Unanswered Question
Giuseppe Larosa Sat, 02/21/2009 - 01:56

Hello Ali,

a well known mandatory attribute is an attribute that every implementation has to support.

BGP next hop, AS path, origin code

an optional attribute is instead an attribute that may be implemented

there are

well-known mandatory

well-known discretionary

optional transitive

optional non transitive



section 4.3

Hope to help


Giuseppe Larosa Sat, 02/21/2009 - 03:16

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


Harold Ritter Sat, 02/21/2009 - 04:32


Just one precision. RFC1771 has been superseded by RFC4271.



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