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3165 Views 14 Replies Latest reply: Oct 20, 2011 6:09 AM by Natraj Babaria RSS

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Autonomous System

Oct 18, 2011 10:43 PM

Natraj Babaria 841 posts since
Jul 20, 2011

Hello All,

 

In interview somebody ask me simple question on Autonomous system. I am jsut wondering what will be the smartest answer for that.

 

Question is  Why we need Autonomous System ?

 

Thanks & Regards,

Natraj Babaria

  • Brian O'Kelly 61 posts since
    Apr 5, 2010
    Currently Being Moderated
    1. Oct 19, 2011 11:25 PM (in response to Natraj Babaria)
    Re: Autonomous System

    Were they referring to IGP or EGP?

     

    For EGP, AS is required for loop prevention. If you see your own AS in an AS_PATH you know not to consider it a best route as it would cause a loop.

     

    For IGP it depends on the protocol. EIGRP requires AS to match to be considered part of a routing domain. RIP does not use the concept of AS. OSPF uses areas.

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  • Conwyn 9,682 posts since
    Sep 10, 2008
    Currently Being Moderated
    2. Oct 20, 2011 1:28 AM (in response to Natraj Babaria)
    Re: Autonomous System

    Hi Natraj

     

    Because one team could not run the Internet.

     

    Regards Conwyn

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  • Conwyn 9,682 posts since
    Sep 10, 2008
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    5. Oct 20, 2011 2:20 AM (in response to Natraj Babaria)
    Re: Autonomous System

    Hi Babria

     

    The AS was designed to seperate the Internet into managable units.

    The EGP produces a list of AS through which the traffic passes. As traffic moves along it removes the front AS. If the AS occured twice in the AS Path you would be looping.

     

    Did you want an intelligent answer or a technical answer?

     

    Regards Conwyn

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  • Brian O'Kelly 61 posts since
    Apr 5, 2010
    Currently Being Moderated
    7. Oct 20, 2011 3:00 AM (in response to Natraj Babaria)
    Re: Autonomous System

    How does BGP carry out loop prevention without the AS Path? There is no other way for BGP to know that it had previously originated a prefix without checking the AS_PATH and seeing its own AS.

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  • Brian O'Kelly 61 posts since
    Apr 5, 2010
    Currently Being Moderated
    9. Oct 20, 2011 4:35 AM (in response to Natraj Babaria)
    Re: Autonomous System

    No, thats its main use. The definition from RFC4271:

     

    The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASes.  Since this classic definition was developed, it has become common for a single AS to  use several IGPs and, sometimes, several sets of metrics within an AS.  The use of the term Autonomous System stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other ASes to have a single coherent interior routing plan, and presents a consistent picture of the destinations that are reachable through it.

     

    Think about it like this. What would happen if you didnt have AS numbers. How would BGP peers know if they were iBGP peers or eBGP peers? How could you prefer internal paths over external paths? How could you prevent routing loops in BGP?

     

    Have a read of this too:

     

    http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-1/autonomous_system_numbers.html

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  • Currently Being Moderated
    10. Oct 20, 2011 4:56 AM (in response to Natraj Babaria)
    Re: Autonomous System

    The definition of an AS is a group of routers under one administrative control.  The administrators of the autonomous system define and outline the routing policy for their own autonomous system(s).  I agree with Conwyn. So while simple, it's still an intelligent answer.

     

    Erick

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  • Currently Being Moderated
    12. Oct 20, 2011 5:49 AM (in response to Natraj Babaria)
    Re: Autonomous System

    See response #2. 

     

    You got a nice response.  Maybe it's not the one you WANT to hear?

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  • Rizwan Ayub 120 posts since
    May 6, 2009
    Currently Being Moderated
    13. Oct 20, 2011 5:51 AM (in response to Natraj Babaria)
    Re: Autonomous System

    @Natraj

               You don't need a defination but you want to know why we need AS?? It is called asking for a defination...AS is clear from it's name but still you can quote Jeff doyle by saying "Laziness is a Virtue..."

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