8.7 The Routing Process
8.7.3 A routing environment example
The Figure illustrates routing in a typical environment. In the Figure, AS5 is receiving routes from both AS1 and AS2, and is originating its own routes (172.16.10.0/24). To simplify, consider just the flow of updates in one direction, left to right. By applying the engine model to AS5, you will get the following.

The routes received from peers (these are the routes coming from AS1 and AS2) follow:

192.213.1.0/24 via AS1
0/0 (this is a default route) via AS1
193.214.10.0/24 via AS2
0/0 (this is a default route) via AS2
192.213.1.0/24 via AS2

Input Policy Engine:

Do not accept default route 0/0 from AS1.
Give route 192.213.1.0/24 coming from AS1 better preference than route 192.213.1.0/24 coming from AS2.
Accept all other routes (this will accept 193.214.10.0/24).

The decision process:

Because 192.213.1.0/24 has better preference via AS1, I will reach 192.213.1.0/24 via AS1.
I will reach 193.214.10.0/24 via AS2.
I will accept 0/0 via AS2.

Routes used by the router:

I will use 0/0 as default from AS2.
I can reach 192.213.1.0/24 via AS1.
I can reach 193.214.10.0/24 via AS2.
Network 172.16.10.0/24 is one of my local networks that I want to
advertise.

Output Policy Engine:

Do not propagate the default route 0/0.
Do not advertise 193.214.10.0/24 to AS4.
Give 192.213.1.0/24 a metric of 10 when sent to AS3.

Routes advertised to peers:

Toward AS3:
192.213.1.0/24 via (AS5 AS1) (this means first AS5, then AS1) with a metric of 10
172.16.10.0/24 (via AS5)
193.214.10.0/24 (via AS5 AS2)
Toward AS4:
192.213.1.0/24 (via AS5 AS1)
172.16.10.0/24 (via AS5)