Explanation and translation of the Spanning Tree Protocol (SpanningTree) FLASH

CISCO's Spanning tree Protocol STP (Spanning-tree Protocol) explains FLASH animation, which is very helpful for understanding STP. I took the time to translate it.

The following content is manually translated by Slyar. The English is not very good. If there is any error, please forgive me...

Click 1 of 21:
Suppose we have three bridges: bridge A, bridge B, and bridge C.

Click 2 of 21:
Any bridge has a link.

Click 3 of 21:
When the bridge starts, all the bridges think they are the root bridge.

Click 4 of 21:
Assume that C sends a bpdu to A and B at the beginning, declare that C is the root bridge, and the root ID is "32768.00-00-00-00-00-02 ", result The root ID of B is changed from 03 to 02, while the root ID of A remains unchanged.

Click 5 of 21:
Considering that the bridge ID of B is greater than the root ID he sees from the BPDU, B considers C as the root. The bridge ID of A is smaller than the root ID he sees from the BPDU, so A still thinks that he is the root. C, of course, thinks that he is the root.

Click 6 of 21:
Assume that the next step is to send a bpdu to B and C, declare that A is the root bridge, and the root ID is "32768.00-00-00-00-01 ", result The root ID of both B and C is changed to 01.

Click 7 of 21:
All bridges reach A consensus that A is the root bridge.

Click 8 of 21:
You must select a root port for each non-root bridge. The root port of a bridge is the port closest to the root bridge. Non-root bridges elect their respective root Ports Based on "root path overhead". "root path overhead" is the sum of the link overhead of data packets arriving at the root bridge.

Click 9 of 21:
Assume that the link between each bridge has the same bandwidth (according to the IEEE Standard, the bandwidth overhead of Mbps is 19), and B and C start to elect their root ports.
(Animation: the root ports of B and C are port1/1)

Click 10 of 21:
Election of the specified port. Select the port with the minimum overhead of each network segment to reach the root bridge path as the specified port.

Click 11 of 21:
A bridge that contains a specified port in a specific network segment is considered to be the specified Bridge of the network segment.

Click 12 of 21:
There are three CIDR blocks: CIDR Block 1, CIDR Block 2, and CIDR Block 3.
Because the ports on A are directly connected to the root bridge, these ports become the specified ports of CIDR Block 1 and CIDR Block 2.

Click 13 of 21:
CIDR Block 3
Because the overhead of the path from B and C to the root bridge is the same, the winning side is the C with a smaller bridge ID.

Click 14 of 21:
Because the bridge ID of C is smaller than the bridge ID of B, the specified port of CIDR Block 3 is port 1/2 on C.

Click 15 of 21:
The root port and the specified port enter the forwarding status.

Click 16 of 21:
The ports that are neither the root port nor the specified port are blocked.

Click 17 of 21:
In this case, the Spanning Tree is fully integrated.

Click 18 of 21:
The priority of BPDU sent by C is higher than that sent by B.
C continuously sends BPDU broadcasts with a priority higher than B.
As long as B continuously receives these high-priority BPDU on port 1/2, the port remains congested.

Click 19 of 21:
For whatever reason, if B does not receive these BPDU within the maximum lifetime (20 seconds by default), it will start to switch to the forwarding status.
Most of the Spanning Tree algorithms define that when a large number of BPDU are lost, the port will be switched from blocked to forwarded.

Click 20 of 21:
An error occurred while generating the tree.
If the blocked port is switched to the forwarding status, the Spanning Tree fails.

Click 21 of 21:
Some situations where the port is switched from blocking to forwarding due to the loss of BPDU:
Rate mismatch
Single Link
Packet destruction
Resource Error
Portfast configuration error
STP Parameter Tuning and Diameter Issues
Software Error

Flash video viewing: