Simple Scenarios

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On the surface, the Contextual Configuration Diff looks like a great idea – you give it two configuration files (they have to be complete and in the format generated by the show running-config command) and the show archive config differences start-configuration target-configuration displays the IOS configuration commands (both removals and additions) needed to change the first file into the second one. You can compare any two configuration files; most commonly, you would compare the running configuration with the startup one with the show archive config differences nvram:startup-config system:running-config command, but you could also compare configurations stored in flash memory or on a remote server accessible via a variety of protocols (FTP, HTTP, SSL, RCP, SCP, and TFTP):

Compare running config with a remote configuration file
fw#copy system:running-config tftp://10.0.0.2/fw-test
!!
2009 bytes copied in 0.592 secs (3394 bytes/sec)
fw#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
fw(config)#no access-list 100
fw(config)#access-list 120 permit ip any any
fw(config)#^Z
fw#show archive config differences tftp://10.0.0.2/fw-test system:running-config
Loading fw-test from 10.0.0.2 (via FastEthernet0/0): !
[OK - 2087 bytes]
Contextual Config Diffs:
+access-list 120 permit ip any any
-access-list 100 permit tcp any any eq www
-access-list 100 permit tcp any any eq telnet
  • If you specify just one configuration file parameter, Cisco IOS assumes that the parameter is the target configuration and displays the differences between the running-config and the target configuration (the opposite of what I would expect).
  • The show archive config differences command will not accept startup-config or running-config as filenames but requires full file path (nvram:startup-config and system:running-config).

Warm-up Tests

I started testing this feature with a few simple test cases. As expected, IOS generated correct differences in all of them:

Startup interface configuration
interface Loopback0
 ip address 10.1.0.1 255.255.255.255
Running interface configuration
interface Loopback0
 description New loopback interface
 ip address 10.1.0.3 255.255.255.255
Configuration difference after the changes made to the Loopback0 interface
config
Contextual Config Diffs:
interface Loopback0
 +description New loopback interface
 +ip address 10.1.0.3 255.255.255.255
interface Loopback0
 -ip address 10.1.0.1 255.255.255.255

Even more interesting, when you remove a complete configuration section, the router lists all previously configured commands in the differences, making it very easy to identify exactly which settings disappeared together with a major configuration object. For example, when I removed the OSPF process with the no router ospf 101 command, Cisco IOS reported these changes:

Reported changes after OSPF process removal
fw#show archive config differences nvram:startup-config system:running-config
Contextual Config Diffs:
-router ospf 101
 -log-adjacency-changes
 -network 10.1.0.0 0.0.1.255 area 2
 -network 0.0.0.0 255.255.255.255 area 0

Continue

Next: Order-Sensitive Configurations

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