Hello Ali,
I found a table
Table 2-5. Path Attributes[*]
Attribute Class
ORIGIN Well-known mandatory
AS_PATH Well-known mandatory
NEXT_HOP Well-known mandatory
LOCAL_PREF Well-known discretionary
ATOMIC_AGGREGATE Well-known discretionary
AGGREGATOR Optional transitive
COMMUNITY Optional transitive
MULTI_EXIT_DISC (MED) Optional nontransitive
ORIGINATOR_ID Optional nontransitive
CLUSTER_LIST Optional nontransitive
atomic-aggregate and aggregator are actually two distinct attributes:
ATOMIC_AGGREGATE is a well-known discretionary attribute that is used to alert downstream routers
that a loss of path information has occurred.
When the ATOMIC_AGGREGATE attribute is set, the BGP speaker has the option of also attaching the
AGGREGATOR attribute. This optional transitive attribute provides information about where the
aggregation was performed by including the AS number and the IP address of the router that
originated the aggregate route
both help to provide advice about a BGP route summarization that occurred, the AS path attribute of the summary route is/can be different then that of the component prefixes
Hope to help
Giuseppe
