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The PC Fails to Obtain the IP Address Allocated by the DHCP Server because the MxU Is Configured with Too Many Broadcast Domains

Publication Date:  2019-07-09 Views:  211 Downloads:  1

Issue Description

Convergence switch――DHCP server――OLT――n*MxU
Connect the OLT to multiple MxUs and a firewall, start the DHCP server, use NAT, and connect to the convergence switch. The user PC connected to the MxU sometimes can obtain an IP address but sometimes fails to obtain an IP address.

Alarm Information

The PC often fails to obtain an IP address. When configured with a fixed IP address from the DHCP IP address pool, the PC, however, can ping the gateway and access the Internet normally.

Handling Process

1. Use a packet header obtaining tool and enable debugging dhcp server packet. It is found on the firewall that the DHCPOFFER packet is already sent. The PC, however, does not receive the DHCPOFFER packet.
2. packet header obtaining on the mirrored port of the MxU and it is found that the DHCPOFFER packet is already sent to the MxU but cannot reach the PC.
3. Reconfigure the data and map each MxU to a VLAN to reduce the number of broadcast domains under the OLT. Test again. All PCs can obtain IP addresses successfully.

Root Cause

The MxU is configured with VLAN conversion. Each port maps a VLAN. As a result, the OLT has too many broadcast domains. The R&D engineer indicates that the DHCP offer packet is lost due to the product architecture problem and excessively large number of broadcast domains.
The customer requires port isolation. Therefore, each port on all DSLAMs is required to map a VLAN, and VLAN conversion is used on the MxU and the OLT to save the VLAN resources. It is recommended to explain to the customer so as to solve this problem. The smart VLAN of Huawei's device can isolate the downstream ports in the VLAN. When this problem occurs, the customer can map each device to a VLAN to avoid the problem.