3com WXR100 3CRWXR10095A User Manual

This chapter describes how to configure IP interfaces and services.

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

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

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

If the path MTU between WX switches 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 WX switch will not be able to 
reassemble the fragments, and the packet is dropped.