About how to understand and obtain evidence of the BackboneFast mechanism of STP to process inferior BPDU

Understand the goal of the Spanning Tree (STP) BackboneFast mechanism for processing inferior BPDU:

1. What is an inferior BPDU?

2. How does BackboneFast process inferior BPDU?

3. effect of the BackboneFast mechanism on the processing of inferior BPDU

What is "bad BPDU ":

In the traditional 802.1D Spanning Tree, if you receive a BPDU that identifies itself as both a root switch and a designated switch, it is called a "bad BPDU". When you receive a "bad BPDU, this indicates that the non-direct connection link of the vswitch is faulty. As shown in Figure 7.69, link A of the vswitch S2 connected to the root bridge S1 fails. At this time, this is A non-direct connection link fault for the vswitch S3, then switch S2 will lose the connection with the root bridge, and then send the BPDU to switch S3, indicating that it is the root bridge, which is the inferior BPDU information, but now switch S3 knows that the root bridge S1 is in good state. It sends an inferior BPDU message to switch S2, and the maximum survival time is 20 seconds by default.) before expiration, ignore the bad BPDU messages sent from vs2, which wastes 20 seconds of convergence latency.

650) this. width = 650; "src =" http://www.bkjia.com/uploads/allimg/131227/0625391040-0.png "title =" 1.png"/>

Therefore, the IEEE 802.1w Organization decided to integrate a BackboneFast mechanism in RSTP. When its port receives a "bad BPDU" message, it immediately accepts it and replaces the current BPDU storage, it does not need to wait for 20 seconds, which speeds up RSTP convergence. As shown in the preceding figure, in the 7.69 environment, switch S3 does not wait for 20 seconds after receiving the BPDU message from S2, when the BPDU message is processed immediately, switch S2 stops sending its own BPDU and considers the port connected to switch S3 as its new root port.

Suggestions for starting and validating the BackboneFast mechanism:

When the BackboneFast mechanism is enabled, you must use the spanning-tree backbonefast command to start it in global configuration mode of each switch in the L2 loop, you can set up an experimental environment for making three switches into loops to obtain the effect of the BackboneFast mechanism. First, debug the switch generation tree event on switch S3 in Figure 7.69, in this way, you can clearly see the status transfer effect of the S3 port. Then, when the BackboneFast mechanism on all switches is not enabled, you can try to cut off the non-direct connection link, as shown in link A in Figure 7.79, at this point, we can see the following Debug event on switch S3, which clearly shows inferior BPDU, but switch S3 does not care about it, it has lasted for a long period of time, to listen, learn, and forward.

650) this. width = 650; "src =" http://www.bkjia.com/uploads/allimg/131227/06253915G-1.png "title =" 2.png"/>

Then, start the BackboneFast mechanism on each switch in the loop and try to cut off the non-direct connection link again, as shown in Figure 7.79. In this case, you can see the following Debug event on switch S3. It is obvious that when the switch receives only one inferior BPDU, it starts to turn to the status, you do not need to wait for the maximum lifetime of 20 s.

650) this. width = 650; "src =" http://www.bkjia.com/uploads/allimg/131227/0625392Q4-2.png "title =" 3.png"/>

This article is from the "unknown Christ" blog. For more information, contact the author!