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Chapter 5
Applying Cisco's Diagnostic Tools
Router_C>show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 172.16.60.130 1 00a0.24a5.0657 SNAP Ethernet0/1
Internet 172.16.60.1 - 0010.7bd9.2881 ARPA Ethernet0/1
Internet 172.16.50.2 - 0010.7bd9.2880 ARPA Ethernet0/0
Internet 172.16.50.1 111 0000.0c09.99cc ARPA Ethernet0/0
Router_C>
Wait a minute! Host Z is listed in the table now. How did that happen?
You must remember that although Cisco understands several different
encapsulation types, its default is ARPA. When the router sent the ARP
request, it was sent using ARPA. Host Z does not understand ARPA, and so
it did not respond to the ARP request.
The process works differently on a Cisco router. When Host Z sent an
ARP broadcast, it was sent with SNAP encapsulation. The difference is that
the router understood the broadcast, recorded the encapsulation type, and
entered it into the router's ARP table, as seen previously. The type allows the
router to override the default encapsulation. Now, when the router needs to
send a frame to Host Z, it uses SNAP encapsulation. Let's test it:
Router_C#ping 172.16.60.130
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.60.130, timeout
is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max
= 4/4/8 ms
Router_C#
It worked great because the router now knows which encapsulation type
must be used when communicating with Host Z. This problem has been
resolved temporarily. To solve it permanently, you must manually change
the encapsulation type used for the interface that Host Z connects to. Now,
let's move on to the next one.
Scenario #2
This is another simple Ethernet problem. Using the example network
depicted in Figure 5.5, you will attempt to solve a less tangible network
problem. The user at Host A complains of very slow throughput to Host Z.
He is able to ping and Traceroute to the destination, but file transfers expe-
rience very slow transfer times.
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