Understanding order of processing a router command

[dan_track]

Hi

I'm curious as to the order of processing when running through the configuration in router mode. i.e:

ip route 10.4.1.0 255.255.255.0 10.5.66.10

interface Serial 0/0

ip summary-address eigrp 100 10.1.2.0 255.255.254.0

router eigrp 100

no auto-summary

redistribute static 1000 1 255 1 1500

network 10.1.0.0 0.0.0.3

network 10.1.10.0 0.0.0.255

eigrp static

exit

In the above scenario when processing the information does the IOS take a top down approach i.e execute the "no auto-summary" command first then the "redistribute" etc..?

If there's any confusion please let me know.

Thanks

Dan

[viyuan700]

i dont think IOS willbe taken a top down approach like Access list.

as per my knowledge when router has to take a decision he refers to the that particular part of config.

In ur config u have defined manual summary where as the no auto summary is after that.

[dan_track]

Hi,

IS this for definite. As a summary then what would this router advertise to teh Hub?

Thanks

Dan

[Giuseppe Larosa]

Hello Dan,

the order in which commands are shown follow some standards so the commands inside a context like router eigrp 100 are not in the order you entered them.

The order of complexity behind a single line can be of several hundreds of line code if you think what happens for a redistribute command.

The router has a process called parser that will organize the commands in a way that is suitable for calling all the IOS subroutines that are involved in the correct order.

But that is different from the configuration file that is a text file for human readers.

These aspects are partially hidden to router administrators and we have to follow configuration standards and guidelines.

For example:

the interface command ip summary-address eigrp 100 10.1.2.0 255.255.254.0 triggers a communication with the EIGRP process that has to take in account this when preparing routing updates to be sent out interface ser0/0, but it is EIGRP not the interface that will decide if the summary has to be sent out ser0/0.

To use successfully manual summarization the auto-summary has to be turned off this is an example of dependency of commands and here are the difficulties of router configuration.

Hope to help

Giuseppe

[Nicholas Matthews]

Hi Dan,

There's usually an easy way to test this - take the commands out and put them back in with a different order. You'll see that for nearly all configuration, it will be printed out in IOS in the same order each time.

For most routing protocols and other configuration that is in a 'section' of configuration, there is a set order that it is stored in the IOS configuration.

Things that the order matters for:

ACLs

Route-maps

Policy-map class statements

Prefix lists

Anything with a sequence number (which includes most of the above)

dial peers (voice)

There are others too, but they become more vague.

Either way - the best way to find out is to test it. You'll learn more that way anyways.