Two switch ports lead to same ethernet MAC address

I will be spending a few hours (or couple of days) with 2 DSL MODEMS connected to a switch (2924). With 2 modems connected to the same BRAS/BAS router (Bell Canada uses Juniper ERX).

With the first modem handling an established PPPoE session, the switch will initially only find 1 path to the ethernet address of the remote BAS router.

But, when that PPPoE session is restarted, the PPPoE negotiations, happening as ethernet broadcasts, will be send to both modems, and the BAS may respond to both requests, at which point, the swicth would find that both ports 1 and 2 can reach the ethernet address of the BAS.

Considering that the BAS probably doesn't handle spanning tree, how will the switch behave ?

Will it just send any/all packets destined to the BAS ethernet address on both ports ? (causing the BAS to receive 2 copies of each packet)

Or will it choose just one port for all traffic destined to that ethernet address ?

Are there IOS commands to tell the switch how to handle such a situation ?

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JF Mezei
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I am not familiar with the remote access technology after it leaves the customer premises so I can't comment on whether your plan will work or not.

However here is how I think an ethernet switch will handle identical source macs on two ports.

There will at any time be only one entry in the forwarding database for the mac.

The most recently received packet will be used to populate the table.

Therefore only one copy of each packet will be sent. It will be sent to the port that most recently received a frame.

If there is a high rate of address switching it is possible that the switch's performance may be affected. In the bad old days I encountered some equipment that could only cope with learning about 20 mac address per second. I think it unlikely that this will actually be an issue but worth watching out for.

You could use static MAC adddress table entries. I forget what it might be called.

You could consider turning off learning and then the packets would be sent on all ports in the VLAN except the receiving port, if that suited your purposes better.

You could also consider configuring etherchannel with no negotiation. Then the switch would choose which single port to use to send particular traffic and there would only be a single learning "virtual" port.

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Many thanks. This is useful to know. As it turns out, the second DSL line connects to a different BRAS router at the telco side, so there are different ethernet addresses.

So when the router here does a PADI request (ethernet broadcast) it would get to both BRAS, and the router here woudl get two offers (PADO) and choose one.

Within a couple of days, the old line will be de-activated so that won't be an issue anymore.

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