10101100000100000000000000000000 <---- the longest prefix--->
    172.16.0.0/24
    >10101100000100000000000000000000

    Per reading page 565 of odom 3rd edition book The correct answer would be 172.16.0.0/13
    Can we get a confirmation here ?????

    Bump ...Bump.. Can someone confirm the answer...????
    “The Only way to do great work is to love what you do. If you haven't found it keep looking. Don't settle” - Steve Jobs
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  • veritas_libertasveritas_libertas Member Posts: 5,746 ■■■■■■■■■■
    This should help everyone get even closer. I remember struggling with this as well. Read the section on making forwarding decisions.

    Route Selection in Cisco Routers - Cisco Systems
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  • chapapachapapa Member Posts: 40 ■■□□□□□□□□
    I think 172.16.0.0/13 is really the correct one.

    on 172.16.1.0/25, the host 172.16.1.130 is on different subnet so the router will not route the packet on that destination
    on the other hand 172.16.0.0/16 is a summary address of 3 subnets and 172.16.1.130 doesn't belong to that 3 other subnets.

    am I analyzing it correctly? I want to know the correct answer :)
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  • NetworkVeteranNetworkVeteran Member Posts: 2,338 ■■■■■■■■□□
    Goldenlight, I can confirm your answer is correct.
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  • DANMOH009DANMOH009 Member Posts: 241
    Ahhhhhhhhh, ive been bagging my head against the wall on this one for ages and finally realized that i was reading the question wrong icon_sad.gif The packet is destined for 172.16.1.130 not 172.16.0.130 what a goof!!!!
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  • workfrom925workfrom925 Member Posts: 196
    veritas_libertas wrote: »
    This should help everyone get even closer. I remember struggling with this as well. Read the section on making forwarding decisions.

    Route Selection in Cisco Routers - Cisco Systems


    I read the link. The link says EIGRP has an administrative Distances of 90. While I'm reading Cisco NetAcademy material, I happen to come cross a diagram showing the AD for EIGRP is 100 in a section explaining AD. Can you explain this? Thanks.

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  • pamccabepamccabe Member Posts: 315 ■■■□□□□□□□
    workfrom925 wrote: »
    I read the link. The link says EIGRP has an administrative Distances of 90. While I'm reading Cisco NetAcademy material, I happen to come cross a diagram showing the AD for EIGRP is 100 in a section explaining AD. Can you explain this? Thanks.
    I believe you can change administrative distances of routing protocols.
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  • DANMOH009DANMOH009 Member Posts: 241
    The AD for EIGRP is 90.

    The picture you have shown here does not show the AD it is showing the AS number, the number you use when creating an instance of EIGRP to run.
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  • workfrom925workfrom925 Member Posts: 196
    DANMOH009 wrote: »
    The AD for EIGRP is 90.

    The picture you have shown here does not show the AD it is showing the AS number, the number you use when creating an instance of EIGRP to run.

    Wow! I have to say to say Ios syntext is very much encrypted that only experts would understand it. Ihe diagram is very misleading to new learners. Thanks Dan.
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