The well-known Border Gateway Protocol (BGP) autonomous system (AS) 23456 facilitates the gradual transition from 2byte ASes to 4byte ASes. Similar to the creation of IPv6, which is intended to combat the threat of exhaustion of IPv4 addresses, 4byte BGP ASes were introduced to combat the eventual exhaustion of 2byte ASes. The well-known 2byte AS 23456, which is also known as AS_TRANS, can be used by a 4byte BGP router to peer with a BGP router that supports only 2byte ASes.

When a 4byte BGP router must advertise an AS value larger than 2 bytes to a 2byte BGP router, the 4byte router will advertise the AS number 23456. Therefore, if the AS number 23456 appears in the output of the show ip bgp command, the router is not compatible with 4byte ASes.

When a 4byte BGP router peers with another 4byte BGP router, the AS is displayed in the output of the show ip bgp command in as plain or as dot format. Asp lain format displays the 4byte AS number as a decimal value from 65536 through 4294967295; this format is used by default. As dot format displays the 4byte AS number as a dotted decimal value from 1.0 through 65535.65535. To change the output to display in as dot format, you should issue the bgp as notation dot command from BGP router configuration mode.

AS 23456 does not facilitate the gradual transition from 4byte ASes to 2byte ASes, because of the impending exhaustion of the pool of 2byte ASes. A 2byte BGP router cannot use AS 23456 as its AS number when transitioning from 2byte ASes to 4byte ASes, because AS 23456 is reserved exclusively for use by 4byte routers peering with 2byte routers. Additionally, AS 23456 is not used by default on any BGP router. It is possible to configure a 4byte BGP router to use a 2byte AS other than 23456 in order to peer with 2byte BGP routers.