STP Discussion

[ITexpert]

Hello Guys,

@Julio E. Moisa @Richard Burts @Joseph W. Doherty @Francesco Molino 

After reading about working of Spanning tree protocol,  I get to know that Switch with lowest Priority ( or lowest MAC) will get elected as Root Bridge and all switches will forward traffic towards Root Bridge .

The ports of all switches which is used to reach Root Bridge are called Root ports and will be in Forwarding state , other ports on switch which create loop will be blocked but listening BPDU's .

In terms of root port election, the port with lowest priority and with lowest cost(means more speed) and if same then lowest port no. will act as root port.  I have questions on this:

1.  what if one port has more priority with lowest cost and other port has higher cost but lowest priority,  which will be root port?

2. what is the order of consideration of port priority vs Cost vs port no. in root port election.

what if we make core switch stack (consisting of two switches ) as root bridge and then each ACL Sw is connected to both switches and STP is blocking  one port on each ACL SW.  then we have chain network from one of ACL SW and loop exist on that.  How STP handles that... I mean to ask does there will be second root bridge for that loop or only our core Sw will act as Root Bridge ?

Thanks 

[paul driver]

Hello

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@ITexpert wrote:

Hello Guys,

@Julio E. Moisa @Richard Burts @Joseph W. Doherty @Francesco Molino 

After reading about working of Spanning tree protocol,  I get to know that Switch with lowest Priority ( or lowest MAC) will get elected as Root Bridge and all switches will forward traffic towards Root Bridge .

The ports of all switches which is used to reach Root Bridge are called Root ports and will be in Forwarding state , other ports on switch which create loop will be blocked but listening BPDU's .

In terms of root port election, the port with lowest priority and with lowest cost(means more speed) and if same then lowest port no. will act as root port.  I have questions on this:

1.  what if one port has more priority with lowest cost and other port has higher cost but lowest priority,  which will be root port? 
Port priority effects any downstream switch connected to that port meaning it is only valid between its directly connect device

Port cost is cumulative throughout stp domain so this will effect the not only the local switch how it elects it root port but also others switches 

2. what is the order of consideration of port priority vs Cost vs port no. in root port election.

Port Cost effects stp domain - Port priority is only locally significant

what if we make core switch stack (consisting of two switches ) as root bridge and then each ACL Sw is connected to both switches and STP is blocking  one port on each ACL SW.  then we have chain network from one of ACL SW and loop exist on that.  How STP handles that... I mean to ask does there will be second root bridge for that loop or only our core Sw will act as Root Bridge ?

Not sure i understand this question? - can you elaborate?

Thanks 

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[ITexpert]

Hello @paul driver,

[paul driver]

Hello

You need check why the loop is occurring I would say investigate between acl 5 -6 

[Richard Burts]

The drawing is pretty clear that from ACL5 there are two paths toward the root bridge, one path through ACL4 and the other path through ACL6. Both of those paths go through ACL1 and that creates a potential loop. Spanning tree should recognize this and will cause ACL5 to put one of those ports into blocking state.

HTH

Rick

[ITexpert]

Hello @Richard Burts @paul driver @Julio E. Moisa

I know that STP will block one port, So it means ACL6 will make root port which has low cost to reach root Bridge which (Which is core stack) and as this is not direct connection with corestack so port priority in root selection will not effect as it is not direct connection. Is it correct ?

the other question,  when STP will block any port , TCN will generated from ACL6, does that TCN's will send to all switches as BPDU's or regular packets ?  does that will make any UP-DOWN on edge ports at ACL 6 ?

Thanks

[paul driver · Accepted solution]

Hello

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@ITexpert wrote:

Hello @Richard Burts @paul driver @Julio E. Moisa

I know that STP will block one port, So it means ACL6 will make root port which has low cost to reach root Bridge which (Which is core stack) and as this is not direct connection with corestack so port priority in root selection will not effect as it is not direct connection. Is it correct ? -Port priority is locally significant 

the other question,  when STP will block any port , TCN will generated from ACL6, does that TCN's will send to all switches as BPDU's or regular packets ?  does that will make any UP-DOWN on edge ports at ACL 6 ?

Depends on what mode of stp you are using -
802.1d  tcn-bpdus are sent from the root switch which will flood to all it downstream switches

802.1w (rstp) doesnt work this way -tcn bpdu are not applicable, edge ports transitioning into a forwarding state dont initiate a tcn the same way as in 802.1d .

In this mode only non edge ports will cause a tcn  and the affected switch itself can flood this tcn throughout the network and not rely on the root switch do it

 - 

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