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FAQ-DSLAM of the Attached DSLAM of Company A Cannot Obtain the Address When the NE40E Serves as a DHCP Server

Publication Date:  2012-07-27 Views:  70 Downloads:  0
Issue Description
The current network uses the NE40E as the DHCP server. Its version is V300R002C06B325.
NE40E (DHCP Server)――> T16000 (Layer 2 switch)――> F4000 (Layer 2 switch)――> DSLAM――> Testing PC
The testing PC cannot obtain the address. 
Alarm Information
Handling Process
1. Check and find that the MAC address of the testing PC can be learned from the Layer 2 switch and the static IP address configured by the testing PC can ping through the sub-interface address with the NE40E being the gateway. These indicate that the Layer 2 network has no problem.
2. Check and find that the data of NE40E DHCP is configured correctly.
3. Capture packets on the testing PC and find that the DHCP discover packet is sent but the DHCP offer packet is not received. Debug the NE40E and find that the DHCP offer packet has been sent by the NE40E, which indicates that the DHCP offer packet is discarded by the device in the network.
4. Attach a testing PC to the Huawei MA5100 to perform the test. DHCP service is normal and the testing PC can obtain the IP address.
5. Mirror and capture the incoming DSLAM packet of company A on the switch. The result shows that this DHCP offer packet is sent to the DSLAM device in the format of broadcast packet.
6. Communicate with the engineer of company A and find that the DSLAM device of the Company A regards the outgoing broadcast packet as the attack packet and discards this packet directly.
7. Consult related documents and find that the DHCP server detects the value of the broadcast response flags in the DHCP discover packet sent by the client to determine that the DHCP offer packet is sent in the format of unicast or broadcast. The value 1 indicates that the packet is sent in broadcast; the value 0 indicates that the packet is sent in unicast. Through capturing packets, it can be seen that the flags sent by the client is in unicast (for detail, refer to the attachment), the NE40E should respond a unicast packet to the client. But opposite to the implementation principle of the protocol, the NE40E defaults the format of the response packet as the broadcast, and this leads that the packet is discarded on the DSLAM. 
Root Cause
1. The Layer 2 network is Down.
2. The NE40E DHCP is configured incorrectly.
3. The interaction of DHCP packets on devices is abnormal.
4. Fault occurs on the intermediate devices.
5. Other reasons. 
1. The NE40E must process the packet according to the RFC protocol. If not, patches can be used to solve this problem.
2. Mirroring and packet capture can be used to locate the fault of abnormal packets interaction.