Learning the particulars of Cisco IOS is one of the most valuable things a network engineer can do. These skills will be the basis of everything you do on the lab and on your network. The following is a collection of things that most everyone uses these days.

Prevent timeout on the serial console. You can also apply this to the VTY lines if you ssh / telnet into the router.

Router(config)#line con 0
Router(config-line)#exec-timeout 0 0         !  0-minutes 0-seconds

The following configuration will allow you to work uninterrupted from system messages.

Router(config)#line con 0
Router(config-line)#logging synchronous

This command will reset an interface, or a range of interfaces back to its default configuration.

Router(config)#default interface fa0/0
Building configuration...

The Cisco default break character of Ctrl+Shift+6 is a pain to type, it takes both hands, and its just plain stupid. Changing this to something easy and quick is a life saver during any lab or in real life. There are two ways to actually make the change thought. The first way demonstrated only applies to the current session. To make the change more permanent you need to configure the escape character on the line.

Router#terminal escape-character 23          ! 23 = Ctrl+W
"^W" is the escape character                 

Router(config)#line con 0
Router(config-line)#escape-character 27       ! 27 = Escape

This is seriously handy for those of us that type faster than we can think at times. Ever been stuck at a Translating “whatever”…domain server (4.2.2.2) ?? The following command will force your router skip the DNS resolution.

Router(config)#no ip domain-lookup

This tip is in 2 parts, first is the do command which allows you to run exec level commands from configuration mode. This is a very handy feature for issuing show commands. The second tip here is combining show ip interface brief with the IOS filtering options and removing lines that contain unassigned interfaces.

Router(config)#do show ip int br | ex una
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            192.168.8.9     YES NVRAM  up                    up
FastEthernet0/0.2          10.1.8.1        YES NVRAM  up                    up

This command will show you the top 5 protocols as reportd by NBAR on the router. You will need to configure ip nbar protocol-discovery on interfaces that you want to see statistics for.

Router#show ip nbar protocol-discovery stats bit-rate top-n 5
 Serial1/0
                              Input                    Output
                              -----                    ------
   Protocol                  5min Bit Rate (bps)      5min Bit Rate (bps)
   ------------------------ ------------------------ ------------------------
   ospf                        536                     345
   icmp                        234                     535
   bgp                          23                      34
   citrix                        0                       0
   cuseeme                       0                       0
   unknown                       0                       0
   Total                         0                       0

How about using extended ping without having to enter the extended ping menu? The so option refers to source, which can also include an interfaces. Other useful options are re or repeat, si or packet size, and df or df-bit which enables the do not fragment bit in the IP header.

Router#ping 10.0.1.2 so 10.0.0.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.1.2, timeout is 2 seconds:
Packet sent with a source address of 10.0.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/12/20 ms

This command will show a sorted CPU usage of all processes running on the router. You can also use show proc cpu history to view an ASCII graphs of CPU utilization.

It may also be usefull to create an alias with filtering. alias exec shproc show proc cpu sort | ex 0.00%__0.00%__0.00%

Router#show processes cpu sort
CPU utilization for five seconds: 1%/1%; one minute: 2%; five minutes: 3%
 PID Runtime(ms)   Invoked      uSecs   5Sec   1Min   5Min TTY Process
   1          40       167        239  0.00%  0.00%  0.00%   0 Chunk Manager
   2        2682    711979          3  0.00%  0.00%  0.00%   0 Load Meter
   3        4846  11865575          0  0.00%  0.00%  0.00%   0 Skinny Msg Serve
   4          16    142564          0  0.00%  0.00%  0.00%   0 EDDRI_MAIN
   5     7707600    542302      14212  0.00%  0.23%  0.19%   0 Check heaps
   6           4         2       2000  0.00%  0.00%  0.00%   0 Pool Manager
   7           0         2          0  0.00%  0.00%  0.00%   0 Timers
   8           0         1          0  0.00%  0.00%  0.00%   0 Crash writer
   9      387639    930516        416  0.00%  0.00%  0.00%   0 ARP Input
  10         460   3706748          0  0.00%  0.00%  0.00%   0 ARP Background
  11           0         2          0  0.00%  0.00%  0.00%   0 ATM Idle Timer
  12           0         2          0  0.00%  0.00%  0.00%   0 AAA high-capacit
  13           0         1          0  0.00%  0.00%  0.00%   0 AAA_SERVER_DEADT
  14           0         1          0  0.00%  0.00%  0.00%   0 Policy Manager
  15           0         2          0  0.00%  0.00%  0.00%   0 DDR Timers
  16          20         2      10000  0.00%  0.00%  0.00%   0 Entity MIB API
  17          28        53        528  0.00%  0.00%  0.00%   0 EEM ED Syslog
  18         220   1065579          0  0.00%  0.00%  0.00%   0 HC Counter Timer
  19           0         2          0  0.00%  0.00%  0.00%   0 Serial Backgroun
  20           0         1          0  0.00%  0.00%  0.00%   0 RO Notify Timers
  21           0         1          0  0.00%  0.00%  0.00%   0 RMI RM Notify Wa
  22           8         2       4000  0.00%  0.00%  0.00%   0 SMART

The following command allows you to reload your router in a set time period. Rather useful for failsafe recovery while making configuration changes in some circumstances. You can view the status of a reload using the command show reload or cancel it using reload cancel.

Router#reload in 5
Reload scheduled for 16:31:15 EDT Fri Jun 8 2010 (in 5 minutes) by admin on vty0 (198.19.8.98)
Reload reason: Reload Command
Proceed with reload? [confirm]

The following tip is from @WannabeCCIE – When you’re labbing and have confirm IP connectivity to 20+ devices, TCL can save your butt. Keep this in a text file and paste it into the CLI.

Router#tclsh
Router(tcl)#foreach IPA {
+>150.100.12.1
+>150.100.12.2
+>150.100.24.2
+>150.100.24.4
+>150.100.41.4
+>150.100.40.4
+>} {ping $IPA timeout 1}
!
! or for a list of interfaces off different VRFs
!
Router#tclsh
Router(tcl)#foreach VRF {
+>"RED 100.100.100.2"
+>"RED 100.100.100.5"
+>"RED 100.100.100.6"
+>"BLUE 200.200.200.2"
+>"BLUE 200.200.200.5"
+>"BLUE 200.200.200.6"
+>} {ping vrf $VRF timeout 1}

The following command is rather useful for diagnosing BGP issues, it shows what is causing any RIB Failures preventing any entries in the BGP table from being installed into the main routing table.

Router#show ip bgp rib-failure
Network            Next Hop                      RIB-failure   RIB-NH Matches
172.16.20.0/24    172.16.20.1        Higher admin distance              n/a

If you have any tips that I may of left out or have any questions, please leave a comment below.

Who writes this crap?

Tony Mattke is a network engineer for a financial institution in Indiana. In the past he has worked for ISPs, data centers, networking manufactures, and the occasional enterprise. For feedback, please leave a comment on the article in question. For everything else including fan mail or death threats, contact him via twitter.

Tagged as: tips