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?

roots...

note the simple switch diagram...

all switches have a priority of 32768, the default

sw1 is root because it breaks the tie with the lowest bridge id of 1111.0000.0000

sw2 is 2222.0000.0000 and sw3 is 3333.0000.0000

sw1 is root, cost is zero to itself and therefore has all ports designated, it doesn't have a root port because it is root

bandwidth is fastethernet which makes the cost of each link 19 directly to the root, whereas sw2 and sw3 using their common segment to get to the root would have an aggregate cost of 38 each way to sw1

so we have determined the root ports of the non-root switches which means designated (forwarding)

now the question is which port is blocking on sw2 or sw3?

the bridge id breaks the tie, sw3's non root port is placed in blocking state

if the link from sw1 to sw3 was changed to 10Mbps, sw1's port to sw3 is still designated because all ports on the root switch are forwarding but the path from sw3 to sw1 is faster via sw2 now... so sw3's port to sw1 is blocking...

rapid-pvst

it's stp per vlan, and faster... go figure...  yes virginia, every vlan has it's own christmas tree with a load balancing ornament on top...  tuning the priority will allow some of the members of the vlan to use one of the supported links while other members of the vlan will use the one's normally blocked by stp, instead of all using the same forwarding link...

all output guaranteed from my NOHO...  (new office home office)

sw2950_02#sh spann vlan 2

VLAN0002
  Spanning tree enabled protocol rstp
  Root ID    Priority    32770
             Address     0009.b73f.ce80
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     0009.b73f.ce80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg BLK 19        128.1    P2p
Fa0/2            Desg BLK 19        128.2    P2p
Fa0/9            Desg BLK 19        128.9    P2p
Fa0/23          Desg FWD 19        128.23   P2p
Fa0/24          Desg BLK 19        128.24   P2p

a minor adjustment gives us this...

sw2950_02#config t
Enter configuration commands, one per line.  End with CNTL/Z.
sw2950_02(config)#int fa0/24
sw2950_02(config-if)#spann vlan 2 port-priority 0
sw2950_02(config-if)#end
sw2950_02#
2w0d: %SYS-5-CONFIG_I: Configured from console by console
sw2950_02#sh spann vl 2

VLAN0002
  Spanning tree enabled protocol rstp
  Root ID    Priority    4098
             Address     0009.b73f.ce80
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    4098   (priority 4096 sys-id-ext 2)
             Address     0009.b73f.ce80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg FWD 19        128.1    P2p
Fa0/2            Desg FWD 19        128.2    P2p
Fa0/9            Desg FWD 19        128.9    P2p
Fa0/23          Desg FWD 19        128.23   P2p
Fa0/24          Desg FWD 19          0.24   P2p

sw2950_02#

aaaaaahhhhhhhhhhhhh...

spanning-tree song...

spanning-tree very pretty and the bpdu's are sweet,
but the root of the poor spanning, is impossible to eat,

Michael the network cat likes that song... reow
 

STP elects a root bridge (switch) and puts all root bridge interfaces into forwarding state

Each non root bridge (switch) determines which of its ports has the least administrative cost (best) to the root bridge and STP makes that port that switch’s root port.

The switch with the lowest (best) cost  to the root is put in forwarding state.

The lowest cost switch on each segment is the designated bridge (switch) and the interface on that switch is called the designated port.

            The root bridge’s (switch) ports are always in forwarding state and the root switch (bridge) is always the designated bridge on all connected segments.

            The non root bridge root port is always forwarding. This port receives the lowest cost BPDU from the root.

            Each LAN’s designated port is always forwarding and the bridge forwarding the lowest cost BPDU is the segment’s designated bridge (switch)

             All other ports are blocking. No forwarding frames, no receiving frames.

         At first each switch claims to be root by sending BPDU’s that contain:

                 The root bridge ID- a combination switch priority and MAC address, lower number, higher priority

             The cost to reach the root- again the lower, the better

             And it’s own bridge ID

can't we just call a root bridge a root switch instead...