Re: Need detailed data on Cat2950 performance

mkaften · ‎11-14-2002

We are planning to use 24-port Catalyst 2950's to distribute critical real-time data to remote clients. Locally, each client will have routers attached to the switch. Our servers would be attached to the switch to provide the data. We want to use multiple VLANs to completely isolate each client's feed from the other. Unfortunately, a very influential person here, who knows very little about switching, is insisting that using multiple VLANs introduces extra overhead into the dataflow, and would "slow it down" to an unacceptable degree. His idea is to buy a separate managed switch for each client connection (each client would have two connections).

Our setup would not use trunking or even routing between the VLANs...nothing that would require frame-tagging...so it's hard to imagine where the "overhead" would come from. We only need 3 ports per client, and even if we bought the 12-port Catalyst 2950, we would be wasting 9 ports per switch. The quantity of data is small enough that we could support 8 clients on a 24-port switch and not even come close to the switch's forwarding capacity. Although this is a very simple configuration, we want the managed switches because they are more powerful than the unmanaged smaller switches, they allow hard-coding of speed and duplex settings, and also because we could monitor the data flow using SPAN if necessary.

I am going to need some data to prove that regardless of any additional latency (if in fact there is any) introduced by the switch using multiple VLANs, it is so small as to be negligible. And I would like to prove just how fast these switches operate. I haven't been able to find any information of this kind on the Cisco site. Cisco talks a lot about "forwarding at wire-speed" and "hardware-based forwarding" but never seems to quantify what that means. The impression I get is that the ASICs forward the frames faster than any machine can place them on the wire. Just how fast is it? I'd like to know what the forwarding speed and latency is, with and without multiple VLANs, assuming 100Mbps Full-Duplex set on all involved machines. Is there any information about how multiple VLAN data moves through the backplane and how each VLAN's data is isolated? I'd also like to know a little more about exactly how the Cisco ASICs do what they do.

Can you help me find these items? Thanks (I hope).

DALE FRANCIS · ‎11-19-2002

To get this info from Cisco might prove a bit hard, all the data sheets talk about are the following:

13.6-Gbps switching fabric

Catalyst 2950G-48: 13.6-Gbps maximum forwarding bandwidth

Catalyst 2950G-24: 8.8-Gbps maximum forwarding bandwidth

Catalyst 2950G-24-DC: 8.8-Gbps maximum forwarding bandwidth

Catalyst 2950G-12: 6.4-Gbps maximum forwarding bandwidth

Catalyst 2950T-24: 8.8-Gbps maximum forwarding bandwidth

Catalyst 2950C-24: 5.2-Gbps maximum forwarding bandwidth (Forwarding rates based on 64-byte packets)

Catalyst 2950G-48: 10.1-Mpps wire-speed forwarding rate

Catalyst 2950G-24: 6.6-Mpps wire-speed forwarding rate

Catalyst 2950G-24-DC: 6.6-Mpps wire-speed forwarding rate

Catalyst 2950G-12: 4.8-Mpps wire-speed forwarding rate

Catalyst 2950T-24: 6.6-Mpps wire-speed forwarding rate

Catalyst 2950C-24: 3.9-Mpps wire-speed forwarding rate

8-MB memory architecture shared by all ports

Up to 16-MB SDRAM and 8-MB Flash memory

Configurable up to 8000 MAC addresses

Configurable maximum transmission unit (MTU) of up to 1,530 bytes (Catalyst 2950G switches only).

Here is an independent test that might shed some more light for you:

http://www.etestinglabs.com/clients/reports/cisco/catalyst_3com.pdf

DALE FRANCIS · ‎11-19-2002

Here is a L2 only test with all the latency figures etc

http://www.etestinglabs.com/clients/reports/cisco/cisco2950.pdf

mkaften · ‎11-19-2002

Thanks VERY much! This looks like exactly what I need. Now, do you know where I can find out how the Cisco ASICs do what they do in a switch? This is really just for my own knowledge, it's much less of a priority than the other information. But you seem to know where to dig this stuff up...

DALE FRANCIS · ‎11-20-2002

There are variatioms as too the deployment of ASIC's within a Cisco switch, for e.g. the 3500XL uses a ring and satelite, tne 6500 uses BUS and Fabric channels. So it is hard to get that specific info out of Cisco and i do not have one to play with either..:((

The best thing i can offer is knowing it has a shared mem pool which can have advantages etc.

Cisco will come out with a white paper on this device once it has been widely delpoyed.

mkaften · ‎11-20-2002

I'm sure I'm not making myself clear here. What I'm looking for is a description of the function of the ASIC, and not a very highly technical description. In other words, I have heard that the ASIC downloads a copy of the CAM table so that forwarding decisions are "hardware-based"...is that true? When does this happen? Is there a (not overly technical) overview of the process by which a packet is forwarded from one switch port to another? Also, if a port is configured as part of a VLAN, is the downloaded CAM table smaller than the whole table? By the way, if a switch has multiple VLANs, is traffic restricted to certain areas of the backplane, or does the traffic intermingle?

DALE FRANCIS · ‎11-20-2002

I understand your frustration..:)) Unfortunately i do not have that information for the 2950 at all, the best bet is to get Cisco involved, please bear in mind i am a partner and us such the info we get is not that great, we rely more on R&D when we get the products and then query TAC.

mkaften · ‎11-20-2002

Thanks for the advice...unfortunately, at the same time I posted my first question here, I opened up a TAC ticket. All I've gotten from them is "we know of no documents that can give you what you want to know". Somehow, you had your finger right on the document. I don't have much faith in them right now.