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4. Operation

A member of a BGP confederation MUST use its AS Confederation Identifier in all transactions with peers that are not members of its confederation. This AS Confederation Identifier is the "externally visible" AS number, and this number is used in OPEN messages and advertised in the AS_PATH attribute.

A member of a BGP confederation MUST use its Member-AS Number in all transactions with peers that are members of the same confederation as the local BGP speaker.

A BGP speaker receiving an AS_PATH attribute containing an autonomous system matching its own AS Confederation Identifier SHALL treat the path in the same fashion as if it had received a path containing its own AS number.

A BGP speaker receiving an AS_PATH attribute containing an AS_CONFED_SEQUENCE or AS_CONFED_SET that contains its own Member-AS Number SHALL treat the path in the same fashion as if it had received a path containing its own AS number.

4.1. AS_PATH Modification Rules​

When implementing BGP confederations, Section 5.1.2 of [BGP-4] is replaced with the following text:

AS_PATH is a well-known mandatory attribute. This attribute identifies the autonomous systems through which routing information carried in this UPDATE message has passed. The components of this list can be AS_SETs, AS_SEQUENCEs, AS_CONFED_SETs or AS_CONFED_SEQUENCES.

When a BGP speaker propagates a route it learned from another BGP speaker's UPDATE message, it modifies the route's AS_PATH attribute based on the location of the BGP speaker to which the route will be sent:

a) When a given BGP speaker advertises the route to another BGP speaker located in its own Member-AS, the advertising speaker SHALL NOT modify the AS_PATH attribute associated with the route.

b) When a given BGP speaker advertises the route to a BGP speaker located in a neighboring autonomous system that is a member of the local confederation, the advertising speaker updates the AS_PATH attribute as follows:

  1. if the first path segment of the AS_PATH is of type AS_CONFED_SEQUENCE, the local system prepends its own Member-AS number as the last element of the sequence (put it in the leftmost position with respect to the position of octets in the protocol message). If the act of prepending will cause an overflow in the AS_PATH segment (i.e., more than 255 ASs), it SHOULD prepend a new segment of type AS_CONFED_SEQUENCE and prepend its own AS number to this new segment.

  2. if the first path segment of the AS_PATH is not of type AS_CONFED_SEQUENCE, the local system prepends a new path segment of type AS_CONFED_SEQUENCE to the AS_PATH, including its own Member-AS Number in that segment.

  3. if the AS_PATH is empty, the local system creates a path segment of type AS_CONFED_SEQUENCE, places its own Member-AS Number into that segment, and places that segment into the AS_PATH.

c) When a given BGP speaker advertises the route to a BGP speaker located in a neighboring autonomous system that is not a member of the local confederation, the advertising speaker SHALL update the AS_PATH attribute as follows:

  1. if any path segments of the AS_PATH are of the type AS_CONFED_SEQUENCE or AS_CONFED_SET, those segments MUST be removed from the AS_PATH attribute, leaving the sanitized AS_PATH attribute to be operated on by steps 2, 3 or 4.

  2. if the first path segment of the remaining AS_PATH is of type AS_SEQUENCE, the local system prepends its own AS Confederation Identifier as the last element of the sequence (put it in the leftmost position with respect to the position of octets in the protocol message). If the act of prepending will cause an overflow in the AS_PATH segment (i.e., more than 255 ASs), it SHOULD prepend a new segment of type AS_SEQUENCE and prepend its own AS number to this new segment.

  3. if the first path segment of the remaining AS_PATH is of type AS_SET, the local system prepends a new path segment of type AS_SEQUENCE to the AS_PATH, including its own AS Confederation Identifier in that segment.

  4. if the remaining AS_PATH is empty, the local system creates a path segment of type AS_SEQUENCE, places its own AS Confederation Identifier into that segment, and places that segment into the AS_PATH.

When a BGP speaker originates a route then:

a) the originating speaker includes its own AS Confederation Identifier in a path segment, of type AS_SEQUENCE, in the AS_PATH attribute of all UPDATE messages sent to BGP speakers located in neighboring autonomous systems that are not members of the local confederation. In this case, the AS Confederation Identifier of the originating speaker's autonomous system will be the only entry the path segment, and this path segment will be the only segment in the AS_PATH attribute.

b) the originating speaker includes its own Member-AS Number in a path segment, of type AS_CONFED_SEQUENCE, in the AS_PATH attribute of all UPDATE messages sent to BGP speakers located in neighboring Member Autonomous Systems that are members of the local confederation. In this case, the Member-AS Number of the originating speaker's autonomous system will be the only entry the path segment, and this path segment will be the only segment in the AS_PATH attribute.

c) the originating speaker includes an empty AS_PATH attribute in all UPDATE messages sent to BGP speakers residing within the same Member-AS. (An empty AS_PATH attribute is one whose length field contains the value zero).

Whenever the modification of the AS_PATH attribute calls for including or prepending the AS Confederation Identifier or Member-AS Number of the local system, the local system MAY include/prepend more than one instance of that value in the AS_PATH attribute. This is controlled via local configuration.

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