Spanning Tree Vs. OSPF/BGP

I think the question you want to ask is, "What is the difference between using a layer 2 vs. layer 3 switch to connect two floors and 170 users?". Spanning tree protocol is designed to close off forwarding loops and prevent broadcast storms. It will work the same on either type of switch.

In short, a layer 2 switch forwards packets based on MAC address only. A layer 3 switch can forward on IP address and provides additional features for filtering and QOS.

As for the effect on DHCP, that would depend upon your connectivity needs and your addressing scheme. All 170 users could be part of the same DHCP scope and use the same default gateway. You could also create vlans on the switches to isolate work groups and assign separate DHCPscopes to each. That works well for in-group communication, but communication between vlans requires a router. I hope this has helped clear things up a little. Write if you've got more questions. Best wishes, CK.

What about using the switch as a bridge between these VLANs?

Spanning tree is NOT a deterrant for Brooadcast storms. It merely provides a system to eliminate bridging loops, or in layman's terms, redundant paths to the same location. If a bridge loop does occur, then a broadcast storm can occur, along with other repeated traffic not limited to broadcasts. ANd broadcast storms can occur w/o loops.

In your scenario, you would use a layer 2 switch. L3 switches are mostly use for Inter-VLAN routing, among other minor uses. But you would not need this since you stated you wanted to remain on the same subnet. A layer 2 Cisco switch, such as stacking 3548XL series would do the trick for you.

Wait, now I see your second comment-VLANs. Yes, however if you create VLANS, that would put your users on seperate subnet (which means you would have a seperate subnet for each VLAN). Dependant upon the number of VLANs and users is the criteria you would use for a L3 switch, with MLS. THis becomes quite costly.

Concerning DHCP: Each VLAN has it's own subnet, therefore you would create a DHCP scope for each VLAN subnet. The clients will automatically pick out of the correct scope.

Spanning Tree is a layer 2 protocol to preven loops in case of an outage, Layer 2 will allow broadcast, OSPF/BGP are routing protocols, also have the capability for a loop free network in case of an outage, this part have nothing to do with your question, about implementing a network with one DHCP server. If you are going to use a Layer 3 vs Layer 2 network this is the main problem you are going to find. Layer 2 allows broadcast, Layer 3 ( routers ) do not allow broadcast to pass through them, DHCP relay is a broadcast, to over come thing problem with routers (at least Cisco Routers ) you need to use an IP Helper, This IP is the IP address to which you want to forward the broadcast to, in your case the DHCP server. Layer 3 solution is moresecure than Layer 2, where you can controll the flow and direction of the packet more effecient. Running OSPF or BGP is not something you need to do for this kind of small network, running the routing protocols as OSPF is for a network that had too many routers that needs to comunicate with each other. in your case you need a small router to segment your network for the 2 floors, each floor can run fine over Layer 2 switches, both floors are connected via the router, also you can creat another segment where all your servers are (i.e DHCP, ftp, DNS) and use IP helpers on the interfaces that had the PC segments.

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