Nortel 2350 用户指南

122 Configuring and managing IP interfaces and services

NN47250-500 (320657-F Version 02.01)

WLAN Security Switch 2300 Series (WSS Software) supports standard maximum transmission units (MTUs) of 1514 
bytes for standard Ethernet packets and 1518 bytes for Ethernet packets with an 802.1Q tag. WSS Software does not 
support changing of the MTU through software configuration, and WSS Software does not do path MTU discovery. 

Communication between WSSs is supported over any path MTU, and the Mobility Domain itself can run over the 
minimum IP path MTU (PMTU). However, tunnels between two WSSs require a path MTU of at least 1384 bytes. 

This minimum MTU path is required because Nortel devices use IP tunnels to transport user traffic between WSSs and 
to transport user traffic and control traffic between switches and APs. Encapsulation of the packets for tunneling adds an 
additional 44 bytes to the packet headers, so WSS Software does fragment and reassemble the packets if necessary to fit 
within the supported MTUs. However, WSS Software does not support defragmentation except at the receiving end of 
an IP tunnel, and only to reassemble fragments created by another Nortel device for tunneling. 

If the path MTU between Nortel devices is less than 1384 bytes, a device in the path might further fragment or drop a 
tunneled packet. If the packet is further fragmented, the receiving WSS will not be able to reassemble the fragments, and 
the packet is dropped.