AS-path override. If non-null, the AS_PATH attribute
of a route matching this filter will be replaced with
the indicated AS path. A BGP-4 AS_PATH is composed of AS
path segments. Each AS path segment is represented by a
triple: <path segment type, path segment length, path
segment value>.
The path segment type is a 1-octet long field with the
following values defined:
1: AS_SET: an unordered set of ASes a route in the UPDATE
message has traversed.
2: AS_SEQUENCE: an ordered set of ASes a route in the
UPDATE message has traversed.
The path segment length is a 1-octet long field containing
the number of ASes in the path segment value field.
The path segment value field contains one or more AS numbers,
each encoded as a 2-octet long field.
Only valid if 'action' is 'announce'.