I understand that only the best path is advertised to eBGP neighbors, but is this true of iBGP? If I peer with two providers, I was under the impression that anyone else peering with both of my providers would have two paths to me. After inspecting a number of looking glasses, it appears this isn't so.
Bgp – Only One Path in the Looking Glass
bgp
Best Answer
BGP always only advertises the best path onward to its peers. This is true for eBGP, iBGP, route reflection (which is merely a special case of iBGP), and any other way you might use BGP.
But, yes, you are correct, if you are sending advertisements to providers A and B, and I have connections and BGP sessions also with providers A and B, then I should see both paths in my BGP tables. I may not see multiple routes created in my routing tables...that's a question of the configuration of my devices and whether I allow multiple routes of equal AS Path lengths to be inserted into the routing table...typically BGP will only install a single route. The BGP tables, though, should show advertisements from both providers.
Actually...even if I'm peering with providers B and C, or even C and D, I should still see multiple paths in my BGP tables as long as you are reachable through B, C, and D. All other things being equal, my devices should pick B because that should be the shortest AS PATH, but I would still see the paths with the longer AS PATHs from C and D. Now, perhaps C and D have both decided that the path through A is better than the path through B, then the paths that I would see would have AS PATHs of "A C" and "A D", so it would appear as if you weren't connected to B at all, but if the path through A went away, then the path through B would get propagated onward through C and D.
If you can give some specifics...maybe even your blocks that you're advertising if you're willing to share that....others can take a look at what we're seeing and perhaps give some more specific guidance.