When would you ever prefer a chassis based switch over stacked 3750s?

mfieldhouse

I'm currently putting together a network design for approx 140 users, each requiring PoE to power their VoIP phone, off of which will hang their desktop machine. We also require sufficient number of ports for servers and other networking devices. The hardware is going to be 5 x 3750s stacked. 2 x 24 port (each server has 2 NICs, one into each switch) and 3 x 48 port PoE, into which the users connect.

In this situation you would need a 6000 chassis with minimum 6 slots, more if you expected the number of users to increase then you've got to buy the additional power supplies, fan tray, whatever else isn't included. And unless you go for the next one up which is 9 slots so you could fit another Supervisor card in - you're going to lose the entire chassis if the Supervisor fails. Whereas if a 3750 stack master fails, another can take over the role easily and at most you would lose 48 users.

So this got me thinking, when would you ever want to buy a chassis based switch over a stack of 3750s?

I don't know much about pricing so maybe that's a factor also. This stuff ties in with BCMSN studies so it's all relevant.

Forsaken_GA

mfieldhouse wrote: »

And unless you go for the next one up which is 9 slots so you could fit another Supervisor card in - you're going to lose the entire chassis if the Supervisor fails.

Virtually all 6500 chassis are deployed with a twin for redundancy, so losing one chassis doesn't immediately mean you're in an outage, though it's very annoying.

So this got me thinking, when would you ever want to buy a chassis based switch over a stack of 3750s?

Because they function more as core routers than as aggregation switches. Depending on the size of your organization, you're usually not using a modular chassis for port density. Then there's also the versatility issue. I can slap line cards for whatever media I need into a modular chassis. I can slap firewall modules, IDS modules, Network Analyzer modules, Wireless Lan Controllers, etc into a modular chassis. Can't get away with that in a fixed configuration chassis.

And then the biggest issue is simply throughput. Take a look sometime at what the crossbar fabric on a Catalyst 6500 with a Sup720-3BXL can do when it has dCef enabled line cards in it.

dtlokee

There are many factors to weigh when making this type of decision. Price is part of it, reliability, performance and so on. You are really looking at it from only the idea that if one switch fails the master operation can fail to a different switch. Cisco generally says at 144 ports (about where you are) it becomes more cost effective to use a chassis based switch over stackable switches. Also you could use the 4500E (4510E 10 slot chassis) with dual supervisors which is a very reliable switch and has a blackplane 4 times taht of the 3750E, and 8 times that of the regular 3750. Another factor when calculating the cost is PoE power requirments, if you exceede the 370 watts per standard switch on a 3750 it gets real expensive to go to the upgraded switch and powersupply.

From my experience I will always attempt to use a chassis over standalone or stacked switches if I can.

One other thing when considering chassis switches is the addition of a UPS system, too many times I have gone to replace some switches in an IDF with a chassis only to find they have all of the existing switches plugged into a APC 750 or something like that.

keenon

it also would depend also on the cost of what the company is willing to spend. i have seem some companies use 4500 or 6500s in the closets. while i have seen others use stackables everywhere including the core of the network due to cost.

Forsaken_GA

Yup, the needs of the network and the budget to support it are what will drive the hardware selections. I think the spirit of the original question is what's the point of a modular chassis instead of just using a bunch of stacked 1RU's, and it seems to be from a port density point of view.

The 3750 stacks are badass and certainly have their place, but I'm with dtlokee, I'll take modulars whenever I can get them. I don't like giving up the versatility that a modular switch allows.

And if I'm remembering right, yeah, a stack can survive the loss of a switch, but it cuts the throughput in half to do so. In a high speed environment, that's a deal breaker

mfieldhouse

Virtually all 6500 chassis are deployed with a twin for redundancy, so losing one chassis doesn't immediately mean you're in an outage, though it's very annoying.

However, in this situation wouldn't you have to rely on fiber or CAT5 in EtherChannel to link the switches together whereas with the 3750s you've got the total 32 Gbps Stackwise backplane?

Also, I appreciate what you say about versatility of the Chassis, but in this situation those additional features would not be needed.

it also would depend also on the cost of what the company is willing to spend

The company is looking purely at the maximum number of ports and network resiliency for the lowest cost possible.

Cisco generally says at 144 ports (about where you are) it becomes more cost effective to use a chassis based switch over stackable switches.

So in this situation where we require 192 ports, would it be more cost effective to go for the Chassis, 2 Sups, 3 x 48 PoE linecards and 2 x 24 non-PoE than a stack of 5 3750s?

Forsaken_GA

mfieldhouse wrote: »

However, in this situation wouldn't you have to rely on fiber or CAT5 in EtherChannel to link the switches together whereas with the 3750s you've got the total 32 Gbps Stackwise backplane?

It depends on what you're doing with it. Our 6500's are core routers, the only thing they do is take links from our transit providers and distribution switches, so they're loaded with line cards that take 10 gig Xenpaks, and that's how they're linked to each other. The cat 6500's have a 32gb backplane on the chassis itself, but with a sup720 and proper line cards, that goes up to 720gb's of throughput, which is nothing to sneeze at.

Also, I appreciate what you say about versatility of the Chassis, but in this situation those additional features would not be needed.

Well sure, but your question is why would you ever prefer a chassis based switch, that's the question I answered

So in this situation where we require 192 ports, would it be more cost effective to go for the Chassis, 2 Sups, 3 x 48 PoE linecards and 2 x 24 non-PoE than a stack of 5 3750s?

In that situation, I'd load up a 4510 with 2 supervisor's for redundancy, and fill the rest with line cards for the port density you want. The 4500 backplane is rated for 64 gbps if I remember correctly. I believe you can also get it up to 240 ports, all PoE enabled

Edit: Wow, the 4500-E's have come along since the last time I looked at them, looks like they can get to 320gb's across the backplane now

dtlokee

Yes the 4500E is up to 320GB/s with 40GB per linecard, that dwarfs the 32Gb/s backplane of a 3750 stack (assuming full duplex, non "E" series switches)

From a redundancy perspective I have had far better success in a chassis with 2 supervisors, and 2 powersupplies than a stack of 3750's. I have found the stacking cables to be somewhat touchy if you over or undertighten them, and I have had switches just drop out of the stack for no reason, remove and reattach the stacking cables and they come back. Kind of another place I have had issues was when building an etherchannel across stack members, never had an issue like that across blades in a chassis.

I am not saying that the 3750 stack isn't an awesome switch to deploy in a network, we use them all the time. The most important thing is to understand the requirements of the network and then make the appropriate selection.

apd123

Have you considered not going with Cisco if one of your major goals is the lowest cost possible? I know the answer is probably going to be that Cisco is required so I will move on.

Parts do fail. A chassis switch without power supply and supervisor redundancy having single points of failure is a flaw in the network design not the switch itself. You could probably do actual calculations based on failure rates of parts, but I am gonna hypothesize that the estimated downtime is greater in the stacked design than the chassis. Some other reasons have already been noted and if you really want to get down to it the 6500 is just overall more capable in a bunch of ways.

joshgibson82

You can also put larger power supplies (2 x 6000w) in the chassis, which will allow you to power up alot of devices (aps, phones,) which may be too much for 3750's to handle. Also some features are only available on the Sup720 platform.