The topology can be referred to the attachment. Before cutover one GSR distributes default route in MAN. In order that two dual-homed links enable load balancing, adjust ospf cost between GSRs as 6 to make S8016 receive two default routes and guide the flow-balancing to uplink to two NE80E.
After the cutover it can be referred to the attachment 2. GSR moves to AS4134 and cancel OSPF process. Two NE80Es distributes default route and then S8016 has only one default route pointing to NE80E_B. Upstream traffic is guided to NE80_B.
Change the cost of GE from S8016 to NE80E_B as default value 10 and then check routing table on S8016. There are two default routes with the cost as 11.
Check routing table and it is found that S8016 points to default route of NE80E_B. ospf cost is 5. According to link bandwidth, it should receive two default routes with cost as 11.
Check ospf LSDB, it can receive two default routes sent by two NE80E. Before cutover the distribution of default route is different, the cost is manually adjusted.