Cisco CCNA 2 Instructor Manual page 96

8.1.4 Unreachable networks
Network communications depend on certain basic conditions:
•
The sending and receiving devices must have the TCP/IP protocol configured.
•
This includes a correct IP address and subnet mask.
•
A default gateway must be set if data will go outside the LAN.
•
Devices must be place to route the data.
•
The router must be configured correctly and the correct routing protocol must be
used.
If these conditions are not met, communication cannot occur. Instruct the students to discuss
problems that could cause a network to be unreachable.
8.1.5 Using ping to test destination reachability
The ICMP protocol can be used to test the availability of a destination. If a destination receives
the ICMP echo request, it formulates an echo reply to send back to the source. If the sender
receives the echo reply, this confirms that the destination can be reached. The process is
initiated with the ping command.
Have the students do an exercise on the ping procedure. Discuss the use of the DNS function.
Explain that the DNS must be available to use a domain name instead of an IP address when
the ping command is used. Also point out that a way to check the function of DNS is to ping
the same destination by domain name and by IP address. If the remote location responds to
the IP address but not to the domain name then this indicates a DNS issue. Explain that a
location may be unreachable because of security restrictions. ICMP may be a blocked
protocol.
8.1.6 Detecting excessively long routes
Situations in a network can occur where datagrams travel in a circle and never reach their
destination. This could occur because no path exists between a source and a destination that
conforms to the requirements of the routing protocol. This could be caused by incorrect routing
information. Explain that paths with too many hops and circular paths create an excessively
long route. The packet will eventually reach the end of its life, known as time to live (TTL). The
TTL is not related to the hop count value of RIP. RIP advertisements are broadcast. That
means they will not go farther than the local segment. The reachability of RIP is controlled by
the routing protocol. It maintains a hop count metric that cannot exceed 15. This means that a
route will not be advertised further than 15 hops. It does not mean that packets cannot travel
more than 15 hops. The process is as follows:
1. As each router processes the datagram, the TTL value decreases by one.
2. When the TTL value reaches zero, the packet is discarded.
95 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 8
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