Possible causes were as follows:
1. The RADIUS or BAS was configured incorrectly.
2. The BAS software had a bug.
Based on the preceding possible causes, Huawei performed the following operations to diagnose the fault:
1. Debugged DHCP and found that DHCP Ack packets carried 32-bit masks.
*0.3883652931 bng0 DHCR/7/debug2:
[ DHCPR send to client ] : =====
[ Xid ]:1795360822
[ cmd ]:2
[ Htype ]:1
[ Hlen ]:6
[ Hops ]:0
[ Secs ]:0
[ Flag ]:0
[ Ciadd ]:0.0.0.0
[ Yiadd ]:22.214.171.124
[ Siadd ]:0.0.0.0
[ Giadd ]:126.96.36.199 chaddr:0022-6b7a-0220
[ Sname ]:
[ File ]:
[ Option ]:-----
Debugged the RADIUS and found that it delivered 24-bit masks.
[Framed-IP-Address(8) ] [6 ] [188.8.131.52]
[Session-Timeout(27) ] [6 ] 
[Framed-IP-Netmask(9) ] [6 ] [255.255.255.0]
[HW-Gateway-Address(HUAWEI-73) ] [6 ] [184.108.40.206]
2. The Framed-IP-Netmask attribute should be delivered to PPP users. For DHCP access users, the Huawei proprietary attribute HW-Subnet-Mask(HUAWEI-72) should be delivered.3. Changed the attribute delivered by the RADIUS, and the issue was resolved.
The attribute delivered by the RADIUS was incorrect.The Framed-IP-Netmask attribute should be delivered to PPP users. For DHCP access users, the Huawei proprietary attribute HW-Subnet-Mask(HUAWEI-72) should be delivered.