Related Documentation
- M, MX, PTX, T Series
- Configuring 4-Byte AS Numbers and BGP Extended Community Attributes
- Configuring 4-Byte Autonomous System Numbers
- Establishing a Peer Relationship Between a 4-Byte Capable Router and a 2-Byte Capable Router Using a 2-Byte AS Number
- Establishing a Peer Relationship Between a 4-Byte Capable Router and a 2-Byte Capable Router Using a 4-Byte AS Number
- Juniper Networks Implementation of 4-Byte Autonomous System Numbers
- Prepending 4-Byte AS Numbers in an AS Path
- Understanding 4-Byte AS Numbers and Route Distinguishers
- Understanding 4-Byte AS Numbers and Route Loop Detection
- Understanding a 4-Byte Capable Router AS Path Through a 2-Byte Capable Domain
4-Byte Autonomous System Numbers Overview
This Technology Overview describes 4-byte autonomous system (AS) numbers and the operation of BGP in a network with a mix of 2-byte and 4-byte AS numbers.
The 2-byte AS number, also known as a 16-bit AS number or 2-octet AS number, provides a pool of 65,536 AS numbers. The 2-byte AS number range has been exhausted. 4-byte AS numbers are specified in RFC 4893, BGP Support for Four-Octet AS Number Space and provide a pool of 4,294,967,296 AS numbers.
As of January 1, 2009 the Internet Assigned Numbers Authority (IANA) only assigns 4-byte AS numbers, unless a 2-byte AS number is specifically requested. The Internet Engineering Task Force (IETF) RFC 4893 defines a method for smooth transition from 2-byte AS numbers to 4-byte AS numbers and for maintaining backward compatibility.
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.
This Technology Overview presents the following topics:
- Juniper Networks implementation of 4-byte AS Numbers
- How to configure 4-byte AS numbers
- The impact on BGP peering sessions with a mix of 2-byte and 4-byte AS numbers
- The impact on BGP paths with a mix of 2-byte and 4-byte AS numbers
- How to configure route distinguishers using 4-byte AS numbers
- How to configure extended community attributes using 4-byte AS numbers
- The impact on BGP route loop detection with a mix of 2-byte and 4-byte AS numbers
Related Documentation
- M, MX, PTX, T Series
- Configuring 4-Byte AS Numbers and BGP Extended Community Attributes
- Configuring 4-Byte Autonomous System Numbers
- Establishing a Peer Relationship Between a 4-Byte Capable Router and a 2-Byte Capable Router Using a 2-Byte AS Number
- Establishing a Peer Relationship Between a 4-Byte Capable Router and a 2-Byte Capable Router Using a 4-Byte AS Number
- Juniper Networks Implementation of 4-Byte Autonomous System Numbers
- Prepending 4-Byte AS Numbers in an AS Path
- Understanding 4-Byte AS Numbers and Route Distinguishers
- Understanding 4-Byte AS Numbers and Route Loop Detection
- Understanding a 4-Byte Capable Router AS Path Through a 2-Byte Capable Domain
Published: 2012-11-14
Related Documentation
- M, MX, PTX, T Series
- Configuring 4-Byte AS Numbers and BGP Extended Community Attributes
- Configuring 4-Byte Autonomous System Numbers
- Establishing a Peer Relationship Between a 4-Byte Capable Router and a 2-Byte Capable Router Using a 2-Byte AS Number
- Establishing a Peer Relationship Between a 4-Byte Capable Router and a 2-Byte Capable Router Using a 4-Byte AS Number
- Juniper Networks Implementation of 4-Byte Autonomous System Numbers
- Prepending 4-Byte AS Numbers in an AS Path
- Understanding 4-Byte AS Numbers and Route Distinguishers
- Understanding 4-Byte AS Numbers and Route Loop Detection
- Understanding a 4-Byte Capable Router AS Path Through a 2-Byte Capable Domain
