Clarification on OSPF routing, two sites with same networks

Answered Question
Jun 23rd, 2009

Hi, can you review this simple sketch and give me your feedback?

Please see attached diagram for details. I also pasted question below.

Note that I have Vlan 5 network range on both sites. No IP address duplication exists.

Question:

Do you see any problem routing traffic from UserA to UserB?

I think it is strange that I am advertising same OSPF networks

On different sites…

Is routing going to interpret this as overlapping networks?

I have this problem too.
0 votes
Correct Answer by Giuseppe Larosa about 7 years 5 months ago

Hello Marlon, Mink

unfortunately 3750 don't support NAT the only multilayer switch that supports it is C6500.

I see your WAN links are GE ports you need to get a L2 service from your provider so that you can extend the vlan over it

the wan links can become L2 trunks carrying the shared vlan and backbone vlan(s) moving L3 config to SVI interfaces

802.1Q tunneling or EoMPLS can be the solutions for the provider side.

Hope to help

Giuseppe

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Dennis Mink Tue, 06/23/2009 - 19:08

By looking at your visio you have the same subnet at both locations. both locations have the same vlan and subnet, yet are devided by Layer 3 boundaries. no way this will work.

few options:

-run a layer 2 connection between teh 2 sites so you do not need to route

-tell users to bite the bullet and change IP addresses

-perform NAT (on your 3750) on either one of the two sides.

Correct Answer
Giuseppe Larosa Tue, 06/23/2009 - 23:27

Hello Marlon, Mink

unfortunately 3750 don't support NAT the only multilayer switch that supports it is C6500.

I see your WAN links are GE ports you need to get a L2 service from your provider so that you can extend the vlan over it

the wan links can become L2 trunks carrying the shared vlan and backbone vlan(s) moving L3 config to SVI interfaces

802.1Q tunneling or EoMPLS can be the solutions for the provider side.

Hope to help

Giuseppe

news2010a Wed, 06/24/2009 - 07:45

In this case, to do 802.1Q and EoMPLS I would need participation from the ISP. Given recent political issues that is not an option.

It sounds like I will have to ask people to change IP - or else just bridge the whole thing then.

Thanks much.

Tony.henry Tue, 06/23/2009 - 20:11

News,

Will be a big problem as other poster said won't work without a layer 2 connection of some kind coming into play.

Since readressing seems to be out of the question according to your diagram.

Another possible solution may be to run NAT out of one of the routers. The user won't have to readdress their network. and they can continue to work. the remainder of the network will see they have got a new address, just not the end user.....

Is it worth the hassle is another question you'll have to answer between yourself and your manager. I would see considerable overhead in doing that.

Tony

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