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RSTP question


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

switch topology.jpg


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?



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
**Please Rate All Helpful Responses *


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? 



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
**Please Rate All Helpful Responses **



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