There is one mechanism built into BGP for influencing route metrics - MED, but this is only transitive to the neighbour AS, so Comcast wouldn't see your MEDs, and it's very common to ignore them anyway.
The other mechanism for influencing this kind of thing is through BGP community signalling, though this method is usually pretty blunt. Typically you can suppress announcements toward a specific AS entirely, or prepend toward them (lengthening the AS path and affecting BGP route selection); usually you can also modify the localpref of your announcement within your peer's network. The goal being, more or less, to encourage remote networks to avoid the problem AS/link entirely. If you're trying to avoid Comcast's messed up internal routing, your only real option is not to announce toward Comcast at all, and hope that Comcast will hand it off quickly to someone that routes it better. I wouldn't really count on it. Prepending might work, if Choopa and Comcast are peers, but if Choopa is a customer, Comcast will likely local-pref their routes above their peering/transit, and AS path length can't defeat this.
Vultr indicates they support some BGP communities to do this kind of manipulation:
More or less you're at their mercy. K Quoting Kyle Drake <k...@kyledrake.net>:
Normally you would use anycast to get you to a DNS server (which doesn't have to be that near), then a geographic DNS server to get you to the right CDN element.That's what I was doing previously, but I need to control the IPs for the CDN, and I only have the budget for one /24, so I'm trying to make the best of it. Aside from some occasional weird routing, the network has worked really well. State has not been an issue for what we're doing (short-lived HTTP connections). I'm just trying to see what the extent of my powers to control weird routing are. It's odd to see Comcast cold-potatoing connections to the wrong routes, sometimes on the other side of the continent ( https://gist.github.com/kyledrake/7a4cd36ea276ec3134b4a51a42a37f48). I'm wondering if there is a way to configure Bird to help steer these sorts of things a little better, even if it's on a case-by-case or region-by-region level. My apologies if these are all dumb questions. Again, not much anycast documentation out there (I'm planning to improve this later by putting together a web resource for people doing this). -Kyle
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