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5415 Views 9 Replies Latest reply: Oct 26, 2009 4:54 AM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

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BGP connections

Oct 23, 2009 3:22 AM

hsnanua 55 posts since
Aug 18, 2009

Here is a question about BGP connection. Please do assist me in this.

 

The attachment shows 6 bgp routers. 2 are EBGP and 4 are IBGP, in the same AS.

 

First question, if sync is off, and there is no IGP in the AS, which routers in the AS 65500 will advertise a route to 172.16.0.0?

 

Second question, if sync is on and there is no IGP, then what will the case be?

 

Third, if sync in on and IGP is present, then what will happen?

 

And finally, if sync is off and IGP is present?

 

The reason being, I am confused about the role of sync in the diagram, but before that, the main question: How does BGP behave in propagating routes? Each book and site explains differently.

 

Help

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  • krait 122 posts since
    Jan 8, 2009
    Currently Being Moderated
    1. Oct 23, 2009 5:34 AM (in response to hsnanua)
    Re: BGP connections

    BGP syncronization function is to make sure that if there are non iBGP routers in the transit path they will not drop traffic since they dont know the destination.

     

    It isnt really a question about an IGP being present or not, its more about if the route in question is redistributed into the IGP.

    Something which is quite unmanagble if you are doing full eBGP peering with a full internet routing database in BGP.

    To many routes to be manageble in an IGP.

     

    In the picture you provided.

    It looks like all the routers in the transit path are talking BGP, so the use of BGP syncronization in this case would fill no use.

     

    Thats my view on the case, im not sure if i explained this in a good maner or not, but im sure one of they usuall braniacs (ment in a good sense )

    will be able to further explain it.

     

     

     

    Anyway to clarify:

    bgp sync on without igp would make sure that no routes where distributed via iBGP.

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    2. Oct 23, 2009 6:31 AM (in response to hsnanua)
    Re: BGP connections

    Sync is on = Router B uses the 172.16.0.0 and installs in its routing table;

    But iBGP routers do NOT advertise or use that network, so Router F have NO idea about the network 172.16.0.0

     

    Sync is off = iBGP router hear about that network, install it and advertise so Router F hears about 172.16.0.0 from Router E and can send traffic to that destination. Router F is very happy.

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    4. Oct 24, 2009 9:45 AM (in response to hsnanua)
    Re: BGP connections

    Not all iBGP routers, just E and B, the border routers, I think that is the "preferred" method.

    I saw an example in the book and did test it on GNS3 with 3 iBGP routers, not 4. If you have GNS3 you can test it too.

     

    I am not too sure on your second Q. I know you should advertise only networks that router can reach with network statement. In my lab, iBGP got networks from eBGP, or advertised by eBGP.  BGP is WEIRD, you need a lot of practice to figure it out.

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,398 posts since
    Oct 7, 2008
    Currently Being Moderated
    6. Oct 24, 2009 7:17 PM (in response to hsnanua)
    Re: BGP connections

    If you're not running an underlying IGP, then you'll need to use directly connected links for peering and will continually have to use next-hop-self or bring in all connected links into BGP to solve reachability.

     

    Have fun with your tests though!  Seeing is the best way to figure it out!

     

    Scott

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    7. Oct 24, 2009 10:29 PM (in response to hsnanua)
    Re: BGP connections
    Also, from all this, the two ways for internal networks to reach the external networks (IBGP to EBGP)  is to define the routes using the network statement and using aggregate routes. Right?

     

    The following summarizes the differences between the main BGP commands:
    • The network command tells BGP what to advertise.
    • The neighbor command tells BGP where to advertise.
    • The aggregate-address command tells BGP how to advertise the networks

     

    Reference: Paquet, C., Teare, D. 2007, Authorized Self-Study Guide: Building Scalable Cisco Internetworks (BSCI), Third Edition,  Cisco Press.

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  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,398 posts since
    Oct 7, 2008
    Currently Being Moderated
    9. Oct 26, 2009 4:54 AM (in response to Martin)
    Re: BGP connections
    The following summarizes the differences between the main BGP commands:
    • The network command tells BGP what to advertise.
    • The neighbor command tells BGP where to advertise.
    • The aggregate-address command tells BGP how to advertise the networks

     

    That's kind of an over-simplified way of looking at things....

     

    The network command is ONE way of telling BGP what to advertise.  The redistribute command is another.  In the Cisco world, network command = internal origin code while redistribute = incomplete/unknown origin code.

     

    The aggregate-address command tells BGP of a CHANGE in how to advertise networks.  (although default behavior is summary + details, though most will use "summary-only" parameter to manipulate)

     

    Scott

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