Cisco Systems 78-11741-02 manual Setting the MTU Packet Size, Enabling IP Source Routing, IPC-84

Configuring IP Services

Managing IP Connections

because the 512-byte router is unable to forward it. All packets larger than 512 bytes are dropped in this case. The second router returns an ICMP destination unreachable message to the source of the datagram with its Code field indicating, “Fragmentation needed and DF set.” To support IP Path MTU Discovery, it would also include the MTU of the next hop network link in the low-order bits of an unused header field.

IP Path MTU Discovery is also useful when a connection is being established and the sender has no information at all about the intervening links. It is always advisable to use the largest MTU that the links will bear; the larger the MTU, the fewer packets the host must send.

Note IP Path MTU Discovery is a process initiated by end hosts. If an end host does not support IP Path MTU Discovery, the receiving device will have no mechanism available to avoid fragmenting datagrams generated by the end host.

If a router that is configured with a small MTU on an outbound interface receives packets from a host that is configured with a large MTU (for example, receiving packets from a Token Ring interface and forwarding them to an outbound Ethernet interface), the router fragments received packets that are larger than the MTU of the outbound interface. Fragmenting packets slows the performance of the router. To keep routers in your network from fragmenting received packets, run IP Path MTU Discovery on all hosts and routers in your network, and always configure the largest possible MTU for each router interface type.

To enable IP Path MTU Discovery for connections initiated by the router (when the router is acting as a host), see the section “Enabling TCP Path MTU Discovery” later in this chapter.

Setting the MTU Packet Size

All interfaces have a default MTU packet size. You can adjust the IP MTU size so that the Cisco IOS software will fragment any IP packet that exceeds the MTU set for an interface.

Changing the MTU value (with the mtu interface configuration command) can affect the IP MTU value. If the current IP MTU value is the same as the MTU value and you change the MTU value, the IP MTU value will be modified automatically to match the new MTU. However, the reverse is not true; changing the IP MTU value has no effect on the value for the mtu interface configuration command.

Also, all devices on a physical medium must have the same protocol MTU in order to operate.

To set the MTU packet size for a specified interface, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip mtu bytes	Sets the IP MTU packet size for an interface.

Enabling IP Source Routing

The Cisco IOS software examines IP header options on every packet. It supports the IP header options Strict Source Route, Loose Source Route, Record Route, and Time Stamp, which are defined in RFC 791. If the software finds a packet with one of these options enabled, it performs the appropriate action. If it finds a packet with an invalid option, it sends an ICMP parameter problem message to the source of the packet and discards the packet.

Cisco IOS IP Configuration Guide

IPC-84