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Four-Octet AS Numbers

BGP speakers that support four-octet AS and sub-AS numbers are sometimes referred to as “ new” speakers. The four-octet AS numbers are employed by the AS-path and aggregator attributes. “ Old” speakers are those that do not support the four-octet numbers.

Two new transitional optional attributes, new-as-path and new-aggregator, are used to carry the four-octet numbers across the old speakers. A new speaker communicating with an old speaker will send the new attributes with the four-octet numbers for locally-originated and propagated routes. The old speaker propagates the new attributes for received routes. The new speaker also sends the AS-path and aggregator attributes with two-octet numbers; any AS number greater than 65535 is replaced with a reserved AS number, 23456.

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