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discontiguous , look please !!!

Discussion in 'General Cisco Certifications' started by kobem, Mar 23, 2007.

  1. kobem

    kobem Megabyte Poster

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    http://img267.imageshack.us/img267/8468/discontiguousua8.jpg

    i forgot to say that it is a rip or igrp network

    there are 172.16.2.0 /24 , 172.16.5.0/24 and 172.20.88.0/24
    networks here

    according to rules of rip : it doesn't have discontiguous ability ,
    doesn't support it .

    and from router1 for sending updates to router2
    172.16.5.0/24 is summarized as 172.16.0.0/16 major network

    from router2 to router1 , 17216.2.0/24 is summarized
    as 172.16.0.0/16 too

    but it looks fine to me , they advertises networks
    into each other , so what is the problem?
     
    Certifications: CCNA
  2. Spice_Weasel

    Spice_Weasel Kilobyte Poster

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    Kobem,

    What you say is correct. There is no problem in the network shown, and you are right about what what networks R1 and R2 will advertise to each other.

    The discontiguous problem will occur when you add a third router. Imagine you add R3, which has an interface in the 172.16.2.0/24 network (connecting it to R2) and an interface in the 172.16.3.0/24 network. R3 will advertise the summary 172.16.0.0/16 network to R2. So, R2 is getting the same 172.16.0.0/16 summary route from both R1 and R3. So if it has a packet destined for 172.16.5.3, which should go to R1, it will not route it correctly. It will have two entries to the 172.16.0.0/16 network, one entry pointing to R1, the other to R3. R2 has no way of knowing which 172.16.0.0/16 summary is the correct route to take to reach the 172.16.5.3 host.

    So, yes, if you only have two routers discontiguous networks are not a problem. The problems occur with 3 or more routers.

    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  3. kobem

    kobem Megabyte Poster

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    yeah , i figured the last one you said

    r2 can't know where it routes the packet to , to r1 or r3.

    ...........

    in fact i thought that if just r1 and r2 are there
    as if i found a problem also there

    listen : r1 advetises 172.16.0.0 to r2 and r2 advertises 172.16.0.0 to r1 too
    but i think r1 already knows 172.16.0.0 (since this is its eth. interface)
    also same thing is avaliable for r2
     
    Certifications: CCNA
  4. Spice_Weasel

    Spice_Weasel Kilobyte Poster

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    If I understand correctly, you are asking if there will be a problem even with just two routers (R1 and R2), since they will both advertise 172.16.0.0/16.

    There will be no routing problems, all packets can be properly routed.

    The reason: Look at R1. It will know about two networks, 172.16.5.0/24 and 172.20.88.0/24, since it has interaces in those networks. It will learn about 172.16.0.0/16 from R2. Now, think about how R1 will build its routing table. Directly connected routes (172.16.5.0/24, 172.20.88.0/24) are preferred over routes learned by rip. Therefore, R1 will select those routes in preference to the route learned from R2, and install them in the routing table.

    For routes to a host in 172.16.0.0/16, but not in one of the connected routes, it will use the routed learned from R2.

    The same reasoning applies to R2 building its routing table. So, there will be no problems routing packets in the R1/R2 network.


    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  5. Spice_Weasel

    Spice_Weasel Kilobyte Poster

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    My apologies Kobem, I made a mistake. :oops:

    I'll blame it on the busy morning (vpn problem :( ) and lack of coffee - for some reason I was thinking the 172.20.88.0/24 network was 172.16.88.0/24 :oops:

    So, yes, the original R1/R2 diagram is discontiguous and R1 will not be able to reach 172.16.2.0/24, and R2 will not be able to reach 172.16.5.0/24.

    Although the R1 and R2 will advertise 172.16.0.0/16, the routers will reject the route since there is already a subnet of the major route in the routing table, known from an interface other than the one that received the rip update.

    Note that if the 172.20.88.0/24 network connecting R1 and R2 was 172.16.88.0/24, as my VPN-addled brain was thinking, things would work fine.

    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  6. kobem

    kobem Megabyte Poster

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    related to your opinion , all networks there was subnets
    of 172.16.0.0/16 and with this they all become contiguous

    so rip works fine

    but can you explain me the sentence more clearly before the "note that ..."
    sentence ?
     
    Certifications: CCNA
  7. Spice_Weasel

    Spice_Weasel Kilobyte Poster

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    Kobem,

    I assume the sentence you are refering to is this one:

    When rip receives a routing update it checks the update before using it to create routes in the routing table. The checks are as follows:

    1) Check if the subnet in the update is part of the same major (classful) network as the interface that received the update.

    1a) If yes, apply the mask of the interface that received the update. If the advertised network has the host bit set, apply a /32 mask.

    1b) If no, then check if any subnets of the classful network already exist in the routing table, from interfaces that did not receive the update.

    - If yes, ignore the update. (This is what happens in your network example)

    - If no, apply a classful mask to the network in the update.


    For example, in your network, when R2 receives a rip update with 172.16.0.0/16 from R1, the following happens, because of the rules, above:

    1) Check if the subnet in the update is part of the same major network as the interface that received the update. <-- Answer: No, because the update is 172.16.0.0/16 but the receiving interface is 172.20.88.0/24

    1b) If no, then check if any subnets of the major network already exist in the routing table, from interfaces that did not receive the update. <-- Answer: Yes, R2 has another interface in the 172.16.2.0/24 network, which is part of the same major (classful) network, so the update is discarded.

    As you can see, even rip, the simplest of routing protocols, has some complexities!

    For a bit of practice, figure out what happens when the network connecting R1 and R2 is changed to 172.16.88.0/24, according to the rules, above.

    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  8. kobem

    kobem Megabyte Poster

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    thanks mate !

    i have two more questions

    1- if they were 192.168.1.0 /24 and 192.168.2.0/24 instead of
    172.16.... , how would they be summarized ?

    192.168.0.0 or 192.168.1.0 ?

    i asked this since major network of class c would be
    n.n.n.host according to knowledges i learned . (so i think
    192.168.1.0/24


    2- discontiguous networks are defined

    ( two or more subnetworks of a classful network connected
    together by different classful network)

    but if we consider this : 172.16... , 10.0 .. , 192.168...
    won't be discontiguous so this will be wrong ?
     
    Certifications: CCNA
  9. Spice_Weasel

    Spice_Weasel Kilobyte Poster

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    1) All the 192.168.x.0/24 networks are class c, and as such rip would summarize as a /24

    e.g. 192.168.1.0/24 summarizes to ... 192.168.1.0/24

    2) Your example would not be discontiguous, because the networks are not part of the same major (classful) network.

    e.g. 172.16.1.0/24 --- 10.10.0.0/16 --- 192.168.5.0/24 is not discontiguous
    172.16.1.0/24 --- 10.10.0.0/16 --- 172.16.128.0/24 is discontiguous

    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  10. kobem

    kobem Megabyte Poster

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    thank you man for your help
     
    Certifications: CCNA

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