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Share a subnet across L3 boundaries?

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Oct 4th, 2006
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We have two campuses which are currently connected @ L2 we would like to convert them over to L3 but would like to share a common subnet between the 2 campuses. We would be using OSPF to connect the 2 campuses w/ 6509's running IOS...Is this possible???

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glen.grant Wed, 10/04/2006 - 17:00
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Not if want to route it , you would have to chop the subnet into smaller pieces with a diffrent mask . It's possible you could do something with bridgeing but have never tried it .

Richard Burts Wed, 10/04/2006 - 18:09
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When you describe your current situation as two campuses connected at layer 2, this means that they share a common address space (subnet/network) and that any end station can ARP for any other end station (in either location) and receive a response from the destination end station. There are certain advantages to this (easy to manage) and certain disadvantages (size of the broadcast domain).

When you talk about changing the connection to a layer 3 connection and running OSPF this means that you are changing several things: there now will be separate broadcast domains (one of the advantages of separating the locations), that each location will have its own subnet/network, that broadcasts from one location will not be forwarded to the other location, and that an end station can ARP successfully only for other end stations in its own location. Part of the definition of a layer 3 connection is that you have different subnets in each location. The only way that I see to share a subnet is to maintain your layer 2 connection.

Glen describes a possible solution with bridging but I suspect that this is really what you have in your current environment.

It looks to me like you must make a choice: keep layer 2 and share a subnet, or convert to layer 3 and have separate subnets.




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