[c-nsp] re-advertising eBGP learned prefixes

[Andrey Koklin]

On 10/20/2011 18:14, Gert Doering wrote:

>> The scheme is a bit more complicated, in fact, something like:
 
>>                --- R4-AS2 ---
>> ClientA---> AS4              R1-AS1--(ibgp)--R2-AS1---> R3-AS3
>>                --- R5-AS3 ---
 
>> We use the same operator (AS3) to organize 2 independent VPN's
>> for magistral and regional services.
>> 
>> The problem here looks that AS3 is present already in the path,
>> so the prefixes are not advertised into another VPN.

> Indeed.  Actually, they are usually advertised these days, but
> then dropped on *import* by AS3.

> You can configure the AS3 router allow incoming paths that already have
> AS3 in the path with "neighbor ... allowas-in" - but that will defeat
> BGP loop-detection, so be careful with what you're asking for.

Yes. But still I cannot persuade R2-AS1 router to advertise prefixes there.
Even without AS3 in the path...

>> Here AS3 = 21017. And what I see on R2:

>> spring#sh ip bgp 10.36.72.32
>> BGP routing table entry for 10.36.72.32/27, version 505812
>> Paths: (1 available, best #1, table Default-IP-Routing-Table)
>>   Not advertised to any peer

> I'm not exactly sure when this behaviour was changed, not doing the
> filter on the sending side of an eBGP link - but we've seen it years
> ago already.

> Your best bet might be "not use AS3 in multiple places".

>> spring#sh ip bgp 10.36.72.32                                   
>> BGP routing table entry for 10.36.72.32/27, version 506076
>> Paths: (1 available, best #1, table Default-IP-Routing-Table)
>>   Not advertised to any peer
>>   20485 30835, (received & used)
>>     10.36.2.22 (metric 3072) from 213.129.126.1 (10.36.1.1)
>>       Origin incomplete, metric 0, localpref 100, valid, internal, best
>>       Originator: 10.36.1.4, Cluster list: 10.36.1.1

> So how's your export policies on that router towards the neighbour
> that you should see the prefix on?

The R3-AS3 router doesn't know these prefixes. It has its own BGP table.