[c-nsp] Same MAC addresses from two ports on different VLAN

[Christopher.Marget at usc-bt.com]

Phil Mayers:
> Nope. All 3Com devices from superstack II onwards have per-VLAN FDBs,
> and all the Extreme's I've ever seen do too.
> 
> Tempting though it is to rag on the cheaper switches, they do usually
> manage the basics ;o)

3com's CoreBuilder 3500 was not endowed with this basic feature when it first shipped.  From the 3com knowledgebase:

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Symptoms: Connecting ethernet card to single device  
Problems when SUN 4 port ethernet card is connected to 3500  
Unable to telnet to all IP addresses  
Unable to ping to all IP addresses  

Causes: Depending on the version of software the SUN system is running the 4 port ethernet card could be using the same MAC address on all of these ports, and a different IP address on each of these ports. If it is using the same MAC address and more than one of these ports are connected to the same 3500 running V1.0 base or extended software, this creates a problem. The problem is that the same MAC address gets learned on more than one port of the 3500. Therefore some packets get to the correct destination and other do not.  

Fixes: The solution to this problem is 3500 v1.1 base or extended software and configuring all the vlans for "allclosed". When using "allclosed" vlans this gives you a separate bridge forwarding table per vlan instead of one bridge forwarding table per 3500.
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I ran into this issue in 1998 with a Solaris server that had a leg in two different VLANs.  V1.1 wasn't available yet.  Manually coding a new MAC address onto one of the interfaces at boot time cleared things up.

/chris