[c-nsp] BGP routes: 207k + 157k = 238k ???...

[Hank Nussbacher]

On Thu, 24 Jan 2008, Tony Li wrote:

>> there is something I can't quite figure out with BGP.
>>
>> Let a bi-homed AS with only two BGP speakers (each of them has one
>> eBGP
>> session with a different upstream, they speak iBGP together).
>>
>> Router 1 receives 238k routes from provider A; so does router 2 from
>> provider B.
>>
>> When looking at iBGP, it can be seen that router 1 sends 207k
>> routes to
>> router 2, while router 2 sends only 157k routes to router 1.
>>
>> I'm surprised by these numbers: the sum of routes from 1 to 2 plus
>> routes from 2 to 1 not only does not equal 238k, but is even greater
>> than 238k (I would have understood it was smaller).
>>
>> What does these numbers mean?...
>
>
> Welcome to BGP.
>
> A BGP speaker is only going to advertise its best external paths via
> iBGP.  If it has selected your other BGP speaker as its best path, it
> will not (currently) advertise the externals that are not the best path.
>
> If you examine your BGP RIB closely, you should be able to find
> routes where the BGP speaker has both paths present.
>
> Regards,
> Tony

In addition to Tony's answer there is one other reason for this to occur -
too little memory in one of the routers.  If memory runs out in one of
them then it can't install all the routes it needs to but you should see
some error message in the log.

But Tony's is the more logical explanation.

-Hank