in my Cisco 6509 Switch , the Spanning Tree for every VLAN shows me a separate mac address for the Designated Root Port
e.x Designated root has priority 32768, address 000b.fc38.f81a
But I don't see any port on this switch that shows this mac address as a result of "Show interface" command
How do i trace, Who actually owns that mac address ??
That's correct, there is a range of mac address reserved in the backplane that the switch is using for its consecutive bridge IDs (it's on the backplane instead of the supervisor so that you can change your supervisor without changing your bridge IDs).
Before 802.1Q was out, Cisco's PVST only supported 1K vlans. We were at that time burning 1K mac addresses for each switch we were shipping just of these bridge IDs. When 1Q defined up to 4K vlans, Cisco had to support this and there was no way we would allocate 4K mac addresses for each piece of hw sold. That's where "extended system ID" or "mac address reduction" was developed: a single mac address is used for all the vlans bridge IDs, but 12 bits are stolen from the bridge priority in order to make this ID unique per vlan. The priority can then only be set by increment of 4096 (2^12). This feature is now enabled by default on some of our latest hardware, which only have 16 or 64 built-in mac addresses reserved. It means that those switches won't even be able to go back to the behavior you observed, i.e. one different mac adress per vlan. It's of course not a problem (and was in fact added to the IEEE standard).