Summary Post – OSPF Network Statement Order and Matching

When you configure OSPF network statements, IOS orders them most-specific to least-specific then does a top-to-bottom match of the interfaces. It doesn’t matter which order you put them in, the configuration will always be ordered with the longest prefix matches first.  Lab time!

I have router R1 with these interfaces.

Let’s add the OSPF configuration where 10.0.0.0/8 is in area 2 then check what OSPF thinks is happening.

All the interfaces are in area 2 as expected. Now let’s add 10.0.0.0/16 into area 1 to see what happens.

Alright. Lo200 stayed in area 2, but the other two interfaces moved to area 1. You can see that the network statement for area 1 was inserted above the area 2 config even though I configured it later.

I also got this interesting log message when I configured the new network statement. I thought it was worth mentioning.

One more to grow on?  10.0.0.0/24 goes into area 0.

Yes, I got the message again when F0/0 moved to area 0.

Send any link state table updates questions my way.

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages.

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