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122 Configuring and managing IP interfaces and services
NN47250-500 (320657-F Version 02.01)
MTU support
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.