I have been tasked with setting up a fairly straightforward rack in a data center (we do not even need a whole rack, but this is the smallest allotment available). In a nutshell, 4 to 6 servers need to be able to reach 2 (maybe 3) vendors. The servers needs to be reachable over the internet.

A little more detail - the networks the servers need to reach are inside of the data center, and are "trusted". Connections to these networks will be achieved through intra data center cross connects. It is kind of like a manufacturing line where we receive data from one vendor (burst-able up to 200 Mbits), churn through it on the servers, and then send out data to another vendor (bursts up to 20 Mbits). This series of events is very latency sensitive, so much so that it is common practice not to use NAT or a firewall on these segments (or so I hear).

To reach the servers over the internet, I plan to use a site to site VPN. (This part is only relevant as far as hardware selection goes).

I have 2 configurations in mind:

  1. Cisco 2911 (2921) (with the additional wan ports module) and a layer 2 switch - in this scenario, I would use the router also for VPN.
  2. Cisco 3560 layer 3 switch to interconnect the networks inside of the data center and an ASA 5510 (which is total overkill, but the 5505 is not rack mountable) as a firewall for the Wan side (internet) and VPN. I envision the setup to be as follows:

Internet -> ASA -> 3560

Vendors -> 3560 -> Servers

The general idea is that the ASA acts as a firewall and VPN device and the 3560 does all the heavy lifting.

The first is a fairly traditional setup but my concern is performance. The second is somewhat unorthodox in that the vendors are directly connected to the layer 3 switch without passing through a firewall. Based on my understanding however, a layer 3 switch will perform substantially better as it will do hardware (ASIC) vs. software switching. (Note that number 2 is a little over the budget, but not unworkable (double negative, ugh))

Since this is my first time dealing with a data center, I am not sure what the IP space is going to look like. I suspect I will retain a block(s) of public IPs, vlan them to individual interfaces for the vendor connections and the servers (which will not reachable from the wan side of course) and setup routing on the switch.

So here are my questionss:

  1. Is there a substantial performance difference between 1 and 2, i.e. hardware based switching on a layer 3 vs a software base on the 2911? I have trolled the internet and found a lot of Cisco literature, but nothing that I could really use to get a good handle.
  2. The vendors we connect to are secure and trusted (famous last words) and as I understand it, it is common practice not to NAT or firewall these connections (because of the aforementioned latency sensitivity). But what what kind of latency are we really talking about if I push the data through a router (or even ASA for that matter)? For our purposes, 5 ms will not kill us, 20 or 30 can be very costly. Others measure in microseconds, but they are out of our league.
  3. Is there any issues with using public IPs on a layer 3 switch?

I am certainly not married to either of these configs, and I am totally open to any ideas. My knowledge (and I use the term loosely) is largely from books so I welcome any advice / insight.

Thanks in advance.


The term "layer 3 switch" is really just a router that happens to be optimized for LAN networks - it doesn't mean anything these days.

The 2900 says: "Circuit-speed WAN performance up to 75 Mbps , plus variety of services" so you can pass about 75mps of traffic "through" this router at software switching speeds.

2) As long as you're not pushing the limits of the forwarding performance of the router, you're talking really <1 ms

If you need to actually route 200mbps of traffic, then you're going to need a 3560 or something to route between the segments of the other cross connects landing in your cage vs the servers in your cage.

3) None at all - routers and switches (plain ones, not new-fangled 'security' or 'enhanced' ones) don't care about public vs private IPs.

I suggest a 3560 and then some device that can terminate a VPN to get you 'inside' of the network.

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