You're pretty much there. To clarify, transit AS's use only fully meshed
IBGP (assuming scalability techniques like Route Reflection and
Confederations also in use) and usually peer internally via loopback
addresses for stability and as you correctly point out, use the IGP to
distribute reachability information for those loopbacks. Using IGP only
routers for transit might have worked at some point years ago, but simply
doesn't cut it anymore due to the sheer volume of paths in the internet.
*********** REPLY SEPARATOR ***********
On 5/17/2001 at 10:04 AM Carroll Kong wrote:
>At 01:16 AM 5/17/01 -0400, Vincent Chong wrote:
>>Hi;
>>
>> For OSPF implementation, an area can be configured in the Loopback
>>interface.
>>But what purpose, when should I do it?
>>
>>TIA
>>Vincent Chong
>
>Well, somewhat off topic, but the router id will lock on to the loopback
>address, which might stabilize the network more. However, I think you
>even
>wrote to the list an email about that so that probably is not what you are
>asking.
>
>Now why would you want to advertise a loopback interface using OSPF or any
>IGP? To teach the IGP how to get their later on for redistribution into
>BGP. Basically only used if you need to use an AS as a transit AS. You
>have basically two choices.
>
>IBGP (full mesh) to the ASBRs of the transit AS. Or, you can redistribute
>the "transit route" through an IGP instead. They tend to use loopback
>interfaces to help the transit ASs achieve more stability to avoid
>flappage. I am somewhat new on this, so if I am wrong, I will happily
>defer to someone with more experience, but this is my take on it from what
>I have read.
>
>
>
>-Carroll Kong
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