Cisco CCNA Exam Training: The Routing Process, Ping, And Traceroute

By Chris Bryant, CCIE #12933

When a PC wants to transmit data, it will first check to see if the destination IP address is on the local network.  (By “local”, we mean “directly connected”).  If the address isn’t on the directly connected network(s), the host will use its forward the packet to its default gateway.

In this example, the host wants to send data to a device at IP address 20.1.1.2.  The host is attached to only one network, 10.0.0.0 /8.  The host will then send the data to its default gateway, which in this case happens to be a Cisco router. When this datagram is sent by the host, the destination IP address will be that of the host at 20.1.1.2.  However, the destination MAC address will be that of the router interface closest to the sending host.

The router receives the datagram, “unpacks” it, and sends it to the routing process.  The router will then match the IP destination address against its routing table.

Using PING And TRACEROUTE To Test Network Connectivity

ICMP (Internet Control Message Protocol) offers two network connectivity testing functions.  The ping command sends five ICMP packets to the intended destination.  Ping will then return one of several symbols to indicate whether IP connectivity exists, and if not, a clue to what the issue is.  The ping is an ICMP Echo Request; the ping reply is an ICMP Echo Reply.

Consider our earlier example.  If the host on network 10.0.0.0 /8 has trouble reaching the host on 20.0.0.0 /8, ping helps diagnose the problem. 

The two general replies from a ping are “!!!!!” and “…..”.  The exclamation points indicate IP connectivity exists between the hosts; five periods indicate that the ping packets were unable to find the destination host.

The second ICMP tool you should be familiar with is traceroute.  Where ping simply tells you whether IP connectivity is or is not present, traceroute displays the path the packet takes to reach its destination.  Traceroute works by sending out ICMP packets with a Time To Live (TTL) of one to find the first step to the final destination.  It will then send a packet with a TTL of two, and the TTL value increases until the final destination is found.  When that happens, the entire path will be displayed on your console.If the final destination cannot be found, asterisks will start to appear on your screen.  This is a sort of error message – it means the TTL has been exceeded.

To abort a ping or traceroute in progress, press <CTRL-SHIFT-6> twice in rapid succession.

A great “1-2-3” method to troubleshoot routing problems:

1. Ping the destination.

2. If the pings don’t come back, run show ip route to make sure your local device has a route to the destination.

3. If a route exists, use traceroute to determine what downstream router can’t route the packet.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com