Install Guide

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.
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 in the system, both the IPv4 and IPv6 traffic are applied to 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 source-interface interface commands in Configuration mode to enable the ICMP error messages
to be sent with the source interface 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 for tunnel interfaces. The traceroute utilities for IPv4 and IPv6 list the
IP addresses of the devices in the hops of the path for which ICMP source interface is configured.
IPv4 Routing
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