OS10 Enterprise Edition User Guide Release 10.4.0E(R3)

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4-Byte AS numbers

OS10 supports 4-byte AS number configurations by default. The 4-byte support is advertised as a new BGP capability - 4-BYTE-AS, in the OPEN message. A BGP speaker that advertises 4-Byte-AS capability to a peer, and receives the same from that peer must encode AS numbers as 4-octet entities in all messages.

If the AS number of the peer is different, the 4-byte speaker brings up the neighbor session using a reserved 2-byte ASN,23456 called AS_TRANS. The AS_TRANS is used to interop between a 2-byte and 4-byte AS number.

Where the 2-byte format is 1 to 65535, the 4-byte format is 1 to 4294967295. You can enter AS numbers using the traditional format.


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