Network design question / discontiguous subnets.

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Oct 10th, 2007
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Hello all. I am in the process of designing a network infrastructure for a DR scenario.

What I need to do is move some existing CIDR (/24) subnets to a new site and make sure all routing (connectivity for management from a third site) is correct.

Essentially I will be creating discontiguous subnets.

As an example:

Currently one 10.A.X.X/16 network with multiple /24 subnets exist in one location.

After the move the new DR location will contain 10.A.10.X/24, 10.A.11.X/24, 10.A.12.x/24 subnets and so on, for a total of about 40 subnets.

The existing location will still contain the rest of the subnets.

Right now the routing is static, I route based on the 10.A.X.X/16 address. And I want to avoid to have to create routes for every subnet that has moved to the new site and for the subnets that have remained at the old site.

I think the EIGRP would help in this situation. And I know that I can specify the EIGP not do subnet summarization...

Anyone had a similar scenario and what is the best way to simplify the routing situation.

Thank you very much.


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lgijssel Wed, 10/10/2007 - 10:13
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You could use EIGRP but this can also be done with static routes. First of all, I would like to point to the URL below about how a routing destination is chosen:

If possible, you should use the subnets 16 - 55, or at least start from any other power of two or a multiple of it. This reduces the amount of routes required.

The route to the remainder is: 10.A.x.x /16

For the DR, you could then configure something like:






dtochilovsky Wed, 10/10/2007 - 10:28
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Thank you Leo, but the problem I have is most of the subnets are already existing and I need to move them as is and not create new subnets.

I thought about creating some sort of summarization along the lines of what you suggest but the subnets are scattered and it would be hard to summarize them all.

But thank you for your suggestion.

lgijssel Wed, 10/10/2007 - 10:36
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With static routes, you need to add remove the routes as needed, that's extra work indeed.

With EIGRP, you must make sure that the networks are removed from the network (to keep EIGRP from advertising them) at the time of their move-over. That's extra work as well and it will be harder to troubleshoot any problems that may occur.

I am an absolute fan of using routing protocols wherever possible but I am not sure if it would be the right solution here. It's your decision.

goodluck with the job!



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