Cisco Systems 78-11741-02 Wireless Office Headset User Manual


 
Configuring IP Services
Managing IP Connections
IPC-83
Cisco IOS IP Configuration Guide
To enable the sending of ICMP redirect messages if this feature was disabled, use the following
command in interface configuration mode:
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:
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
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
Command Purpose
Router(config-if)# ip redirects
Enables the sending of ICMP redirect messages to learn routes.
Command Purpose
Router(config-if)# ip mask-reply
Enables the sending of ICMP mask reply messages.
"Unreachable" sent
Packet dropped
S1014a
MTU = 512
Packet = 800 bytes
Don't fragment
MTU = 1500