STP port roles help

Unanswered Question
Jun 25th, 2010

Hi, I am having lot of confusion understanding STP Port roles.

I know, How STP determines the root bridge, but having understanding in Root port determination, and designated/Non-designated ports.

I have uploaded a topology, please see.

1. In topology, Sw1 is the root bridge.Sw1 sends root path cost 0 to Sw2, and Sw3. When Sw2, and Sw3 receives the BPDU, Do they add cost of 19 on incoming interfaces or outgoing interfaces? I am really confused in this.

2. The segment between Sw3, and Sw4, Sw3 fa0/2 is designated, and Sw4 fa0/3 is root port? why is this? Why Sw4 fa0/3 is root port? Is it because of lower root path cost to root bridge? If that's the case, Why it didn't select Sw4 fa0/4 as root port?

3. When STP considers lower BID, What exactly it compares inside that BID?  As I know, When choosing Root bridge, It considers priority+ mac.

    What about when choosing designated and non-designated port? Does it consider also priority+mac address too?

Could you please answer my questions in good explanation?

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Giuseppe Larosa Fri, 06/25/2010 - 05:08

Hello Sandy,

1) cost of elected root port is added to the received cost in the received BPDU. BPDUs sent out a switch on  non root ports in STP forwarding state all carry the same cost to the root that of the root port

cost to root bridge = received cost to root bridge + cost of elected root port

2)

>> The segment between Sw3, and Sw4, Sw3 fa0/2 is designated, and Sw4 fa0/3 is root port? why is this?

this may happen, different criteria are used to elect DP and RPs on switches once the root bridge is elected:

lowest cost to root bridge

Lowest BID of BPDU sender

lowest STP priority of received BPDU

lowest port index of received BPDU

for example if SW1 is the root bridge Sw4 has two paths of equal cost to root bridge one via SW3 and one via SW4, if SW3 has lowest BID in comparison to SW2 SW4's port to SW3 is elected root bridge and SW4's port to Sw2 will be placed in blocking  state to break the loop.

3)

BID = STP priority + MAC address

each switch can use a single MAC address for all Vlans/ STP instances or a different MAC address for each Vlan/STP instance in PVST+

in first case STP priority = base value + vlan-id

and base_value has to be a multiple of 4096  (possible Vlan range in 802.1Q)

this is called MAC reduction or extended system-id

for vlan 10 is base+10 for vlan 22 is base+22

example:

sh spanning-tree vlan 500

VLAN0500

  Spanning tree enabled protocol rstp

  Root ID    Priority    8192

             Address     0005.741b.1f74

             Cost        2

             Port        129 (TenGigabitEthernet3/1)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

>>>  Bridge ID  Priority    33268  (priority 32768 sys-id-ext 500)

             Address     0022.55be.4740

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 300

sh spanning-tree vlan 510

VLAN0510

  Spanning tree enabled protocol rstp

  Root ID    Priority    8192

             Address     0005.741b.1f7e

             Cost        2

             Port        129 (TenGigabitEthernet3/1)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

>>>>  Bridge ID  Priority    33278  (priority 32768 sys-id-ext 510)

             Address     0022.55be.4740

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 300

as you see for two different vlans two values 33268 and 33278 are used with the difference equal to 510-500.

A single MAC is used

Hope to help

Giuseppe

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