sh spann bridge id...

wow...

dls1#sh spann vlan 1 bridge                                                    
                                                                               
                                                   Hello  Max  Fwd             
Vlan                         Bridge ID              Time  Age  Dly  Protocol   
---------------- --------------------------------- -----  ---  ---  --------   
VLAN0001         28673 (28672,   1) 000f.8ffe.0980    2    20   15  ieee       
dls1#

dls2#sh spann vlan 1 bridge                                                    
                                                                               
                                                   Hello  Max  Fwd             
Vlan                         Bridge ID              Time  Age  Dly  Protocol   
---------------- --------------------------------- -----  ---  ---  --------   
VLAN0001         24577 (24576,   1) 000b.5fc9.0000    2    20   15  ieee       
dls2#

als1#sh spann vlan 1 bridge                                                    
                                                                               
                                                   Hello  Max  Fwd             
Vlan                         Bridge ID              Time  Age  Dly  Protocol   
---------------- --------------------------------- -----  ---  ---  --------   
VLAN0001         28673 (28672,   1) 0009.b73f.ce80    2    20   15  ieee       
als1#




als2#sh spann vlan 1 bridge                                                    
                                                                               
                                                   Hello  Max  Fwd             
Vlan                         Bridge ID              Time  Age  Dly  Protocol   
---------------- --------------------------------- -----  ---  ---  --------   
VLAN0001         32769 (32768,   1) 0009.b752.d780    2    20   15  ieee       
als2#

switch 03...

transparent bridge
a transparent bridge is the idea of a layer 2 device which segments a network and passes frames between    
these segments to nodes by learning  information about the nodes addresses, and without the nodes
knowledge of their existence (hence, the bridges are seemingly transparent to the nodes)

bridging loop
a bridging loop is a condition which can occur if more than one layer 2 path exists in a network between
two endpoints.  layer 2 is helpless to stop such occurrences for lack of a mechanism such as ttl

Spanning Tree Protocol
spanning tree is such a mechanism, designed solely to shut down a redundant path that might cause a
loop

BPDU
the unit of communication between switches to establish the rules for the effective topology of the tree, sent
every 2 seconds

root bridge
a root bridge is determined after a topology election in the tree; this is effectively the hub of the layer 2    
network. the ports of the root bridge are always designated

root port
every switch in the network determines a root port; the port with the least cost path to the root switch

root path cost
the cost determined by bandwidth to the root switch.  this cost is often cumulative

designated port
a non-root port on the switch that is the least cost path to the root connecting to another segment

Hello Time
the time between each transmission of a bpdu; default 2 seconds

Forward Delay
the cumulative time for a port to transition to the forwarding state

Max Age time
the time for the prior information learned from a bpdu to expire because a new bpdu has not been received

TCN
a topology change notification bpdu will be initiated by a switch upon port transitioning

Common Spanning Tree
a single instance (common) tree for the network

PVST
a tree instance for every vlan

PVST+
the same as pvst but ieee open standard (without isl)

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...

spanning-treed...

i asked michael the network cat exactly what it is that spanning-tree does...
he replied, it prevents switching loops...
precisely...

so in case you don't have a meshed network, what then michael?

it causes a lot of traffic, slows down your ports, doesn't block any ports because there are no redundant links, but gives you nice output and bridge and port status information, lots of debugging stuff if you want to see it, and many other features that will be tested by cisco...

ok then... the right answer is the cisco answer...