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5. Error Handling

A BGP speaker MUST NOT transmit updates containing AS_CONFED_SET or AS_CONFED_SEQUENCE attributes to peers that are not members of the local confederation.

It is an error for a BGP speaker to receive an UPDATE message with an AS_PATH attribute that contains AS_CONFED_SEQUENCE or AS_CONFED_SET segments from a neighbor that is not located in the same confederation. If a BGP speaker receives such an UPDATE message, it SHALL treat the message as having a malformed AS_PATH according to the procedures of [BGP-4], Section 6.3 ("UPDATE Message Error Handling").

It is a error for a BGP speaker to receive an update message from a confederation peer that is not in the same Member-AS that does not have AS_CONFED_SEQUENCE as the first segment. If a BGP speaker receives such an UPDATE message, it SHALL treat the message as having a malformed AS_PATH according to the procedures of [BGP-4], Section 6.3 ("UPDATE Message Error Handling").

5.1. Common Administrative Issues​

It is reasonable for Member Autonomous Systems of a confederation to share a common administration and Interior Gateway Protocol (IGP) information for the entire confederation. It is also reasonable for each Member-AS to run an independent IGP. In the latter case, the NEXT_HOP may need to be set using policy (i.e., by default it is unchanged).

5.2. MED and LOCAL_PREF Handling​

It SHALL be legal for a BGP speaker to advertise an unchanged NEXT_HOP and MULTI_EXIT_DISC (MED) attribute to peers in a neighboring Member-AS of the local confederation.

MEDs of two routes SHOULD only be compared if the first autonomous systems in the first AS_SEQUENCE in both routes are the same -- i.e., skip all the autonomous systems in the AS_CONFED_SET and AS_CONFED_SEQUENCE. An implementation MAY provide the ability to configure path selection such that MEDs of two routes are comparable if the first autonomous systems in the AS_PATHs are the same, regardless of AS_SEQUENCE or AS_CONFED_SEQUENCE in the AS_PATH.

An implementation MAY compare MEDs received from a Member-AS via multiple paths. An implementation MAY compare MEDs from different Member Autonomous Systems of the same confederation.

In addition, the restriction against sending the LOCAL_PREF attribute to peers in a neighboring autonomous system within the same confederation is removed.

5.3. AS_PATH and Path Selection​

Path selection criteria for information received from members inside a confederation MUST follow the same rules used for information received from members inside the same autonomous system, as specified in [BGP-4].

In addition, the following rules SHALL be applied:

  1. If the AS_PATH is internal to the local confederation (i.e., there are only AS_CONFED_* segments), consider the neighbor AS to be the local AS.

  2. Otherwise, if the first segment in the path that is not an AS_CONFED_SEQUENCE or AS_CONFED_SET is an AS_SEQUENCE, consider the neighbor AS to be the leftmost AS_SEQUENCE AS.

  3. When comparing routes using AS_PATH length, CONFED_SEQUENCE and CONFED_SETs SHOULD NOT be counted.

  4. When comparing routes using the internal (IBGP learned) versus external (EBGP learned) rules, treat a route that is learned from a peer that is in the same confederation (not necessarily the same Member-AS) as "internal".

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