[j-nsp] internal BGP necessary ?

[Patrick Okui]

Hi, Sorry I got caught up today.

On  24-Mar-2013 13:46:31 (+0300), Piotr wrote:
> 
> There is some problem with upload diagram, even small, here is link:
> 
> http://pokazywarka.pl/mg6gtf/
> 
> [url=http://pokazywarka.pl/mg6gtf/]diagram[/url]
> 
> W dniu 2013-03-24 11:36, Piotr pisze:
>> Hi
>>
>> I attached diagram. I have to connect together two different companies.
>> They have ethernet leased lines, so i can use bgp on private AS numbers.
>> Each have own OSPF. There is max 1k routes in igp. In this moment i use
>> only ebgp, i removed ibgp. I made redistrubutions ospf-(e)bgp and
>> (e)bgp-ospf on A1 A2 B1 B2,  i have also tags against mutual
>> redistributions. Ebgp has 200 ad on cisco, For now all works, but first
>> time without ibgp. I'm affraid about some problems like loops..

Ok, the short version is your design should work with a few issues but
loops shouldn't be one of them especially if your tagging is working
correctly.

The long version with an example of a possible problem is as follows:

If an interface on B3 flaps, then very likely router A4 is going to do a
link-state recalculation since these will be external LSAs that are
generated for whatever OSPF in AS1 injected into the BGP in B1 and B2
and was subsequently re-announced into AS2's OSPF..

So, another design would be where B1, B2, would generate aggregate
prefixes that describe what's in the OSPF in AS1, and routers A1 and A2
would also generate aggregate prefixes to describe what's in the OSPF in
AS2. (probably with null pull-up static routes or similar). MED could
still control preference of one link over another.

At this point you'd need iBGP between A1, A2 (and A3) as well as B1, B2,
(and B3). This is because A1 - A4 would only have information regards
the loopbacks and interface addresses that are in AS2 and B1 - B4 would
have the same for what's in AS1.

If you are generating a default route towards each ospf area1 then
that'd be it.

--
patrick


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