Spanning single subnet on single OSPF area over multiple routers

[yong khang NG]

Hi all

Would like to get some comment on my propose logical network design on OSPF

Basically, it's using single area OSPF but span across 3 router.

Condition is this 10.10.0.0 / 22 network should span over 3 router, and all end user PC behind router B and router C able to reach APP Server 10.10.0.10

Question

01. should i write a specific static route on router B and router C make sure there's connection reach App server at 10.10.0.10? Then redistribute the static route to OSPF.

02. As i know, OSPF and VLANs are independent. So segment 10.10.0.0 /22 span over the routers' with different VLAN ID wouldn't cause any problem?

03. If i not like to advertise network 11.11.0.0/24, so it is able to let whole network locate is by adding static route on every router?

Router A OSPF

redistribute connected subnets

redistribute static subnets

Router B OSPF

redistribute static subnets

Router C OSPF

redistribute static subnets

[Masoud Pourshabanian]

Hello,

Are those switches layer2 or layer3?

If your switches are layer 2,  you do not need to set any distribute commands. Just put the network under OSPF configuration section.

Router OSPF 1

network 10.10.0.0 0.0.X.X   [interface ip]

If your switches are layer 3 and are not part of OSPF, you need to set static route to those switch, then distribute those static route into OSPF.

B. OSPF and vlan are not related as you said. OSPF will distribute all routes(specified for distribution) regardless of vlan number.

C: I did not understand you third question. If you do not advertise a route, you need to have static route.

Masoud

[yong khang NG]

Hi Masoud,

Thanks for the reply. Forgive for my statement that mislead on you.

Back to questions

The switch is Layer3 switch, C6500. Some of the interface was set as routed port and participate on the OSPF dynamic routing.

My question is very simple, i just want to span network 10.10.0.0/22 across the 3 routers, within single OSPF area0. And behind each router, there's a Layer 3 which might segerate this network using different VLAN ID.

Is it feasible for this to work?

Million thanks

Noel

[Masoud Pourshabanian]

Yes, it works.

If your layer 3 switch is not part of OSPF, you need to have static route toward your switch in your router. Distribute those static routes in your router.

If OSPF is set in your layer 3 switches, you do not need to set any distribution. Your switches will propagate routes to the network.

Masoud

[Jon Marshall]

Based on your diagram not it won't work because you are trying to use the same subnet mask at three sites separated by a L3 WAN.

Jon

[Masoud Pourshabanian]

Depending on the type of service you get from your service provider, it is possible wan routers use the same subnet mask.
 

[Jon Marshall]

The only way that would work is if the WAN was VPLS for example but there is no mention of that and the diagram clearly shows a different IP subnet in use on the WAN.

You cannot route to the same network ie. each site will think all the IPs are local and they would never be routed across the WAN.

You could possibly do something to extend the vlans at each site but the OP makes no mention of that although perhaps he hasn't included that information.

Jon

[Masoud Pourshabanian]

I think it was not concern of Youn because he was familiar with IP subnneting. And you can not exactly define the WAN IP address before you get service from your service provide. I think there are enough IP addresses for WAN and can be split if need. However, you are right if we want to be very accurate.

[Jon Marshall]

No problem.

I am not trying to prove anyone wrong here, I just looked at the diagram and it clearly shows each site using an SVI with the same IP subnet and subnet mask which just won't work unless there is something the OP has not mentioned.

Just didn't want to give the impression it would work as in the diagram and then when it came to implementation it didn't.

Jon