Supported Platforms
Related Documentation
- M, MX, PTX, T Series
- 4-Byte Autonomous System Numbers Overview in the Using 4-Byte Autonomous System Numbers in BGP Networks Technology Overview
- Prepending 4-Byte AS Numbers in an AS Path in the Using 4-Byte Autonomous System Numbers in BGP Networks Technology Overview
Prepending AS Numbers to BGP AS Paths
You can prepend one or more autonomous system (AS) numbers at the beginning of an AS path. The AS numbers are added at the beginning of the path after the actual AS number from which the route originates has been added to the path. Prepending an AS path makes a shorter AS path look longer and therefore less preferable to BGP.
In Junos OS Release 9.1 and later, you can specify 4-byte AS numbers as defined in RFC 4893, BGP Support for Four-octet AS Number Space, as well as the 2-byte AS numbers that are supported in earlier releases of the Junos OS. In plain-number format, you can configure a value in the range from 1 through 4,294,967,295.
If you have a router that does not support 4-byte AS numbers in the AS path, the prependend AS number displayed in the AS path is the AS_TRANS number, AS 23456. To display the route details, use the show route command. For more information, see Prepending 4-Byte AS Numbers in an AS Path in the Using 4-Byte Autonomous System Numbers in BGP Networks Technology Overview .
In the following 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 the configuration for AS 1, prepend multiple AS numbers:
Related Documentation
- M, MX, PTX, T Series
- 4-Byte Autonomous System Numbers Overview in the Using 4-Byte Autonomous System Numbers in BGP Networks Technology Overview
- Prepending 4-Byte AS Numbers in an AS Path in the Using 4-Byte Autonomous System Numbers in BGP Networks Technology Overview
Published: 2012-12-04
Supported Platforms
Related Documentation
- M, MX, PTX, T Series
- 4-Byte Autonomous System Numbers Overview in the Using 4-Byte Autonomous System Numbers in BGP Networks Technology Overview
- Prepending 4-Byte AS Numbers in an AS Path in the Using 4-Byte Autonomous System Numbers in BGP Networks Technology Overview
