Each large community is a 12-byte value, formed from the logical concatenation of three 4-octet values: a Global Administrator part, a Local Data part 1, and Local Data part 2. The Global Administrator is a four-octet namespace identifier, which should be an Autonomous System Number assigned by IANA. The Global Administrator field is intended to allow different Autonomous Systems to define large communities without collision. Local Data Part 1 is a four-octet operator-defined value and Local Data Part 2 is another four-octet operator-defined value.
In SR OS, a large community value is configured using the format <ext-asnum>:<ext-comm-val>:<ext-comm-val>; the colon is a required separator character between each of the 4-byte values. In route policy applications, it is possible to match multiple large community values with a regular expression in the format <regex1>&<regex2>&<regex3>, where regex1, regex2 and regex3 are three regular expressions, each evaluated one numerical (decimal) digit at a time.
Large communities can be added to or removed from BGP routes using BGP import and export policies. When a BGP route is locally originated by exporting a static or aggregate route into BGP, and the static or aggregate route has one or more large communities, these community values are automatically added to the BGP route.
To remove all the standard communities from all routes advertised to a BGP peer, use the disable-communities large command.