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2134 Views 11 Replies Latest reply: Dec 20, 2011 4:42 AM by tnewshott RSS

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BGP Advertised Distance

Dec 18, 2011 6:48 AM

Bourbon, CCNP, CCIE Candidate 149 posts since
Dec 11, 2008

Studying the Advertised Distance chart and BGP - why is BGP external AD so low - 20, and the BGP internal distance so high - 200? 

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    Re: BGP Advertised Distance

    BGP is external gateway protocol as it carries Internet routes;  so you want to get outside as soon as possible (shortest way);

    If you have internal BGP peers, you do not want to go that way, if there is shorter external path;

     

    It also AD of 20 is better than any other AD's of routing protocols;

  • Beau Yancy 148 posts since
    Dec 10, 2010
    Currently Being Moderated
    2. Dec 18, 2011 4:55 PM (in response to Martin)
    Re: BGP Advertised Distance

    Martin beat me to it! I agree.

     

    With that though, be careful of EIGRP summary routes as those will default to AD of 5 if not defined when summarizing. **See Martin comment below**

     

    Message was edited by: Beau Yancy

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    3. Dec 19, 2011 1:04 AM (in response to Beau Yancy)
    Re: BGP Advertised Distance

    but EIGRP summary AD of 5 is not advertised;  it just locally significant, right ?

  • Beau Yancy 148 posts since
    Dec 10, 2010
    Currently Being Moderated
    4. Dec 18, 2011 4:50 PM (in response to Martin)
    Re: BGP Advertised Distance

    Ahh, right you are! Thanks for keeping me on my toes Martin and for preventing the possible spread of misinformation!

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    5. Dec 18, 2011 5:17 PM (in response to Beau Yancy)
    Re: BGP Advertised Distance

    Even they made BGP special when you redistribute routes into OSPF.

    When you redistribute routes into OSPF,  Default metric is 20 except BGP which is 1.

    BGP is the King;

  • Currently Being Moderated
    Re: BGP Advertised Distance

    If you look at the purpose of BGP, it is to enable vector-based routing based on Path Attritubes tied to prefixes, which make up the NRLI that is advertised between BGP neighbors. These neighbors can be eBGP neighbors - those with an AS number that is dissimilar from our own, or those peers can be internal - with an AS that matches our own.  Our own AS number is defined as the number in the BGP process configuration line "router bgp <AS>". 

     

    With that said, if you have multiple peers, with at least one internal and one external, and you receive the same route from both, return back to the purpose of BGP - to route between discrete AS's.  This would dictate that you would want the shortest path between one AS and another - so if you have a direct link to the other AS, you want to prefer that path as opposed to a path that has you traveling further within your own AS before you reach the next AS. 

     

    This can get pretty complicated when you have multiple exit paths, even more so when you redistribute at multiple points within a network and learn routes from several different neighbors. 

  • Brian 2,968 posts since
    Aug 17, 2009
    Currently Being Moderated
    7. Dec 19, 2011 1:23 AM (in response to Beau Yancy)
    Re: BGP Advertised Distance

    No, you are right in your initial comment.  None of the routing protocols advertise the AD, AD is locally significant.  So, you could have a route to a destination learned through eBGP with an AD of 20 and if you then create a summary route for this same destination back through your EIGRP domain to another router that has reachability to the destination as well then you basically create a suboptimal routing path.  Not good.  So, as you said, watchout when creating EIGRP summary routes.

     

    Just my 2-cents.

     

    Brian

  • Beau Yancy 148 posts since
    Dec 10, 2010
    Currently Being Moderated
    8. Dec 19, 2011 4:12 PM (in response to Brian)
    Re: BGP Advertised Distance

    Right, thanks. It's almost like you would have to go out of your way to end up in that situation but still possible.

  • Currently Being Moderated
    9. Dec 19, 2011 6:18 PM (in response to Beau Yancy)
    Re: BGP Advertised Distance

    You would be surprised how easy it is to wind up in that situation.

     

    Engineer 1: Hey - looks like we're good.

    Engineer 2: Nah, lets do a summary route, clean up the advertisements.

    Engineer 1: I'm not so sure it works that way....

    Engineer 2: I'm sure it does, I just entered it in. My routing table looks great!

     

    <outage ensues>

  • Beau Yancy 148 posts since
    Dec 10, 2010
    Currently Being Moderated
    10. Dec 20, 2011 3:12 AM (in response to tnewshott)
    Re: BGP Advertised Distance

    Haha, yea...

     

    Boss: You're fired!

     

    Good stuff though.

     

     

  • Currently Being Moderated
    11. Dec 20, 2011 4:42 AM (in response to Beau Yancy)
    Re: BGP Advertised Distance

    Yep....the famous "write resume" command.

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