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4
C
M
IP
ONFIGURING AND
ANAGING
I
S
NTERFACES AND
ERVICES
This chapter describes how to configure IP interfaces and services.
MTU Support
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.
However, tunnels between two WX switches require a path MTU of at
least 1448 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 MAP access points.
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 1448 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.
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