CCNA - Spanning-Tree

When 3 switch connect together, make a circle. Without spanning-tree, there will be a loop.

Here is how switches make spanning-tree, it is enabled by default on the switches.

1). Elect the Root bridge, I like to call it Root switch.

- Switches send BID(Bridge ID) out of its connecting ports.

- BID composed with Bridge Priority, Extend System ID and MAC.

- Default BID priority is 32769, but can be changed switch-globally per vlan.

- With BID, switches compare with each other, which who has lowest BID will be the Root bridge, the BOSS.

2). Elect the non-designated port on the other switches.

- There are 3 kinds of port roles, root port, designated port and non-designated port.

- Root port is the port that has the direction toward the root bridge(the BOSS), designated port is just the opposite.

- Non-designated port is the port that the traffic will be blocked on this port except BPDU.

- How to decide which switch's port to be blocked:

- They use port cost and BID to decide. The port cost is the first priority.

- Post cost is the how fast it go toward the Root bridge.

- Here are the cost definitiion: 10G - 2, 1G - 4, 100M - 19, 10M - 100.

3). States of a switch port without configurated portfast.

- Blocking: Only receivce BPDU, no other traffic

- Listening: Send and receive BPDU, preparing to participate in the active topology.

- Learning: Prepares to participate in frame forwarding and begins to populate the MAC address table.

- Forwarding: All traffic allowed.

- Disable: Port is down, administratively disabled.

4). BPDU Timers

- Hello time: By default, 2 seconds to send a BPDU frame. Can be changed between 1 and 10.

- Forward delay: The time spent in the listening and learning state. Default 15 seconds for each state, can be changed between 4 and 30.

- Maximum age: The time a switch port saves BPDU information. 20 seconds by default, but can be changed between 6 and 40.