Comparing Config Differences

When I started as a network engineer, whenever I configured something and forgot what it was, I usually do "show start" and "show run" to compare the difference manually before I do a "write mem". Believe me, it was an eyesore comparing the start-up and running-config line by line. There is a easy way though to compare the configs and the command is "show archive config differences" for the router to compare the start-up and the running-config. Check the output below.

Router#show archive config differences
Contextual Config Diffs:
+no aaa new-model
interface FastEthernet0/0
+description to LAN
+ip ospf cost 100
+duplex half
+mpls ip
-aaa new-model
-aaa session-id common
interface FastEthernet0/0
-description to Building 2
-ip ospf cost 800
-duplex full

The commands prepended with a "+" means that this commands are in the "start-up config". The ones with "-" are in the running config. Of course when you do a "write mem" after this and you issue the command, you won't be seing any differences :). Good day!

Show Command Multiple Filtering

Normally when we do show command we make use of the "|" to filter and put in keywords after like include, exclude, begin and section. As we all know "include" means show only that matches the string like for the example below.

R1#sh run | inc CISCO
 neighbor CISCO peer-group

We can do some multiple command filtering like the example below using the "include" keyword. Let's say we want to see the interface name, then the description, the OSPF cost and if its configured with the "mpls ip" command.

R1#sh run | inc interface |^ description |^ ip ospf cost |^ mpls ip
interface FastEthernet0/0
description towards LAN
ip ospf cost 100
mpls ip

The trick is to use multiple "|" and then the regular expression "^". Then put a space before the string because the configurations under the interface configuration if you do a "show run" has a space before the line. This also applies to the "exclude" keyword but who the heck uses "exclude" that much? There goes another stupid blog entry post. :)

Simple TCL Ping Script

Doing ping tests for lots of IP addresses can be tiring since you can't paste all the ping commands at the same time. You have to do it one at a time. All you need is Patience or you can opt for a TCL scripting language which is already available in Cisco IOS 12.2(25). At first, I thought you need to learn TCL scripting to be a CCIE but nah! :) All you need is the basics and if you are interested learning more about TCL you can click here. To access the tcl command line in Cisco router issue the "tclsh" command. For the ping script just modify the ip addresses what is shown below and it should be good.

foreach address {
1.1.1.1
2.2.2.2
3.3.3.3
4.4.4.4
5.5.5.5
6.6.6.6
} { puts [ exec "ping $address" ] }

The word "address" here is just a variable, you can substitute this with anything you want. Now lets try applying this to the Cisco router.

R0#tclsh
R0(tcl)#foreach address {
+>(tcl)#1.1.1.1
+>(tcl)#2.2.2.2
+>(tcl)#3.3.3.3
+>(tcl)#4.4.4.4
+>(tcl)#5.5.5.5
+>(tcl)#6.6.6.6
+>(tcl)#} { puts [ exec "ping $address" ] }

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/44/100 ms

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/27/56 ms

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/26/96 ms

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 4.4.4.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/27/52 ms

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/27/96 ms

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 6.6.6.6, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/32/84 ms

How cool is that!!! This script can be useful when you are doing your CCIE lab exams as they say, when you want to verify that every subnet in your lab routers are reachable. You can try this in your own Dynamips lab. This should be safe to be done in the production routers but where I work, I doubt if this will be allowed by the company's IT policy. :P