Understanding Autonomous System Path-Based Actions

You can prepend or add one or more autonomous system (AS) numbers at the beginning of an AS path. The AS numbers are added after the local AS number has been added to the path. Prepending an AS path makes a shorter AS path look longer and therefore less preferable to the Border Gateway Protocol (BGP).

For example, from AS 1, there are two equal paths (through AS 2 and AS 3) to reach AS 4. You might want packets from certain sources to use the path through AS 2. Therefore, you must make the path through AS 3 look less preferable so that BGP chooses the path through AS 2. In AS 1, you can prepend multiple AS numbers.

Related Topics

• Junos OS Feature Support Reference for SRX Series and J Series Devices
• Routing Policies Configuration Overview
• Understanding Routing Policy Match Conditions and Actions
• Example: Configuring a Routing Policy to Prepend the AS Path