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MAC Address Table Size

Posted on 2004-04-07
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I´d like to know the relation between the number of active points inside a LAN and the minimum size of mac addresses that a switch inside such LAN must support.

E. g., if a LAN has 100 access points what´s the minimum size of the mac address table of their switches?
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Question by:mtnaka
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by:Pete Long
ID: 10775504
As far as Im aware there is no minimum size? all these tables are dynamic anyway, if they were to big (or had a long time to live - depending on which way you look at it) then they would be holding stale records, Using that perspective a smaller Dynamic table will perform better, but a switch is working at Layer TWO

It doesnt matter to a switch how big the LAN is, its only concered with whats on the end of each port admittedly there could be MANY things on one portlike in my crude doodle below

Ie

client--------------------switchA-----------------switchb--------------switchc

so switch C might have 30000 objects on that one link

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by:pseudocyber
pseudocyber earned 30 total points
ID: 10775811
Agree with Pete - Offhand, I believe I've seen tech spec sheets saying that switches are capable of 65K+ table sizes.  There is no "mimimum" size - if there aren't many entries, there just aren't many entries.

Also, most pro switches are built with ASIC's - Application Specific Integrated Circuits - and looking up MAC addresses in a table is as basic a function as switches are built to do - they do it very fast, normally.
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by:mtnaka
ID: 10776769
Actually, my question is more like whats the recommended size. Someone says they need 64k mac address table for a network with 200 points lets say, could I tell them that you dont need that big of a table? 12k or even 8k would be enough to have traffic flowing normaly?

thanx,
Naka
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by:Pete Long
ID: 10776824
Hello Naka

This is not something you are going to need to worry about? unless you are going to build switches in your garage, Enterprise class switch designers have thought about this for you

Its a little like saying

"Well if your buying a land rover your going to need a 90 litre fuel tank, cause a 60 litre fule tank isnt big enough"

If a 60 Litre tank wasnt big enough they wouldnt be selling it?
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by:pseudocyber
ID: 10776977
I've never heard of anyone specifying a mac address table size - most people don't even know what they are.
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by:mtnaka
ID: 10777335
Well, thats where the problem is. I actually re-sell network equipment, usually we get part numbers so its an easy decision on the equipment, but this customer sent some definitions that dont go with the particular brand. Now the big question is, does he need a MAC table with 64K considering he is implementing it for a location with 200 computers + whatever routers / modems he's got? Or could is it in his interest to go with the 12k option considering they dont expect the location to expand for at least 5 years?

thanx for all the help so far!
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by:pseudocyber
ID: 10777400
Well, why not the 12K since you're talking about 200 +/- devices?  What difference does it make besides price?  200<12000<64000.
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PennGwyn earned 90 total points
ID: 10777568
The customer is always right.

He may have specified a minimum MAC table size as a way to weed out "toy" devices, figuring the class of device he wants will have no trouble meeting this level.

You *might* score some points with him if you demonstrate that you know what you're doing by asking to confirm that spec.  From the information available, it doesn't sound like he will actually need anything like that much.  Perhaps there's some additional information that he hasn't provided....

A switch will provide best performance if its table is big enough to hold information about every active MAC address in the all of the VLANs/broadcast domains present on the switch (even if the actual devices are attached to some other switch).  It will work -- with some risk of a slight performance hit -- if the table is smaller than that.  12K should be enough for 200 devices, and more will not, in and of itself, provide any direct benefit.

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by:Pete Long
Pete Long earned 30 total points
ID: 10778167
>>VLANs

Whoa there! when did we introduce Layer 3 switching to the equation? We were at layer two when I left, this is a pointless argument 200 network nodes is nothing? they will run quite happily on a 10baseT Dumb Hub, you can thell happily tell your customer you dont have dynamic mac address table as these devices handle this at Layer one instead of layer two LOL

>>A switch will provide best performance if its table is big enough to hold information about every active MAC address in the all of the VLANs/broadcast domains present on the switch

I dont agree, in that case it would be better and faster to run ARP resolution on NVRAM, the whole point af a cache is that its a cache, the most often used is first in the queue and those not being used drop of the bottom. using this argument is like saying I need a 1Gb L2 Processor Cache on my Processor cause then I can cache every memory location?
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by:Pete Long
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ThanQ
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