Path Mtu Discovery - Alcatel Omni Switch/Router User Manual

Path MTU Discovery

Path MTU Discovery
All Gigabit Ethernet modules and all Mammoth-based Ethernet modules on the Omni
Switch/Router in Release 4.0 and later support path Maximum Transmission Unit (
discovery. In path
that does not require fragmentation anywhere along the path from a source host to its desti-
nation. This frame size, known as a Path
of each hop in the path.
MTUs
Path discovery is active all of the time and is part of the switch's operating system; you
MTU
do not need configure it.
The source host initially assumes that the
all datagrams with the "Don't Fragment" (
a datagram that is too large to forward without fragmentation, the following steps will be
executed:
1. The switch/router that cannot forward these datagrams (i.e., the constricting hop) will
discard them.
2. The constricting hop will send
with a code that indicates fragmentation is needed and the "Don't Fragment" (
the Internet Protocol (
Big" message) contains the
3. After receiving a "Datagram Too Big" message, the source host reduces the size of the
so it matches the
MTU
4. The
MTU
However, the source host will not reduce the size of a datagram below 68 octets.
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discovery, the Ethernet frame (datagram) size is set to the largest size
MTU
discovery is not supported on token ring,
MTU
, or non-Mammoth Ethernet modules. However,
WAN
token ring and
FDDI
(e.g., trunking or bridging) between remote switches.
ICMP
) header has been set. This message (known as a "Datagram Too
IP
PMTU
of the constricting hop.
PMTU
discovery process ends when datagrams can be sent without fragmentation.
( ), is thus equal to the minimum of the
MTU PMTU
o Note o
can be used as intermediate links
of a path is the
PMTU
) bit set. If a switch/router along the path receives
DF
destination unreachable messages to the source host
of the constricting hop.
)
MTU
,
FDDI
of the first hop. It sends
MTU
) bit in
DF