Services Are Interrupted Due to Different MTU Values on Huawei S5700 and Vendor R's Switch

Publication Date:  2013-12-31 Views:  1089 Downloads:  0
Issue Description
Two Huawei S5700s are configured in a stack. Each switch is connected to a vendor R's switch through an Eth-Trunk that is aggregated by three Ethernet links. The 5700s, functioning as access switches, use Eth-Trunk interfaces to transparently transmit VLAN packets to vendor R's switches. The gateway is configured on vendor R's switch. Service interruption occurs from time to time, but the interface status is normal and no alarm information is generated. Such symptoms cannot be simulated.
Alarm Information
Handling Process
The MTU value on the S5700 cannot be changed. Therefore, change the MTU value on interfaces of vendor R's switch to 1600. After the configuration, services are recovered.
Root Cause
When the fault occurs, capture packets on the S5700 and analyze the Eth-Trunk. It is found that the size of response packets is 1500 bytes or nearly 1500 bytes, and the DF value of these large packets is 1 indicating that the packets cannot be fragmented. Capture packets on the uplink device. It is found that the uplink device cannot receive large packets of 1500 bytes or nearly 1500 bytes. The fault may occur on the Eth-Trunk. After communication with vendor R, it is found that the MTU value is set to 1492 on member interfaces of the Eth-Trunk on vendor R's switch. Change the MTU value to 1600. Services are recovered.