Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.14.1.0 - Page 444

IPv4 Path MTU Discovery Overview 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. To use the PMTD functionality, you must enter the ip unreachables command on a VLAN interface to enable the generation of ICMP unreachable messages in the intermediate nodes. The PMTD functionality is based on the ICMPv4 destination unreachable message which is generated by the intermediate device only when the ip unreachables command is configured. PMTD is supported on all the layer 3 VLAN interfaces. Since all of the Layer 3 interfaces are mapped to the VLAN ID of 4095, you cannot 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 the system, both the IPv4 and IPv6 traffic are applied the same MTU size; you cannot specify different MTU values for IPv4 and IPv6 packets. Packet handling during MTU mismatch When you configure the MTU size on an interface, ensure that the MTU size of both ingress and egress interfaces are set to the same value for IPv4 traffic to work correctly. If there is an MTU mismatch between the ingress and egress interface, there may be a high CPU usage. If egress interface MTU size is smaller than the ingress interface, packets may get fragmented. Using the Configured Source IP Address in ICMP Messages 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. Configuring the ICMP Source Interface You can enable the ICMP error and unreachable messages to contain the configured IP address of the source device instead of the previous hop's IP address. This configuration helps identify the devices along the path because the DNS server maps the loopback IP address 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 unreachable messages and in the traceroute command output. Use the ip icmp source-interface interface or the ipv6 icmp 444 IPv4 Routing