Cisco Systems 78-11741-02 Enabling Icmp Mask Reply Messages, Understanding Path MTU Discovery

Configuring IP Services

Managing IP Connections

To enable the sending of ICMP redirect messages if this feature was disabled, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip redirects	Enables the sending of ICMP redirect messages to learn routes.

Enabling ICMP Mask Reply Messages

Occasionally, network devices must know the subnet mask for a particular subnetwork in the internetwork. To obtain this information, such devices can send ICMP mask request messages. ICMP mask reply messages are sent in reply from devices that have the requested information. The Cisco IOS software can respond to ICMP mask request messages if this function is enabled.

To enable the sending of ICMP mask reply messages, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip mask-reply	Enables the sending of ICMP mask reply messages.

Understanding Path MTU Discovery

The Cisco IOS software supports the IP Path MTU Discovery mechanism, as defined in RFC 1191. IP Path MTU Discovery allows a host to dynamically discover and cope with differences in the maximum allowable maximum transmission unit (MTU) size of the various links along the path. Sometimes a router is unable to forward a datagram because it requires fragmentation (the packet is larger than the MTU you set for the interface with the ip mtu interface configuration command), but the “don’t fragment” (DF) bit is set. The Cisco IOS software sends a message to the sending host, alerting it to the problem. The host will need to fragment packets for the destination so that they fit the smallest packet size of all the links along the path. This technique is shown in Figure 17.

Figure 17 IP Path MTU Discovery

MTU = 1500

Packet = 800 bytes

Don't fragment

MTU = 512

"Unreachable" sent Packet dropped

S1014a

IP Path MTU Discovery is useful when a link in a network goes down, forcing the use of another, different MTU-sized link (and different routers). As shown in Figure 17, suppose a router is sending IP packets over a network where the MTU in the first router is set to 1500 bytes, but the second router is set to 512 bytes. If the “Don’t fragment” bit of the datagram is set, the datagram would be dropped

Cisco IOS IP Configuration Guide

IPC-83