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Need Advice/Assistance w/ Resegmenting a Network


I'm a new hire at my place of employment, in which I work at a branch office. Upon arriving, I was told that the network wasn't exactly running up to par with occasional hitches in the network. After doing some investigation, I found the that all the Cisco networking equipment is using VLAN 1 with multiple physical links between everything. Eventually, the connections go back to a C3640 router.

The C3640 router uses EIGRP to connect to other routers outside our location in which we also get our Internet connection. The main office has configured the router and I'm mainly responsible for everything internal to our branch. We have 4 subnets that we use internally:

176 - Datacenter

177 - Device Management

216 - Tech/Servers

229 - Users

The physical link to the router is defined as follows:

interface FastEthernet0/0

description LAN segment

ip address secondary

ip address secondary

ip address secondary

ip address secondary

I believe the problem is that all traffic, especially heavy traffic from the datacenter, has to go through this old router before reaching the other subnets. We have several gigabit capable switches here capable of routing. My idea is to move all the subnets to the C3750 & C4948 switches and allow the routing to be processed on those two switches. That leaves the C3640 to be only used for connections outside out network, such as to corporate or the Internet.

I want to resegment the network to 4 individual VLANs. I have a C3750 that I would like to be the main router for subnets 229 & 177 but would also provide a hub-spoke connectivity to all the other switches in the office. I also have a C4948 switch that I would like to be the router for subnets 176 & 216. If I am to do this, could I potentially just define everything via static routes or would you suggest using a routing protocol?

Any help would be appreciated.

21 Replies 21

You usually do have to specify a metric but not with redistribute static. However if you want to then the above is fine. As there is only one path to your site then it's not such a big deal what metric you specify.



Just letting you know I have yet to make the changes. I'm waiting on the main office to give me TACACS access to the router (I was using my manager's access).

In the meantime, we got our replacement router for the C3640 (a C3845). Long before I decided to move the subnets to the C3750 switch, my boss recommended that we go ahead and get a new router (C3845). We didn't expect to get the new router until mid-to-late November, but it arrived today.

The router was spec'd out to 512MB RAM & 256 MB Flash (compared to the C3640's 64 MB RAM, 32 MB Flash). Now that we got it, I'm curious about the design aspect of our branch. Since we have a new router that is fairly powerful, should I move the subnets back to the C3845 (as with the original design with the C3640 router) or should I still use the C3750 for internal routing and use the new router specifically for outside access.

Any opinions would be appreciated.

A 3845 is rated at 500 Kpps for minimum size packets (good for about 336 Mbps, Ethernet, 64 byte sized packets). I.e. 3845 should support 100 Mbps line-rate, duplex, but unable to guarantee single gig line-rate.

Both your 3750 and 4948 switch support line-rate all ports, so if there's more than about 100 Mbps LAN routing, you would want it on the L3 switches.

Thanks Joseph. Much appreciated. I shall utilize the C3750 and still use the routers for routing out of office purposes.

Joseph W. Doherty
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Hall of Fame Master

From what you've described, indeed it makes a whole lot of sense to get LAN routing off the 3640 and onto one or both of your L3 switches.

As to which L3 switch to select for routing (or both?) the 4848 is a wire-speed device, the 3750 models often not (although the 12 port fiber version you have is - it also supports special SDM templates). As both your switches support wire rate, and assuming most routing will be between server subnets and user subnets, it wouldn't make much of a difference which switch you route on but if there's also much traffic between the data center subnet and tech/server subnets, and these all will be connected to the 4948, it would make sense to route there (to avoid needlessly passing traffic up/down to the 3750).

Whether to use both L3 switches as routers would only make sense if there much traffic to route between subnets that naturally converge on the 3750. From what you describe, it doesn't seem like there would be much need to route on the 3750 (this assuming most traffic is from downlinked user switches to the port that connects to the 4948).

From later posts, unclear how you would use HSRP without some additional links between switches. If you do have those, then having both L3 switches active while using HSRP makes sense, but this also assumes you have sufficient ports to deal with lost of either the 3750 or 4948.

A good next step might be a dual stack of 3750 12 port SFP models and somewhere to connect the 4948 connected devices if it fails.

Hey Jose, I've been reading this thread and at the end where you discuss using L3 switches as routers; I was wondering if you could advise me as well...HERE's MY QUESTION >>

I need to determine if I should go with a (new) L2 switch and mult VLANS and use router-on-a-stick for intervlan routing. Its conducive to what the cust has at his multiple sites.

Or should I convince him to go with a L3 switch to the router instead ?

My concern is that its a mobile wireless site with two different radio signals (CDMA and GSM) operating in the same site...and I don't want a bottleneck on the trunk between the switch and the router when intervlan routing using the L2 switch. That is, one of the vlans consuming all the 10/100 trunk bandwidth would not work in the L2 switch env - or is it ?

Suggestions please ??

Actually, I had already replied to these questions in your original post "Getting Started with LANs: bandwith considerations w/router-on-a-stick vs L3 switches" although I didn't reply to your 2nd (duplicate) post "Voice over IP: Help with Layer 2 vs Layer 3 switch in mobile wireless env". Was there something to my original reply that was unclear or confusing?

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