Cisco Systems Understanding What Protocol is Used to Synchronize Time Between Various Cisco IOS Devices in Your Network

•The route for the redirect is not another ICMP redirect or a default route.

•The router is configured to send redirects.

Make sure students understand default gateways. Instruct students to look at the router lab setup and visually determine the default gateway of the host attached to it.

8.2.3 Clock synchronization and transit time estimation

Networks that connect to each other over vast distances choose their own method of clock synchronization. As a result, hosts on disparate networks who attempt to communicate with software that requires time synchronization can encounter problems. The ICMP timestamp message type is designed to help alleviate this problem.

The ICMP timestamp request message allows a host to ask the remote host for the current time. The remote host uses an ICMP timestamp reply message to respond to the request. The type field on an ICMP timestamp message can be either a 13 or 14 timestamp reply. The code field value is always set to zero. The ICMP timestamp request contains an originate timestamp, which is the time on the requesting host just before the timestamp request is sent. The receive timestamp is the time that the destination host receives the ICMP timestamp request. The transit timestamp is filled in just before the ICMP timestamp reply is returned. Originate, receive, and transit timestamps are computed in numbers milliseconds elapsed since midnight (00:00), Universal Time.

The host that originated the ICMP timestamp request can use these timestamps to estimate transit time across the network. The host can subtract the originate time from the transit time to guess the transit time. However this can vary widely based on traffic and congestion. The host that originated the ICMP timestamp request can also estimate the local time of the remote computer. This is an important concept for the students to understand. Make sure that they also understand that NTP, which is a UDP protocol, is used to maintain the time between systems.

8.2.4 Information requests and reply message formats

ICMP information requests and reply messages were originally intended to allow a host to determine the number of the network it resided on. However, BOOTP and DHCP are now used to allow hosts to obtain the network number to which they are attached.

8.2.5 Address mask requests

A subnet mask is important to identify network, subnet, and host bits in an IP address. If a host does not know the subnet mask, it may send an address mask request to the local router. The router responds with an ICMP address mask reply. If the address of the router is known, this request may be sent unicast. If the address is not known, the request will be a broadcast. When the router receives the request, it will respond with an address mask reply. This reply will identify the correct subnet mask. This is an important concept for the students to understand. This is also a good time to review IP addressing.

8.2.6 Router discovery message

When a host on a network boots and has not been manually configured with a default gateway, it can learn the available routers through the process of router discovery. This

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