Howmany routers for 19 subnets? or howmany ports.

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Apr 11th, 2007
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We are having a discussion about how to best setup our network. We have 19 subnets and are having some questions about it.

If we choose for the 3845 router will it support up to 19 (some more is ok to be on the safe side). subnets or do we need to buy more then one router? What modules do we need to provide routing for all the 19 subnets and won't it be slow if we connect 19 subnets to one router?

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paolo bevilacqua Wed, 04/11/2007 - 05:57
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which kind of subnets are these ? WAN or LAN?

If the latter, today is better to use layer3 switches for routing at wire speed and no CPU impact.

Consider that today a good L3 swtich like 3560 - 8 ports costs about USD 1000.

Of course if it is WAN you need a real router, depending on the speed of your circuits and the features applies you can in many cases use even smaller models than the 3845.

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jarco5000 Wed, 04/11/2007 - 06:12
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This is aqtually a lan but the use of a router is not optional.

There is also a lot of fiber used in this network. What type of router is recommended for a network of this size with high bandwith requirements?

And also it should support voip

paolo bevilacqua Wed, 04/11/2007 - 07:03
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As i said I would recommend a layer3 switch to route your LAN subnets. Specifically a 3560 or 3750, there are so many models to choose from. Some models are all-SFP so you can connect as much fiber you want.

The 38xx and 28xx ISR are great routers but are not layer3 switch in itself, although they can host some network module (NME-X-xxx) that are true layer 3 switch. In fact, one of them is a true 3750!

If you do that, you end with a very very powerful and flexible box that is a true IOS router, plus L3 switch, plus VoIP gateway / PBX, in short, it does everything!

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jarco5000 Wed, 04/11/2007 - 07:15
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is their a website or tool available to help me make the dissision to what module i need or to what modules are available with a little axplanation?

Craig Balfour Wed, 04/11/2007 - 07:04
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The Cisco 3845 router supports a wide range of network modules including ethernet switching network modules with a wide range of port options. For more information see

There is no reason why you cannot route all 19 (and more) subnets on the same router.

Your main issue is physical connectivity. If you have 19 VLANs and each is a different building and you have a cable coming from each building that needs to plug in somewhere you will need 19 physical ports. These physical ports can be:

a) in the router if you purchase and install a network switching module with sufficient ports; or

b) on a layer 2 switch with the VLAN's trunked up to and routed on the router. This configuration is called a router-on-a-stick configuration and is quite flexible.

Obviously, in terms of throughput, dedicating a port per VLAN will give you much better throughput than routing all of your VLAN's through a single router port in a router-on-stick configuration but if buying the required number of physical ports for your router proves too expensive you can create a hybrid solution by routing say 2 or 3 vlans per router port and buying a network switching module with half or a third of the required physical ports.

cdunmood Wed, 04/11/2007 - 10:36
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The previous solution is good, a couple questions that I have, are there any remote locations included within your 19 subnets or are all 19 subets local to one location?

Are you running GE Fiber? To ISP, Corporate Site, etc..?

Running router on a stick is not a bad idea for a cost effective solution. You would have to run a second router for redundancy.

jarco5000 Sun, 04/15/2007 - 23:46
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the problem is that we are required to use one router only. (router on a stick is not an option). I looked at the modules for the 3845 router but i cant figure out how i will do routing with fiber and utp. we need at least 4 (prefer 8 ) utp ports and 19 fiber ports.

Can someone plz do a suggestion about what modules and what router to use?

paolo bevilacqua Mon, 04/16/2007 - 04:17
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What about 2 x Catalyst 3750G-12S.

This will give you 24 SFP ports, by virtue of that which you can choose the media that you want.

You can then add more switches with the backbone cable @32 Gb/sec. All the stacked devices will appear as a single unit for IP addressing and management.

Above that there is the 4500 and 6500 switches with the respective all-SFP line cards. These are of an upper class devices.

Hope this helps, if so please rate post!

cdunmood Mon, 04/16/2007 - 05:26
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You may need a L3 switching solution with a routing fabric at the core running dual sups. This will give you your subnets, your fiber, and your port denstity and redundancy.

You will need a WAN FLEX mod card to route traffic up to an isp or pop.

jarco5000 Mon, 04/16/2007 - 05:37
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i am forced to use a router in this scenario. (its required by the costumer). I think i am best of with a 7603 chassis with a 24 ports fiber module of the 6500 switch in it. (its compatible i think

paolo bevilacqua Mon, 04/16/2007 - 10:45
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Hi jarco,

the 7600 and the 6500 are the actually the same machine and run the same software. Just like the 45xx and the 3750, these are in fact L3 switches, that is, hardware-based routers, or how you want call them. The 7600 is marketed primarily as a router and support also WAN cards.

The 7600 / 6500 is a top class device. But also the 45xx and 3750 would work perfectly unless it is a service provider network core.

This can be a good chance to explain to the customer that the difference is not like 10 yrs ago, router and switch are actually kinda blurred into the same thing now.


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