Say there is ISP-Alice (ISP-A) and ISP-Bob (ISP-B). They both serve neighboring areas, but they currently only peer with ISP-Charlie. They want to change that and peer directly with each other to speed up service.

Given this situation I would like to know the steps ISP-A and ISP-B would take to peer with each other. Specifically:

  1. Is dedicate fiber normally used to connect their data centers or do they leverage fiber that is already servicing customers at the perimeter?
  2. Once the PHY path is established, how does each start sending BGP advertisements through the Path?
  3. Assuming they are using IP; how did they set up the initial IP addresses? Are those initial IP addresses public or just meant to talk between them (some /31 p2p connection?)?
  4. Can the customer equipment see any of these advertisements?

1 Answer 1


Some connection between the two ISPs is needed. Dedicated fibers (for example in a datacenter they're both present in) is very common. But there are other possibilities: a layer 2 connection (for example a MPLS circuit provided by a carrier) can be used, or a BGP session can be established over an internet exchange point. Basically, you need some way to be able to pass traffic between those routers, without any intermediate routers.

BGP advertisements are sent when a BGP session is configured on a router. IP addresses are agreed upon when the peering is arranged. Either one of the parties provides IP addresses (and typically a a /127 for IPv6 or a /31 for IPv4 is used for point to point connectivity) or IP addresses from a peering LAN IP range on an IXP are used.

The actual BGP messsages are only visible for the routers involved, but of course the effects of these advertisements can be seen because of traffic using those paths (assuming for a moment these BGP updates actually result into routes being used).

  • A direct connection isn't necessary, though desirable. You can establish a multihop session over the Internet as well.
    – chepner
    Oct 9, 2021 at 16:42

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