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Spanning-tree example with explanation

Spanning-Tree roles and states



Spanning-Tree algorithm
1. Elect a Root Bridge (lowest Bridge ID)
2. Elect Root Ports (one root port per bridge)
3. Elect Designated and Non-Designated ports (one designated port per segment)

STP Tiebreakers:
1. Root Path Cost
2. Sender Bridge ID
3. Sender Port ID

Spanning-Tree example topology



Spanning-Tree convergence process explained

1. Elect a Root Bridge (lowest Bridge ID)
Since the priority of all switches is 32768 (32778 = 32768 + VLAN ID 10), the switch with the lowest Base MAC address will become the Root Switch.
Root Bridge = CAT1

2. Elect Root Ports (one root port per bridge)
Every switch (except the Root Bridge) will elect one Root Port.
CAT2 election tiebreakers:
Ports "d" and "f" have the same root path cost (19 = FastEthernet ports used). Root path cost on other ports is higher than 19.
BPDU on both ports is received from the Root Switch = same Sender Bridge ID.
Sender port ID received on port "f" is lower than Sender port ID received on port "d" and thus port "f" becomes the Root Port.
(CAT 1 port F1/0/24 has Priority 0 and this is lower than port F1/0/23 default priority 128).
CAT3 election tiebreakers:
Port "b" has the lowest Root path cost and thus becomes the Root Port.
CAT4 election tiebreakers:
Switch CAT4 has Gig ports, however connected to Fa ports. Spanning-Tree is able to detect the speed of the link, thus the cost of links will be 19.
However, the cost of one of the links (ports "p" and "q") has been manually set to "1".
Port "q" becomes the Root Port because the Root path cost (20) is lower than port "s" (38).

3. Elect Designated and Non-Designated ports (one designated port per segment)
Imagine you are a frame in the middle of the link. What way would you take to reach Root Bridge?
Link "a-b" - "a" becomes DESIGNATED port.
Link "c-d" - "c" becomes DESIGNATED port.
Link "e-f" - "e" becomes DESIGNATED port.
Link "p-q" - "p" becomes DESIGNATED port - lower root path cost.
Link "s-r" - "r" becomes DESIGNATED port - lower root path cost.
Link "t-u" - "u" becomes DESIGNATED port - lower sender bridge ID (001A lower than 0024).

All other ports with unassigned roles become BLOCKED - ports "d", "t" and "s".


Spanning-Tree example topology - solution



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