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  1. #1
    Join Date
    Sep 2008
    Location
    Durban,South Africa
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    102

    Default Discontigous networks

    Hi guys,

    Can anyone give me clarity on what discontigous network is.

    Thanks.

  2. #2
    Join Date
    Nov 2008
    Location
    Birmingham, UK
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    1,424

    Default

    A discontiguous network is one that contains 2 or more subnets from the same class address, separated by another network. For instance

    172.16.0.0-----10.0.0.0-----172.16.1.0

    This can cause problems with routing because if auto-summary is used, it will advertise the same classful network to both sides of the network.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  3. #3
    Join Date
    Sep 2008
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    Durban,South Africa
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    Default

    Thanks Fuzz

  4. #4
    Join Date
    Mar 2008
    Location
    Winston Salem, NC
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    432

    Default

    this is why you must turn off auto-summ in EIGRP, or if your using RIP use Version 2
    Cisco---------------------- CompTIA
    -CCNA ----------------------Security +
    -CCNP (BSCI,BCMSN,ISC W)

  5. #5
    Join Date
    Nov 2008
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    Default

    RIPv2 also auto-summarises by default, right? It's just OSPF that doesn't?
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  6. #6
    Join Date
    Mar 2008
    Location
    Winston Salem, NC
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    432

    Default

    Quote Originally Posted by Fuzz View Post
    RIPv2 also auto-summarises by default, right? It's just OSPF that doesn't?
    i belive so, but RIPv1 doesnt support vlsm design
    Cisco---------------------- CompTIA
    -CCNA ----------------------Security +
    -CCNP (BSCI,BCMSN,ISC W)

  7. #7
    Join Date
    Mar 2008
    Posts
    2,888

    Default True.

    RIP Version 2 supports automatic route summarization by default.
    Maddox Thomas-Clark 14/10/2008
    Bean Thomas-Clark 18/09/2007
    Big Evils Cisco World
    Linkedin

  8. #8
    Join Date
    Nov 2008
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    Birmingham, UK
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    1,424

    Default

    You don't need to be using a vlsm design to have discontiguous networks.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  9. #9
    Join Date
    Sep 2008
    Location
    UK
    Posts
    62

    Default

    I'm a little confused as to what the default state is for EIGRP and auto-summarization of networks.

    Initially, I was under the impression that if a discontiguous network is present and EIGRP is being used, then "no auto-summary" would be a pre-requisite in getting the likes of RIPv2 or EIGRP to work.

    Yet, the following URL on Cisco, appears to imply that the default behaviour for EIGRP has changed since 12.2(T.

    http://www.cisco.com/en/US/docs/ios/...html#wp1011150

    But I'm not seeing this behaviour with my 12.4 routers, and auto-summary appears to be enabled by default.

    Any ideas...?

    Thanks
    Currently studying for CCNA.
    Home Lab:
    Routers (1841, 1721, 3660, 2522)
    Switches (2950G-48, 2950G-24, 2950G-12, 2924-XL)

  10. #10
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    Nov 2008
    Location
    Birmingham, UK
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    Default

    As far as all the texts I've read, including CiscoPress, it seems that auto-summary is enabled by default, at least for the CCNA exam. Just remember that if you are using a discontiguous network design with RIPv2 or EIGRP, you should see no auto-summary in the config.
    CCNP R&S, CCNA DC
    Currently studying: CCIE R&S, CCNP Data Centre
    Follow my CCIE progress with study notes on my blog: http://beyondccna.blogspot.co.uk/

  11. #11
    Join Date
    Dec 2006
    Posts
    2,186

    Default auto-summary

    RIP, RIPv2 and EIGRP auto-summarize classful boundaries. It's stupid that these routing protocols still do this. You need to do manual summarization.
    So, under the routing protocol, type no auto-summary. However, RIPv1 cannot do this, so RIPv1 can only be used in classful networks, which makes it useless. OSPF does not auto-summarize so it will work in discountiguous networks by default.
    Cheers!
    Todd Lammle

  12. #12
    Join Date
    Jun 2009
    Posts
    31

    Red face Classless and Classful network!

    Can anybody give me a little skinny on Classless and Classful networks?I am already on Chapter.7 bah these terms sometimes confuse me a bit!so any clarification would be greatly appreciated.

    Thanks
    Michele

  13. #13
    Join Date
    Mar 2008
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    2,888

  14. #14
    Join Date
    Jun 2009
    Posts
    31

    Smile

    Thanks bigevil!

    I'll reply if it helps

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