I got two CISCO switches of same model. (they're connected for trunking and spanning-tree is enabled (by default)) - I tried to leave the configuration as default as possible.

I got a server with two NICs, NIC teaming is configured.

Each NIC is connected to each switch. (the NIC teaming app sets up a random MAC address and is using it for both NICs instead of their factory assigned MACs)

Switches complains about MAC address flapping.. But why? Isn't the switch smart enough to know that my server have fault tolerence and is using two network cables? What configuration am I missing?

EDIT: The actually question is highlighted.. The rest is just ranting.. hehe


Unless your switches are Cisco VSS capable (typically the high-end boxes) then you have to switch the NIC teaming to working in an active/passive mode rather than active/active.

The reason is that as it stands both switches have their own, completely separate, CAM table, but your team announces the same MAC down each physical port, in this case to different switches unaware of the team - so ST/portfast just flaps which switch owns the MAC address.

As I said if you had VSS switches they share one CAM table so you can split a team like this, that's what it's for in fact - but not for regular non-VSS switches sorry.

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  • Thanks for the reply that's good to know and it makes lots of sense, I will do some more research regarding the CAM table.. Maybe the active/passive option exists in that crappy Realtek teaming software. (Ps. if anyone knows a 3rd party NIC teaming software please speak up, although I doubt it exists) – ToastMan Apr 11 '11 at 15:01
  • basically all switches have a CAM table, it just lists what MACs it knows on what ports, very VERY few switches are setup to work in pairs and share a CAM table - so active/active is usually to the same switch, with active/passive for most dual-switch setups. – Chopper3 Apr 11 '11 at 15:08
  • I forgot the name of it, but I know it's possible to connect 2 switches together and have it act as 1 switch.. – ToastMan Apr 11 '11 at 15:11
  • In a Cisco world it's called VSS, no idea about other makes sorry – Chopper3 Apr 11 '11 at 15:13
  • Our switches support stocking.. we still have this issue ... any ideas? – ToastMan Apr 11 '11 at 20:55

Nope... switches aren't smart enough to randomly guess that your server has teamed interfaces. You must tell it so.

Think from the perspective of the switch (port X is the port directly attached to server... port Y goes to the other switch connected to the server) Ok... port X reports MAC x... ok... add that to the ARP cache... ok... port Y says it has MAC X... ARP cache says it's on port X... that server can't be in 2 places at once... someone might be trying to do some arp-poisoning or other similar man-in-the-middle attack... or a bunch of other things. An error would always be appropriate.

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  • Um yup I already know that, thanks :p Is there any special configuration I need to perform on the switches? – ToastMan Apr 11 '11 at 14:33

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