Ipv4 Path Mtu Discovery Overview; Using The Configured Source Ip Address In Icmp Messages - Dell Force10 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.

Using the Configured Source IP Address in ICMP Messages

This feature is supported on the Z9000 platform.
ICMP error or unreachable messages are now sent with the configured IP address of the source interface instead of the front-end
port IP address as the source IP address. Enable the generation of ICMP unreachable messages through the ip unreachable
command in Interface mode. When a ping or traceroute packet from an endpoint or a device arrives at the null 0 interface configured
with a static route, it is discarded. In such cases, you can configure Internet Control Message Protocol (ICMP) unreachable
messages to be sent to the transmitting device.
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IPv4 Routing
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