Configuring a Local AS

You can configure BGP with a different local AS number for each EBGP session, which allows BGP to configure a local AS for each EBGP session. Configuring a local AS simulates a virtual AS for the router. The AS paths for the routes from that EBGP peer have the configured local-as prepended before the peer AS for that session. This is useful if ISP A has acquired another ISP B, but does not want to change the configurations of ISP B’s customer routers. ISP B’s AS is the AS that is configured as the local AS.

Note: If the local AS for the EBGP/IBGP peer is the same as the current AS, do not use the local-as statement to specify the local AS number.

To configure a local AS, include the local-as statement:

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

For autonomous-system, you can specify a number from 1 through 4,294,967,295 in plain-number format. Beginning with JUNOS Release 9.1, the range for autonomous system (AS) numbers is extended to provide BGP support for 4-byte AS numbers as defined in RFC 4893, BGP Support for Four-octet AS Number Space. The JUNOS software continues to support 2-byte AS numbers.

Beginning with JUNOS Release 9.2, you can also configure a 4-byte AS number using the AS-dot notation format of two integer values joined by a period: <16-bit high-order value in decimal>.<16-bit low-order value in decimal>. For example, the 4-byte AS number of 65,546 in plain-number format is represented as 1.10 in the AS-dot notation format. You can specify a value in the range from 0.0 through 65535.65535. in AS-dot notation format.

If you include the private keyword, the local AS is not prepended before the peer AS. This means that the AS paths do not show details of such a configuration, and ISP A’s EBGP peers and IBGP peers do not see any difference from before the local AS configuration.

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