Cisco CCNA 2 Instructor Manual page 99

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The route for the redirect is not another ICMP redirect or a default route.
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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
98 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 8
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