Dell Configuration Guide for the S4048–ON System 9.14.2.6

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 in the physical interface, you must allocate and activate the fedgov CAM ACL space using the cam-acl command. The fedgov CAM ACL space is defined as a value (0-8) and you can select the required value to define the space. When the space is activated, you can send the ICMP type 3 messages through the physical port.

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.