RSTP question

[todorn123]

Hi guys,
Have a question regarding spanning tree and way its supposed to work when there is a redundant path in fiber daisy-chained switches. Root switch for all vlans is connected via fiber link to the first of the daisy-chained switches. Below is same config on each switch in regards to RSTP. See attached for switch topology.
We just installed redundant fiber between switch A and D. Correct me if I am wrong but if RSTP is working right on switch A and switch D, it should put trunk ports between the two to blocked since there is redundant path A<->B <->C<->D.
Thank you!  

spanning-tree mode rapid-pvst
spanning-tree loopguard default
spanning-tree portfast edge default
spanning-tree extend system-id
udld enable 

interface GigabitEthernetX/X/X
switchport trunk allowed vlan X,Y,Z,...
switchport mode trunk

[MHM Cisco World]

from first view 
D-A is one cost calculate 
D-C-B-A is three cost calculate 

so I think that C-B is block not D-A,
please can you config that?

[Martin L]

Its cost to reach Root based on speed of links (or manually set cost); I assume Fiber link is faster then other links, (faster speeds = lower cost which is better),  Sw D Root Cost via A is better then via C and B.

Segment B-C will have 1 port Blocking (Alt mode) based on Cost to Root, then lower MAC (if cost is the same).

Regards, ML
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[todorn123]

Yes, I confirmed that B-C segment have one port blocked, not D-A. Thank you both for your help.

Since we have same fiber link speed (1Gb) and port speed, port cost should equals 4 on all switches A, B, C and D, does RSTP uses MAC address of the switch to calculate the path cost and choose the lower to reach root bridge? 

[Martin L]

Cost is cumulative and it increments by each switch. MACs matter only when there is a tie.  Root Sw sends its cost of 0 to all neighbors.  Next switch down adds its own cost (link to Root) and sends it to his neighbors.  

So, Sw A cost to Root is 19; Sw B cost to Root is 2 times 19 while Sw D cost is 4 times 19 OR 4 + 19.

STP/RSTP tie breaker is as follow:

1. Lowest root path cost to root bridge

2. Lowest sender bridge ID

3. Lowest sender port ID

you could add step 0 for Root elections: lowest Priority of Switch and lowest MAC if priority is the same

Regards, ML
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