Test: CST fundamentals

Check your understanding of Modules 1 and 2.

Intermediate · 10 min read

CST (Common Spanning Tree) is the original IEEE 802.1D form of Spanning Tree, in which a single spanning tree instance runs for the whole switched network, whatever VLANs it carries. Switches exchange BPDUs, elect one root bridge and block redundant ports so that exactly one loop-free path connects any two switches.

In simple terms: Extra cables between switches keep the network up when one fails, but they also create loops. CST chooses one set of paths for all VLANs and keeps the spare ports switched off until they are needed.

Ten questions on Modules 1 and 2. Answer each one and read the explanation. If you get one wrong, go back to the lesson it came from.

Predict · scenario 1

1. Why does a Layer 2 loop cause a broadcast storm?

Predict · scenario 2

2. What does a blocked port still do?

Predict · scenario 3

3. A switch has priority 4096 and the extended system ID for VLAN 30. What priority does show spanning-tree display?

Predict · scenario 4

4. Which bridge ID wins the root election: 32778/…0001 or 28682/…00FF?

Predict · scenario 5

5. With CST, how many root bridges are there for VLANs 10, 20 and 30?

Predict · scenario 6

6. On a switch that is not the root, which port becomes the root port?

Predict · scenario 7

7. Two ends of a link tie on root path cost. What decides the designated port next?

Predict · scenario 8

8. How many root ports does the root bridge have?

Predict · scenario 9

9. With default timers, how long does a newly connected non-PortFast port take to forward?

Predict · scenario 10

10. After the root sets the TC flag, how long do switches keep MAC entries?