What should i do for the routers to be ospf neighbors via switch?

[lnxtenko]

Hello! For a final project we have this part of the topology to work on. We have to set OSPF, network, router-id and all that stuff which I've done with success.

However, it is asked we remove the links between R14-R13-R9 and to connect them through a switch, I believe like this.

But when i do show ip ospf neighbor R9 and R13 dont detect R14, probably because of the different subnets? How can i fix it? R9 and R13 recognize each other as neighbors even though they connect via the Switch. Can anyone help? Thanks a lot!!

[MHM Cisco World]

The three routers must use same subnet in link connect to SW

MHM

[lnxtenko]

So should i just change the subnets arbitrarily even though R4 used 195.130.1.0/24 and 195.130.2.0/24 ?

[MHM Cisco World]

R9/13/14 link to SW must use same subnet like 192.130.1.0/28 <<- dont use /30 mask because it give you only two host IP and ypu need three host at least so use /29 or /28

MHM

[lnxtenko]

Sorry for the reply again, this would work even if i switched R4 to 195.130.3.3 instead of switching R13 and R9 both to 192.130.1.x right ?

[MHM Cisco World · Accepted solution]

No friend 

R4 Need also use 192.130.1.x .

MHM

[M02@rt37 · Accepted solution]

Hello @lnxtenko 

Since you plus R9,R14 and R13 on a swtich, it is no more P2P connection but a "Multi-access network" ; Multi access Networks are networks that consist of more than 2 devices sharing the same media.  So, connect R9,R13 and R14 each other trhough a single network space ; 192.30.1.0/24.

https://www.ccnablog.com/ospf-part-iv/

[lnxtenko]

Hi! It won't be considered arbitrary if I do that, considering R4 used 195.130.1.0/24 and 195.130.2.0/24 ? Thanks a lot for your reply!

[Torbjørn · Accepted solution]

You either need to put the interface of R14 into the same subnet as the others, or configure routing on the switch(I would probably use an SVI for adjacency to R13 and R9 and have a routed interface towards R14). You are correct in that OSPF will never establish an adjacency without a common subnet with it's neighbors.