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while i was studying about IXP route-servers, i read that they do not forward any traffic. I get that a route server is a "control plane" entity and it is used for easier bgp peering. However, i cannot understand how the traffic is forwarded through the IXP between two ASes that peer, then.

Any help?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 10:26
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In the IXP, the routers still have connections to each other, so traffic is sent between the routers. You may be confusing the (logical) TCP connections used by BGP to exchange routing information with actual links between the routers over which traffic is sent. Remember, routing protocols do not route packets, the router does that based on its routing table. Routing protocols are one way to populate a routing table.

The point of a route server is that a BGP full mesh in an IXP would involve many TCP connections. A route server can reduce the number of TCP connections to one per router to a route server. This works much like iBGP route reflectors, but unlike route reflectors, the route servers are not routers, they only supply the routing information. It is still up to the routers to send traffic to the other routers, but the decision of to which router is made based on the information from the route server.

  • So in a way, we could say that control plane information is propagated through the route server, while data plane is forwarded via the links of the routers? – konstantinosAR Dec 17 '18 at 22:01
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    Yes. Routing protocols do not route, they only exchange routing information, and they are one of the three ways to populate a routing table. – Ron Maupin Dec 17 '18 at 22:02
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To add to @RonMaupin's answer: a route server will receive routes from many peers, and advertise the routes it learns to those peers, advertising the next-hop of the peer as the way to reach routes learned. So ASN1 will learn routes to another ASN2 through the IXP route server, but forwarding will go directly from ASN1 to ASN2, not via the route server.

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An IXP typically presents itself to it's members as an ethernet network. Smaller IXPs may be built out of simple Ethernet switches, big IXPs will likely use advanced features to present themselves as an Ethernet network while allowing higher performance and redundancy.

A router has a table of currently active routes. For each route the table normally holds both an interface and a next-hop IP address.

On ethernet networks the next hop IP addresses is translated to a MAC address through the ARP table (and if nessacery by making an arp request). This means that a router on an IXP can forward data to any other router connected to that IXP.

Route servers are not fundamentally required for an IXP, providers can and often do set up BGP sessions directly between their routers and larger providers often choose not to use the route servers to give themselves more control over their peering. The actual data path through the IXP remains the same whether the route was received from a route server or a direct BGP session.

The benefit of route servers is for members who want to peer as widely as possible. They can buy their port on the IXP, establish a single BGP session to the route server and immediately start exchanging traffic with like-minded peers without having to go through the hassle of making individual arrangements.

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