Fault Symptom
The AP is connected to the BRAS in VLAN access mode; the BRAS functions as the gateway of the AP. The AP is configured to obtain an IP address from the DHCP server through the BRAS. After the configuration, the AP cannot obtain an IP address from the DHCP server.
Fault Analysis
1. The ping from the BRAS to the AC is successful.
2. Run the trace mac enable command to globally enable MAC trace.
3. Run the trace mac mac-address vlan vlan-id command to check the connectivity between the BRAS and AP.
-[2010/5/22 16:34:41-][DHCPR][0023-8902-5120]:Receive OFFER packet successfully
(Ciadd:0.0.0.0 Yiadd:172.16.32.3 Siadd:0.0.0.0 Giadd:172.16.32.1 chaddr:0023-8902-5120
RouteIP:172.16.32.1 SubMask:255.255.255.0 ServerId:1.1.1.1 lease:1800s
The command output shows that the BRAS has received a DHCPOFFER message sent from the DHCP server.
4. Run the display aaa online-fail-record interface interface-type interface-number command to check the cause of user access failure.
--------------------------------------------------------------------------
User name : SD-WH-GQHW-BS-2.M-02001000002...
Domain name : fit-ap-nm
User MAC : 0023-8902-5120
User access type : IPoE
User access interface : GigabitEthernet1/0/1.1
Qinq Vlan/User Vlan: 0/2512
User IP address : 255.255.255.255
User ID : 100734
User authen state : Authened
User acct state : AcctIdle
User author state : AuthorIdle
User login time : 2010/05/22 17:12:48
User online fail reason: DHCP server no response
-------------------------------------------------------------------
Are you sure to show some information?(y/n)[y]:
5. Run the debugging ip packet command, and you can find that the source IP address of the DHCPOFFER message is 222.175.193.178. The IP address of the DHCP server in the DHCP server group configured on the BRAS, however, is 222.174.192.22.
*2.2206331108 SD-WH-GQHW-BS-2.MAN IP/7/debug_case:Slot=1;
Receiving, interface = GigabitEthernet1/0/1.1, version = 4, headlen = 20, tos = 96,
pktlen = 369, pktid = 2298, offset = 0, ttl = 255, protocol = 17,
checksum = 17582, s = 2.2.2.2, d = 172.16.32.1
prompt: Receiving IP packet from GigabitEthernet1/0/1.1
After the BRAS receives the DHCPOFFER message, it finds that the source IP address of the message is not the IP address of the DHCP server. Therefore, the BRAS considers the message invalid and discards the message. In this manner, the AP cannot obtain an IP address.
Procedure
1. Run the system-view view to enter the system view.
2. Run the dhcp-server group group-name command to enter the DHCP server group view.
3. Run the dhcp-server 2.2.2.2 command to configure the IP address of the DHCP server to be the source IP address of the DHCPOFFER message.
After that, the AP can obtain an IP address from the DHCP server through the BRAS.
Or, you can set the IP address of the actual DHCP server to 222.174.192.22. After that, the AP can obtain an IP address from the DHCP server through the BRAS.
Summary
If a user cannot obtain an IP address from the DHCP server through the BRAS, you can check whether the IP address of the DHCP server is the same as that configured on the BRAS. If the IP addresses are different, configure them to be the same.
