IOS - AS-Override

AS-Override is often used on PE to CE eBGP sessions where the same AS number has been used across multiple customer CE devices with Site-of-Origin to prevent routing loops. In this case AS-Override is configured on the PE device on the BGP session facing the CPE. The PE looks for the CPEs AS number in the AS_PATH of routing updates and replaces any occurrence of the CPEs AS number with the PEs AS number.

When SoO is used this work without creating loops, including for dual-homed end sites. If a site is dual-homed without SoO then there is the possibility for a routing loop. This is usually visibility by the long AS_PATH in the BGP table on the receiving CPE with it's own AS number in the path two or more times instead of the expected one (from the originating CPE/site).

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