Spanning tree block port selection

[morialpesh008]

Hi,

       Refer below  STP scenario, Can someone tell me why switch-1 port Fa0/2 block and not port Fa0/1?

all switch has connected with fast-ethernet port, priority of all bridge is default. you all are see all switches MAC in attached snap.

also is it true that in PVST+ or RPVST+ only root bridge send BPDU packet?

what is the concept of sending BPDU packet in cisco or IEEE STP?

Thanks.

alpesh

[azibnaseem]

Hi morialpesh008  '

from the diagram what I have assumed that Switch0 is the root bridge. Please post the output of command, show spanning-tree from switch0.

Regards'

[morialpesh008]

Hi Azib,

   Switch 3 is root bridge, i have leave all configuration default so Switch-3 is root bridge because it has lower MAC then other switches.

here is output of Switch-3 show spanning-tree command.

S3#show spanning-tree

VLAN0001

Spanning tree enabled protocol ieee

Root ID Priority 32769

Address 0005.5E63.E7CE

This bridge is the root

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

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)

Address 0005.5E63.E7CE

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

Aging Time 20

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/3 Desg FWD 19 128.3 P2p

Fa0/4 Desg FWD 19 128.4 P2p

[azibnaseem]

Hey morialpesh008  

STP elect the root bridge on three ways.

- Lowest Cost

- Lowest Bridge ID

- Lowest Port Number

As from the diagram Switch1 interface fa0/1 & fa0/2 have same cost to the Root Bridge then it is elected by the lowest bridge ID. Switch0 is the lowest bridge id to the root than switch2. Thus it's block the port fa0/2 & not fa0/1.

Regards'

[morialpesh008]

hello azi,

       first of all thanks for your clarification, as you said there are three things to be consider when we are running on STP agreed with you, but then what is the role of port priority? all STP port has default priority is 128, what does it mean?

is STP port modes depend on port priority?

Thanks

Alpesh

[azibnaseem]

Hey morialpesh008  

First of all sorry for the late reply. After electing the root bridge STP has to be choose the root port to the root bridge & the lowest port-priority become the root port & other redundant port will be blocked. So port-priority is used to elect the root port & block the the redundant link in the network.

Regards'

[morialpesh008]

Hello Azi,

   can u give me an example or topology based on port priority. i mean in which circumstances port priority role will come.

Rgds,

[azibnaseem]

Hey morialpesh008  

The best way I can explain is that 2 switch are connected to each other using redundant links where Cost is the same, Bridge ID is the same then Port-priority is going to break the tie between both switches in order to select the root port. 

In simulator Port-Priority is the same where as Port Id is different. However on real environment port numbers are added into default port priority. 

I have attached a GNS 3 topology for your review and also attached screen shot from real switch.

Kind Regards'

[paul driver]

Hello

All switchs have the same default BID priority but it looks like Switch 3 has the lowest base mac address thus will become root and both its links become designated

Switch 0/2 are directly connected to the root thus both have and lowest cost to the root
Switch 1 has longest stp cost and highest base mac address to the root via either switch 0/2 but chooses switch 0 link as it root path due its lowest base mac address.

Please review the link regards the stp tcn process regards 802.1d and 802.1w stp convergence -  Here

res
Paul

[mcardinal]

In my deep dive of this subject today, what I found is switch 1 picks Fa0/1 instead of Fa0/2 because the upstream switch's BID is lower.

Additionally, the reason Switch1 blocks port Fa0/2 instead of Switch2 blocking Fa0/2 is because Switch2's BID is lower. The blocked side of the link will always be on the switch with the higher BID.

HTH

Matthew

"When you teach someone what you know, you gain a greater understanding of the subject, which increases your own retention of the knowledge."