Problem ospf

[blaucournet]

Hello,

I have a weird problem with my switch.

I have an ospf and I wanted to add 2 networks:

network 172.27.3.0 0.0.0.255 area 0.0.0.0
network 172.27.7.0 0.0.1.255 area 0.0.0.0

But in my run config is no write network 172.27.7.0 0.0.1.255 area 0.0.0.0 but network 172.27.6.0 0.0.1.255 area 0.0.0.0.
network 172.27.3.0 0.0.0.255 area 0.0.0.0 is good

I can remove network 172.27.6.0 0.0.1.255 area 0.0.0.0 with a "no" but when I write network 172.27.7.0 0.0.1.255 area 0.0.0.0, it come back.

I already tested :

remove network 172.27.6.0 0.0.1.255 area 0.0.0.0 and wr but my problem is still here
remove network 172.27.6.0 0.0.1.255 area 0.0.0.0 and reload the switch but my problem is still here

Thank you

[balaji.bandi]

If you looking to annouce 172.27.7.0/23 then that will change in to 172.27.6.0/23 only.

example :

All 128 of the Possible /23 Networks for 172.27.*.*

[blaucournet]

Hello,

So it's no possible to take a network in 172.27.7.0/23 ?

[MHM Cisco World]

when you config NET under ospf with summary prefix the router will not advertise the summary it will advertise the prefix of interface UP within the summary range, 
here you have link have 172.27.6.0
and hence the OSPF advertise it.
for summary you need area range command between Area (config in ABR) to advertise the range. 

[MHM Cisco World]

simple lab 
two router LO in R1 have 1.1.1.1/32 but under ospf I use 1.1.0.0/16 

we see the prefix appear in R2 as the Lo connect to R1 not as we enter under ospf 
the prefix is 1.1.1.1/32

[balaji.bandi]

I believe OP Looking  why he is not seeing 172.27.7.0/23 in the config when he configured the config in the OSPF network.

when he look show run  ( he only seeing 172.27.6.0/23  instead 172.27.7.0/23)

or am i missing here something ?

[MHM Cisco World]

let wait his reply, because config ospf should not change when show running but what ospf show route can different. 

[blaucournet]

It's no possible to take a network 127.72.7.0/23 because 7 it's odd ?

[Seb Rupik]

Hi there,

The behavior you are experiencing is the OSPF process only permits network statements which list a network ID and wildcard mask. Given 172.27.7.0 0.0.1.255 , the network ID of 172.27.7.0/23 is 172.27.6.0 . OSPF is making the correction for you.

It is not because the network octet is odd. You could for example use:

network 172.27.7.0 0.0.0.255 area 0

...which would remain unchanged in the running configuration.

cheers,

Seb.

[MHM Cisco World]

I repeat the same config in my lab, and same 
I add 127.72.7.0 0.0.1.255 
and the router automatic correct to to the network ID of the 127.73.6.0/23

so this is not issue with your config it default behave of router IOS.