BGP speaker sends an update packet to NSX containing AS4_aggregator with ASN 0.
On packet capture for update send by BGP Neighbour should see a packet containing the following :
Path Attribute - AGGREGATOR : AS : 0 origin : 0.0.0.0
NSX 6.3.X includes the feature for ASN-4 , and fully supports RFC 4271 " A Border Gateway Protocol 4 (BGP-4)". RFC 6793, RFC 7606 and RFC 7607 are not supported.
Based on RFC 4721 and since we NSX doesn't supports RFC NSX version running 6.3.X will reset BGP sessions when AS4_aggregator attribute contains ASN 0 and renegotiate them.
You may found entries on NSX Edge logs like the example below where X.X.X.X is the IP for the BGP neighbour and YYYY is the name of the Switch :
2018-12-14T12:21:25+00:00 YYYY routing[988]: [user.emerg] EXCEPTION 0x4103-95 (0000): UPDATE message contains AS4_AGGREGATOR attribute with AS number 0.
2018-12-14T12:21:25+00:00 YYYY routing[988]: [user.err] PROBLEM 0x4102-46 (0000): NM has received an UPDATE message that failed to parse.
2018-12-14T12:21:25+00:00 YYYY routing[988]: [user.debug] DEV 0x4102-54 (0000): BGP Connection FSM state changed. Remote address: X.X.X.X ld state: Established New state: Idle)
2018-12-14T12:21:25+00:00 YYYY routing[988]: [user.emerg] EXCEPTION 0x4102-71 (0000): UPDATE Message Error (Optional Attribute Error): BGP NOTIFICATION sent to neighbor X.X.X.X (A
S: 65331)
2018-12-14T12:21:25+00:00 YYYY routing[988]: [user.info] AUDIT 0x4102-49 (0000): BGP: A connection's FSM state has deteriorated. Remote address: X.X.X.X (Old State: Established New State: Idle)
Packet captures for BGP updates received on NSX Edge contains on Path Attribute field Aggregator AS : 0 as per image below :