stp root switch...

from cisco's 3560/3750 guide which of course you can download...

If all switches in a network are enabled with default spanning-tree settings, (and when they come out of the box they are wearing their birthday suits) the switch with the lowest MAC address becomes the root switch.


that is gospel...

priority is default at 32768...

that is gospel...

this is the BID of this switch on vlan 10...

dsw1#sh spann                                                                 
                                                                              
VLAN0010                                                                      
  Spanning tree enabled protocol rstp                                         
  Root ID    Priority    32778        

32768 + 10 is 32778...

the priority can change all that ONLY if it has been configured to do so... and if it has been configured to do so, the mac doesn't matter...

again, that is if it has been configured to do so...

which means the priority is an afterthought, although we all know you want to be deterministic about the placement of the root switch...

out of the box, MAC is king...

your witness, counselor...

stp topology change capture...

packet capture on the live gear depicting a topology change notification bpdu, after spann vlan 1 root primary...
also note eigrp hellos on dls2... click on it to enlarge...

dls2#sh arp                                                                    
Protocol  Address          Age (min)  Hardware Addr   Type   Interface         
Internet  10.10.10.2              -   000b.5fc9.0000  ARPA   Vlan101           
Internet  10.1.1.102              -   000b.5fc9.0000  ARPA   Vlan1             
Internet  10.1.1.101             22   000f.8ffe.0980  ARPA   Vlan1   

port authority...

root port - every switch in the diameter has a root port; the port with the least cost path to the root switch

designated port - every switch in the diameter has at least one designated port; it is a port on a LAN segment (non-root port) with the least cost path to the root switch

blocking port - a port which is neither root port nor designated port

alternate port - candidate root ports although currently blocking but close to the root switch and identified by uplinkfast as suitable

forwarding port - just that, forwarding; no other stp activity expected

my members...

etherchannel configuration...  it's a good idea if both sides match...


interface FastEthernet0/1
 switchport mode trunk
 speed 100
 duplex full
 channel-group 1 mode on
!
interface FastEthernet0/2
 switchport mode trunk
 speed 100
 duplex full
 channel-group 1 mode on
!
interface FastEthernet0/3
 switchport mode trunk
 speed 100
 duplex full
 channel-group 2 mode on
!
interface FastEthernet0/4
 switchport mode trunk
 speed 100
 duplex full
 channel-group 2 mode on

nice way to keep track of membership...


sw2950_01#sh int port-chan 1 | inc Members
  Members in this channel: Fa0/1 Fa0/2
sw2950_01#


sw2950_01#sh int port-chan 2 | incl Members
  Members in this channel: Fa0/3 Fa0/4
sw2950_01#

don't let spanning tree decide root... the root id is the id of the root and the bridge id is the id of this switch... if  the priority of the two is equal, this is the root... if the mac's are the same, this is the root... if it says this is the root, this is the root... if all ports are forwarding, this is the root... how do you like my root...


sw2950_01(config)#spann vlan 50 root primary

sw2950_01(config)#do sh spann vlan 50

VLAN0050
  Spanning tree enabled protocol rstp
  Root ID    Priority    24626
             Address     0009.b752.d780
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24626  (priority 24576 sys-id-ext 50)
             Address     0009.b752.d780
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po2              Desg FWD 12        128.65   P2p
Po1              Desg FWD 12        128.66   P2p

sw2950_01(config)#

and some aggregation up inside..


sw2950_01(config)#do sh int port-channel1
Port-channel1 is up, line protocol is up (connected)
  Hardware is EtherChannel, address is 0009.b752.d781 (bia 0009.b752.d781)
  MTU 1500 bytes, BW 200000 Kbit, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Full-duplex, 100Mb/s, media type is unknown media type
  input flow-control is off, output flow-control is off
  Members in this channel: Fa0/1 Fa0/2

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 very pretty...

STP summary

1.  all bridge (switch ports) stabilize at forwarding or blocking.  Forwarding ports are considered part of the spanning tree.

2.  one switch is elected root, and its ports will all move to forwarding state.

3.  each switch receives hellos from the root, directly or through another switch. The port that receives the least cost BPDU is placed in forwarding and becomes that switch’s root port

4.  for each segment one switch forwards the BPDU with the lowest cost.  That switch becomes that segment’s designated bridge.

5.  the other interfaces are placed in blocking

6.  the root sends BPDU’s every 2 seconds. This time interval can be modified and will be noted in the BPDU.

7.  if max-age elapses (20 seconds, default) and no BPDU, panic ensues and the spanning tree changes.

8.  forward delay (default 15 seconds) is the time it takes for a port to transition through the dumbass states to forwarding. (listening, 15 seconds, learning 15 seconds)

9.  when a switch goes into listening, it sends a TCN BPDU (topology change notification) through the new path to the root. Other switches will refresh their tables with the new entry.

10. spanning tree creates these delays to prevent transitional loops because…

spanning tree, very pretty and the bpdu’s are sweet,

but the root of the poor spanning, is impossible to eat…

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...