On Wed, Apr 9, 2025 at 8:31 AM William Herrin via NANOG < nanog@lists.nanog.org> wrote:
nanog@lists.nanog.org (Sriram, Kotikalapudi (Fed) via NANOG) wrote:
Does the following ever happen in reality? Do you think it is strange and unlikely? The lateral (i.e., non-transit) peer of an AS is also the transit
On Mon, Apr 7, 2025 at 8:10 AM Elmar K. Bins via NANOG <nanog@lists.nanog.org> wrote: provider of the AS's transit provider. Example: AS A has AS B as a transit provider and AS C as a lateral peer, and AS C is a transit provider of AS B.
Yes, and it's very often a mess traffic-engineering wise...
Hi Elmar,
Would you mind discussing this further and offering examples of some of the traffic engineering challenges?
[...description of default BGP behaviour elided...]
Now, if one of the networks is playing local pref games, which they shouldn't be doing, then they may misroute packets the long way around the planet. But that's their fault for playing local pref games.
Bill--can you clarify why you feel setting localpref values for peers differently from customers is something ISPs "shouldn't be doing?" If I'm an ISP (call me ASN Ishmael....er, ASN A), and I have a customer, ASN B, who has a customer, ASN C. I also have a peer, D, that provides service to their customer, ASN C. I hear ASN C's prefixes with equal AS-PATH length from ASN D and ASN B; default BGP behaviour looks for tie breakers further down the decision pathway. However, if I send the traffic through ASN D, as a peer, I make no revenue from carrying that traffic. If I send the traffic via ASN B, however, I earn revenue for passing the traffic along the customer, assume 95th percentile billed, link. Is it your assertion that as an ISP, as a provider of services to my customers, I should leave it up to tie-breaking heuristics further and further down the BGP decision tree as to whether I can earn money by carrying traffic or not? Or do you perhaps see some wisdom in preferring to send traffic through my customer's links when possible, so that revenue can be earned to keep the company afloat, rather than passing it for free through a (probably competitor's) peering link, who then hands it to the destination? If peer ASN D is also a competitor of mine, if I let BGP toss the coin for me, there's a good chance in doing so I've taken money out of my bank account, and handed it to my competitor. To paraphrase Randy Bush, "I highly encourage my competition to follow your advice." ;)
Regards, Bill Herrin
Thanks! Matt PS--yes, I know the alternative is to put filter lists on your peering sessions that actively drop any ASNs that happen to be within your downstream customer cone, rather than messing with localprefs. However, that increases the fragility of your connectivity mesh, because if ASN C's link to ASN B drops, you're now black-holing them; they can reach the rest of the internet via ASN D, but they can't reach you, because you're filtering out their announcements at your peering edge. Whups. Maybe localprefs aren't such an evil thing after all... ^_^;