Using the show Command
Obviously, show running-config would be the best way to verify your configuration, and
show startup-config would be the best way to verify the configuration that will be used the
next time the router is reloaded, right?
Table 1.12 shows some basic command you might use every day in a production environment.
TABLE 1 . 1 2 Everyday Commands
Command Meaning
show running config This shows the configuration that router is using.
show startup-config This shows the configuration that the router will use when the
router is reload.
show interface This shows the status of all interfaces.
show ip interface This shows the default IP configuration on all interfaces.
show ip interface brief This command provides a quick overview of the router’s
interfaces, including the logical address and status.
show protocols This shows the status of layers 1 and 2 of each interface as well
as the IP addresses used.
show controllers This shows whether an interface is a DCE or DTE interface.
The show running-config command is important and could very well be one of the most
used commands on a Cisco router. The show running-config command shows the configuration
that the router is running. The show startup-config shows the backup config, or
the configuration that will be used the next time the router is booted.
The show interface command is important, and that’s what I’ll discuss in this section.
Here’s what it looks like:
Router#sh int s0/0/0
Serial0/0 is up, line protocol is down
If you see that the line is up but the protocol is down, as shown earlier, you’re experiencing
a clocking (keepalive) or framing problem—possibly an encapsulation mismatch. Check the
keepalives on both ends to make sure that they match, that the clock rate is set (if needed), and
that the encapsulation type is the same on both ends. The previous output would be considered
a Data Link layer problem.
If you discover that both the line interface and the protocol are down, it’s a cable or interface
problem. The following output would be considered a physical layer problem:
Router#sh int s0/0/0
Serial0/0 is down, line protocol is down
If one end is administratively shut down (as shown next), the remote end would present as
down and down:
Router#sh int s0/0/0
Serial0/0 is administratively down, line protocol is down
To enable the interface, use the command no shutdown from interface configuration mode:
Router#config t
Router(config)#int s0/0/0
Router(config-if)#no shutdown
You can get a nice brief overview of the interfaces with the show interface brief
command:
Router#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset up up
FastEthernet0/1 unassigned YES unset up up
Serial0/0/0 unassigned YES unset up down
Serial0/0/1 unassigned YES unset up up
Serial0/1/0 unassigned YES unset administratively down down
Todd#sh protocols
Global values:
Internet Protocol routing is enabled
FastEthernet0/0 is up, line protocol is up
Internet address is 192.168.21.1/24
FastEthernet0/1 is administratively down, line protocol is down
Serial0/0/0 is up, line protocol is up
Internet address is 172.16.10.1/24
Serial0/0/1 is administratively down, line protocol is down
Todd#
The show controllers command displays information about the physical interface itself.
It’ll also give you the type of serial cable plugged into a serial port. Usually, this will be only
a DTE cable that plugs into a type of data service unit (DSU).
Router#sh controllers serial 0/0
HD unit 0, idb = 0x1229E4, driver structure at 0x127E70
buffer size 1524 HD unit 0, V.35 DTE cable
cpb = 0xE2, eda = 0x4140, cda = 0x4000
Router#sh controllers serial 0/1
HD unit 1, idb = 0x12C174, driver structure at 0x131600
buffer size 1524 HD unit 1, V.35 DCE cable
cpb = 0xE3, eda = 0x2940, cda = 0x2800