[c-nsp] bgp config [continue]

[Pete Templin]

Shaun Reitan wrote:
>>He is announcing you his network and this is why you are able to use DS3
>>to reach him, but you are only announcing him the default. In his
>>routing table he has a more specific path to your network through his
>>ISP that is why he uses it. To fix this announce him default and your
>>network.
> 
> right but the route-map on his end for me has as-path prepends, and my side
> still chooses the ds3 regardless, i have tryed putting 10 as 10 times in the
> prepend and i get the same thing... same with my provider and there upstream
> (as choosing default through me).

Rule #1: Most specific prefix wins.
Rule #2: Lowest administrative distance wins.
Rule #3a: Highest BGP weight wins, if BGP is the lowest AD.
Rule #3b: Highest BGP local preference wins, if BGP is the lowest AD.
Rule #3c: Shortest AS path wins, blah blah blah.
...

In this case, you're losing the battle at rule 1.  The most specific 
route in his table for your prefix is the exact-match for your prefix 
learned from the 'Net.  He is not receiving an exact-match for your 
prefix from your BGP session, so there is no way for him to know to 
reach you that way.  We haven't gotten to Rule 3c at all.

 >>> I highly doubt my provider and gblx both have problems with there 
 >>> config so I'm assuming it's my config or the router config at the 
 >>> other end of the ds3.  Any advice?
 >>
>>When he starts announcing his network to you, you also propogate this
>>announce to GBLX, so they install it into their tables, because his
>>network has a shorter path through you, then other paths that they have.
>>This is normal, to make the path through you look less preferable, you
>>need to announce his network with prepends, number of prepends that will
>>do the job may vary.
> 
> I'm already doing as-path prepends

It's not a 'problem' per se.  It's Rule 3b biting you.  If you were 
marooned on a desert router console, which would you pick?

a) Pay money to send traffic to destination X.
b) Exchange traffic for free to destination X.
c) Get paid money to send traffic to destination X.

Hopefully, you'd pick C, and GBLX&InterNAP are doing this.  Almost every 
provider around puts a higher local preference on routes learned from 
customers (get paid for traffic) than peers (no dollar$ exchange) than 
transit providers (pay for traffic).

If you want to change this behavior, you'll need to announce the routes 
to GBLX & InterNAP with their respective peer-level local preference 
community on each prefix.  Prepends have NOTHING to do with this (except 
they'll finally have something to do with it once you accomplish the 
stuff below).  Since you don't appear to own the connections to those 
providers, you'll need to work with your upstream to see if they can 
help you achieve this goal.  FYI, I believe the InterNAP community to 
request peer-level local pref is 65020:0 and according to 
https://robin.gblx.net/GBLX_Customer_BGP_Communities.html#utilization 
the GBLX community would be 3549:100.

pt