Ipv4 Path Mtu Discovery Overview - Dell Z9000 Configuration Manual

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IPv4 Path MTU Discovery Overview

This functionality is supported on the Z9000 platform.
The size of the packet that can be sent across each hop in the network path without being fragmented is
called the path maximum transmission unit (PMTU). This value might vary for the same route between
two devices, mainly over a public network, depending on the network load and speed, and it is not a
consistent value. The MTU size can also be different for various types of traffic sent from one host to the
same endpoint.
Path MTU discovery (PMTD) identifies the path MTU value between the sender and the receiver, and uses
the determined value to transmit packets across the network. PMTD, as described in RFC 1191, denotes
that the default byte size of an IP packet is 576. This packet size is called the maximum transmission unit
(MTU) for IPv4 frames. PMTD operates by containing the do not fragment (DF) bit set in the IP headers of
outgoing packets. When any device along the network path contains an MTU that is smaller than the size
of the packet that it receives, the device drops the packet and sends an Internet Control Message
Protocol (ICMP) Fragmentation Needed (Type 3, Code 4) message with its MTU value to the source or the
sending device. This message enables the source to identify that the transmitted packet size must be
reduced. The packet is retransmitted with a lower size than the previous value. This process is repeated in
an interactive way until the MTU of the transmitted packet is lower or equal to the MTU of the receiving
device for it to obtain the packet without fragmentation. If the ICMP message from the receiving device,
which is sent to the originating device, contains the next-hop MTU, then the sending device lowers the
packet size accordingly and resends the packet. Otherwise, the iterative method is followed until the
packet can traverse without being fragmented.
PMTD is enabled by default on the switches that support this capability. To enable PMTD to function
correctly, you must enter the ip unreachables command on a VLAN interface to enable the
generation of ICMP unreachable messages. PMTD is supported on all the layer 3 VLAN interfaces.
Because all of the Layer 3 interfaces are mapped to the VLAN ID of 4095 when VLAN sub-interfaces are
configured on it, it is not possible to configure unique layer 3 MTU values for each of the layer 3
interfaces. If a VLAN interface contains both IPv4 and IPv6 addresses configured on it, both the IPv4 and
IPv6 traffic are applied the same MTU size; you cannot specify different MTU values for IPv4 and IPv6
packets.
IPv4 Routing
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