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Would like to ask a BGP question; in the first link picture there is no direct link on the two bottom layer3 switches but they form a ibgp neighbor with each other.

no bottom link between layer 3 switches

In the second link picture, the layer3 switches are connected in a ring topology and iBGP is run between all routers.

enter image description here

My question is which is the best topology? I will be running MPLS onwards.

Kind Regards

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    as a general rule, it's better to embed the pictures in the question rather than link them, first it's more readable and second your links will likely disappear in the future. Stack Exchange aims at retaining (good) questions and answers over time for the community benefit.
    – JFL
    Apr 27 '16 at 9:25
  • OK sure thing sir. and Done :) Apr 27 '16 at 9:29
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BGP does not care if the peer is directly attached or not, as long as peers can communicate with each other, so from BGP point-of-view both topologies are strictly identical.

This is why there's an IGP running within the AS so each BGP router knows how to reach its peers.

The fact that the IGP can be iBGP may cause confusion.

Design your iBGP topology as an IGP, the same way you would do with OSPF for example, put it in place, then add the eBGP layer.

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  • Hi, thanks for the reply. Currently I have the 1st setup and the IGP running to connect the network is static routes, which I intend to keep, since it will be simpler to manage. Unless I get more routers then I will migrate to a dynamic routing protocol.Could you please give me more detail on what you mean by Design my iBGP as an IGP please?Thanks. Apr 27 '16 at 11:37
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    If you intend to have fault-tolerance, static routing is not really adequate...
    – JFL
    Apr 27 '16 at 11:53
  • I agree that in general static routing is frowned upon. Wouldn't static routes be Ok in the second drawing where it is a ring and there is minimum downtime ? Apr 27 '16 at 12:07
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    @BeachSamurai, static routing doesn't scale, and there may be a lot more routers and/or routes in the AS than the BGP speakers shown.
    – Ron Maupin
    Apr 27 '16 at 12:20
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    @BeachSamurai, what if each router had dozens of directly connected networks? The directly connected routes do have a lower AD, on the router to which they are connected, but now you want to share those routes with the other routers. That is a pain with static routing, especially when you want to add or delete networks, and it can lead to errors when manually entering the static routes since they need to manually entered on each of the other three routers. You may then find yourself banging your head on the wall, troubleshooting a very subtle error of transposed digits in a static route.
    – Ron Maupin
    Apr 27 '16 at 15:50
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If you can directly attach 2 hosts, do it. If the link goes down, all your routing protocol will react instantly.

Otherwise you will rely on routing protocols timeouts.

Moreover a more densely meshed network will guarantee a shorter path.

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