IPv4 Path MTU Discovery OverviewThe size of the packet that can be sent across each hop in the network path without being fragmented is called the path maximumtransmission unit (PMTU). This value might vary for the same route between two devices, mainly over a public network, depending on thenetwork 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 hostto the same endpoint.Path MTU discovery (PMTD) identifies the path MTU value between the sender and the receiver, and uses the determined value totransmit packets across the network. PMTD, as described in RFC 1191, denotes that the default byte size of an IP packet is 576. Thispacket size is called the maximum transmission unit (MTU) for IPv4 frames. PMTD operates by containing the do not fragment (DF) bit setin the IP headers of outgoing packets. When any device along the network path contains an MTU that is smaller than the size of thepacket that it receives, the device drops the packet and sends an Internet Control Message Protocol (ICMP) Fragmentation Needed (Type3, Code 4) message with its MTU value to the source or the sending device. This message enables the source to identify that thetransmitted packet size must be reduced. The packet is retransmitted with a lower size than the previous value. This process is repeated inan 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 packetwithout 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 thepacket 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 ipunreachables command on a VLAN interface to enable the generation of ICMP unreachable messages. PMTD is supported on all thelayer 3 VLAN interfaces. Because all of the Layer 3 interfaces are mapped to the VLAN ID of 4095 when VLAN sub-interfaces areconfigured on it, it is not possible to configure unique layer 3 MTU values for each of the layer 3 interfaces. If a VLAN interface containsboth IPv4 and IPv6 addresses configured on it, both the IPv4 and IPv6 traffic are applied the same MTU size; you cannot specify differentMTU values for IPv4 and IPv6 packets.Using the Configured Source IP Address in ICMPMessagesICMP error or unreachable messages are now sent with the configured IP address of the source interface instead of the front-end port IPaddress as the source IP address. Enable the generation of ICMP unreachable messages through the ip unreachable command inInterface 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 thetransmitting device.Configuring the ICMP Source InterfaceYou can enable the ICMP error and unreachable messages to contain the configured IP address of the source device instead of theprevious hop's IP address. This configuration helps identify the devices along the path because the DNS server maps the loopback IPaddress to the host name, and does not translate the IP address of every interface of the switch to the host name.Configure the source to send the configured source interface IP address instead of using its front-end IP address in the ICMP unreachablemessages and in the traceroute command output. Use the ip icmp source-interface interface or the ipv6 icmpsource-interface interface commands in Configuration mode to enable the ICMP error messages to be sent with the sourceinterface IP address. This functionality is supported on loopback, VLAN, port channel, and physical interfaces for IPv4 and IPv6 messages.feature is not supported on tunnel interfaces. ICMP error relay, PATH MTU transmission, and fragmented packets are not supported fortunnel interfaces. The traceroute utilities for IPv4 and IPv6 list the IP addresses of the devices in the hops of the path for which ICMPsource interface is configured.438 IPv4 Routing
