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peer-as (Protocols BGP)



Hierarchy Level

Release Information

Statement introduced before Junos OS Release 7.4.


Specify the neighbor (peer) autonomous system (AS) number.

For EBGP, the peer is in another AS, so the AS number you specify in the peer-as statement must be different from the local router’s AS number, which you specify in the autonomous-system statement. For IBGP, the peer is in the same AS, so the two AS numbers that you specify in the autonomous-system and peer-as statements must be the same.

The AS numeric range in plain-number format has been extended in Junos OS Release 9.1 to provide BGP support for 4-byte AS numbers, as defined in RFC 4893, BGP Support for Four-octet AS Number Space. RFC 4893 introduces two new optional transitive BGP attributes, AS4_PATH and AS4_AGGREGATOR. These new attributes are used to propagate 4-byte AS path information across BGP speakers that do not support 4-byte AS numbers. RFC 4893 also introduces a reserved, well-known, 2-byte AS number, AS 23456. This reserved AS number is called AS_TRANS in RFC 4893. All releases of the Junos OS support 2-byte AS numbers.

In Junos OS Release 9.2 and later, 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.

With the introduction of 4-byte AS numbers, you might have a combination of routers that support 4-byte AS numbers and 2-byte AS numbers. For more information about what happens when establishing BGP peer relationships between 4-byte and 2-byte capable routers, see the following topics:

  • Using 4-Byte Autonomous System Numbers in BGP Networks Technology Overview.


autonomous-system—AS number.

Range: 1 through 4,294,967,295 (232 – 1) in plain-number format for 4-byte AS numbers

Range: 1 through 65,535 in plain-number format for 2-byte AS numbers (this is a subset of the 4-byte range)

Range: 0.0 through 65535.65535 in AS-dot notation format for 4-byte AS numbers

Required Privilege Level

routing—To view this statement in the configuration.

routing-control—To add this statement to the configuration.