Traffic routing basics (BGP focus) – White paper

Some important basic information to begin with

  • You define where packets leave your network
  • The other party defines which route is used to route the packets back to you
  • In most cases, the Internet is not routed symmetrically

Routing outbound

Routing packets from your network to the other network are defined by you.

  • Your router sends outgoing packets on the basis of its routing table
  • The routing table is built up based on learned (=incoming) prefixes from neighbors
  • Prefixes are typically learned from:
    1. Transit sessions
    2. Peering sessions (IXP or PNI)
    3. Customer sessions
  • If you have more than one router, your routers synchronize their learnings via IBGP
Inbound outbound traffic path

First, you have to learn a route from outside; then you can use it to send packets there.

  • In an advanced network, your router sees the prefixes from the target network on different paths
  • As there is more than one way of learning prefixes, you have to define where the outbound packet should be sent

How a router decides which path is best

Let’s take a Cisco router as an example. Cisco’s best path algorithm (1 is the strongest):
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Download the pdf to read more!

To learn more about traffic routing basics, please download the pdf. For customers, the pdf is available in the DE-CIX customer portal.