HP ProCurve Switch 6120G/XG Management Manual page 41

Static Virtual LANs (VLANs)
Multiple VLAN Considerations
2-18
drops the packet. This is not a problem for a switch with a multiple forwarding
database because the switch allows multiple instances of a given MAC
address; one for each valid destination. However, a switch with a single
forwarding database allows only one instance of a given MAC address. If (1)
you connect the two types of switches through multiple ports or trunks
belonging to different VLANs, and (2) enable routing on the switch having the
multiple forwarding database; then, on the switch having the single forwarding
database, the port and VLAN record it maintains for the connected multiple-
forwarding-database switch can frequently change. This causes poor perfor-
mance and the appearance of an intermittent or broken connection.