Concept Guide

network load and speed, and it is not a consistent value. The MTU size can also be dierent for various types of trac sent from one host
to the same endpoint.
Path MTU discovery (PMTD) identies 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 congured. 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 congure unique layer 3 MTU values
for each of the layer 3 interfaces. If a VLAN interface contains both IPv4 and IPv6 addresses congured on the system, both the IPv4 and
IPv6 trac are applied the same MTU size; you cannot specify dierent MTU values for IPv4 and IPv6 packets.
Packet handling during MTU mismatch
When you congure 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 trac 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 Congured Source IP Address in ICMP
Messages
ICMP error or unreachable messages are now sent with the congured 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 congured with a static route,
it is discarded. In such cases, you can congure Internet Control Message Protocol (ICMP) unreachable messages to be sent to the
transmitting device.
Conguring the ICMP Source Interface
You can enable the ICMP error and unreachable messages to contain the congured IP address of the source device instead of the
previous hop's IP address. This conguration 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.
Congure the source to send the congured 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 Conguration 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 congured.
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IPv4 Routing