| 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) |
|