extended system id...

a fixed switch shares it's mac address with it's ports...

Base ethernet MAC Address: 00:09:B7:52:D7:80
the mac address of port 1 on this switch is:


asw2#sh int f0/1
FastEthernet0/1 is up, line protocol is up (connected)
  Hardware is Fast Ethernet, address is 0009.b752.d781 (bia 0009.b752.d781)

if your comfortable with hex, you can easily determine the rest of the ports mac addresses by counting up...


asw2#sh int f0/10
FastEthernet0/10 is down, line protocol is down (notconnect)
  Hardware is Fast Ethernet, address is 0009.b752.d78a (bia 0009.b752.d78a) = 10 hex

you knew that...

the default switch priority is 32768...

we know that the combination of priority and mac address gives us the BID
because 1 or many ports may belong to a vlan, we need more distinction...

the extended system id makes that yet more granular...


asw2#sh spann vlan 20

VLAN0020
  Spanning tree enabled protocol ieee
  Root ID    Priority    32788
             Address     0009.b73f.ce80 (base ethernet)
             Cost        38
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32788  (priority 32768 sys-id-ext 20) 32768 + 20

the extended system id allows the vlan for the switch to have a unique BID

graphic below from http://adelzalok.wordpress.com/2011/09/29/anatomy-of-a-mac-address-bpdu-bid-and-the-802-1q-ethernet-frame-tag/  great site by the way... you haven't clicked on it yet?