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The Address Is Inaccessible If the NE40E Serves as the DHCP Server

Publication Date:  2012-07-27 Views:  79 Downloads:  0
Issue Description
The networking diagram is as follows:
NE40E-1----NE40E-2----T16000 (Layer 2 switch)----testing PC
The versions of the two NE40Es are V300R002C06B21B. NE40E-1 and NE40E-2 serve as the DHCP server and the DHCP relay respectively. NE40E-2 uses the same sub-interface to terminate multiple VLAN accesses. After the configuration, the address is inaccessible through the testing PC. 
Alarm Information
Handling Process
1. The testing PC randomly chooses an address in the address pool and configures it as static. If the static address and the interface gateway address of NE40E-2 (DHCP relay) can ping each other successfully, the fault does not occur on the Layer 2 network. After the testing PC captures packets with ethereal, the PC can send the dhcp discover packet normally, but fails to receive the dhcp offer packet.
2. On NE40E-2 (DHCP relay), the interface address and the relayed address can ping each other successfully.
3. Debug the DHCP packets on NE40E-2 (DHCP relay) and NE40E-1 (DHCP server). The packet is sent to the testing PC.
4. Continue to analyze the dhcp offer packet sent by NE40E-2. When terminating multiple VLAN data at the same sub-interface, NE40E only carries the first VLAN tag, one of which configured continuously. Therefore the dhcp offer packet cannot find the VLAN corresponding to the testing PC, and thus is discarded on the switch.
5. On the switch, add the interface connected to the testing PC to the first VLAN configured with terminate continuous VLAN on the NE40E, the testing PC can access the address.
6. Why is only the first VLAN tag carried? Because the common DHCP is enabled on NE40E-2 (DHCP relay) and the dhcp discover packet does not record the VLAN tag of the packet. When the dhcp offer packet returned by NE40E-1 (DHCP server) arrives at NE40E-2 (DHCP relay), the packet cannot judge which VLAN to go, and can only go to the first VLAN by default. Therefore, the address is inaccessible through other VLANs. The DHCP Option field can be used to store some control information and parameters undefined by protocol. If you attempt to carry the VLAN tag from the user with the packet, enable Option82 at the DHCP Relay Agent. The solution is as follows:
Open the access sub interface view and perform the following configuration:
dhcp option82 insert enable 
Root Cause
1. The Layer 2 network of the testing PC is disconnected. The dhcp discover packet cannot be sent out or the packet fails to arrive at the DHCP Server.
2. The address of NE40E-2 (DHCP relay) and that of NE40E-1 (DHCP server) cannot be pinged.
3. NE40E-1(DHCP server) does not send the dhcp offer packet, or the device between NE40E-2 (DHCP relay) and NE40E-1(DHCP server) discards the packet.
4. Others.