Question on limit to dynamically learned MAC addresses per vlan

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Jan 26th, 2010
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I'm trying to troubleshoot connectivity problems between a virtual server at a central site and PCs in the same vlan at a remote site. At the central site is several VMWare servers connected to a 3560 switch. The PCs at the remote site need to reach this virtual server, and while most do, some have trouble seeing it.

A common thread seems to be whether the 3560 has been able to learn the MAC address of the PC. If it hasn't, we can put in a static address and everything is OK.

I'm wondering... is there a limit to the number of dynamic MAC addresses that a switch like the 3560 can learn? If so, can that limit be changed, and is the situation the same for static addresses?

Also, is the limit of total MAC addresses for the switch fixed, or can that be changed?


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spfister336 Tue, 01/26/2010 - 09:08
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Yes, I've seen that, but is there any limit per vlan?

And it says "configurable"... how is it configured? I think the total limit must be around 5500 right now, based on the output I've seen...


Giuseppe Larosa Tue, 01/26/2010 - 11:35
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Hello Spfister,

cam table size is per platform, there is no limit for a single vlan.

The limit  is hardware based, some changes can be performed by changing the TCAM profile with sdm prefer command


see table 7-1

after change you need to reload for change to be active

you may be interested in

VLANs—The VLAN template disables routing and supports the maximum number of unicast MAC addresses. It would typically be selected for a Layer 2 switch.

you can check this with

sh mac address-table address count


sh mac-address-table address count

the command accepts also a vlan parameter to show MAC addresses per a specified vlan

Hope to help


spfister336 Tue, 01/26/2010 - 13:24
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Just wanted to follow up with some more details on this network set up...

[remote side 4500] ----> (CSME) ----> [central side 4500] ----> (ATM) ---->  [central side 8540] ----> [vmware 3560] ----> [vmware server]

the remote side has a vlan, let's call it 321, and the vmware server has a virtual machine set up for vlan 321. Most remote machines find the vmware server, and some find it one minute and not the next.

The remote side has about 330 MAC addresses in the vlan in question. The central side 4500 never seems to learn more than about 200 or so. I'm assuming that the central side 4500 learns mac addresses from the remote side, and passes it through the 8540 (configured for IRB) to the 3560. None of the central side devices seem to learn much more than 200 of the MAC addresses and I'm not sure where that limitation would be. Does anyone have any idea what's going on here?

Giuseppe Larosa Wed, 01/27/2010 - 07:05
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you have added details, are you using LAN emulation LANE or are you bridging over ATM ethernet frames?

I wonder if there is any device in the middle with a smaller CAM table (IRB)

Hope to help


spfister336 Wed, 01/27/2010 - 07:33
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It is using bridging... I think I've gotten the answer, though. It seems that AT&T has a set limit on the number of mac addresses they'll learn. The limit for that site turned out to be 200.


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