Spanning Tree Basics – Part 2 – The STP root

Posted on Posted in Routing and Switching

In part one we looked at the simplest spanning tree decision that a switch can make when it has a single up-link to multiple switches. In part two I will look at spanning tree from the perspective of the root bridge

The spanning tree root bridge in the image above is CORE-1. The root bridge can be selected for all VLANs in a STP domain or a single VLAN. This can be achieved using the following command:

On the spanning tree root ALL ports will always be in a forwarding state.

The image above displays “this bridge is the root”. This is the main indicator that a switch is the spanning tree root for a STP domain or selected VLANs. Notice that all ports are in the designated (DESG) and forwarding (FWD) state.

The spanning tree root’s ports will ALWAYS be forwarding and labeled as designated

Now let’s look at CORE-2 to see its understanding of the spanning tree domain.

CORE-2 knows that it is NOT the spanning-tree root. How can this be identified from the output above?

First, “this bridge is the root” is NOT displayed in the output. The second indicator is that the root ID /MAC address and the bridge ID are NOT identical.

The MAC address of the “root” ID is that of CORE-1 and the “bridge” ID is that of CORE-2. The 3rd indicator is the port, G0/0 is the outgoing/incoming interface where BPDUs will be received, it is also the up-link to the spanning-tree root.

The fourth indicator is the fact that some of the interfaces are in the blocking state. The STP root will NEVER have ports in a blocking state.

Now CORE-2 has two up-links to CORE-1, at this time the ports are not part of 802.3ad so both ports CANNOT be in a forwarding state. One port MUST be blocked. The decision making process on which port to block can be found here. CORE-2 has to do the following:

  1. The interface associated to lowest path cost is more preferred.
  2. The interface associated to the lowest system priority of the advertising switch is preferred next.
  3. The interface associated to the lowest system MAC address of the advertising switch is preferred next.
  4. When multiple links are associated to the same switch, the lowest port priority from the advertising switch is preferred.
  5. When multiple links are associated to the same switch, the lower port number from the advertising switch is preferred.

Again, let’s look at the output of the “show spanning-tree vlan 1979 detail” command.




4. When multiple links are associated to the same switch, the lowest port priority from the advertising switch is preferred.

CORE-2 has two up-links to CORE-1, in this case criterion # 4 was met so port G0/0 became the root port and is moved to the forwarding state.

CORE-2’s root port and blocking port will cause the connection to CORE-1 will look like this:


Conclusion – spanning-tree is a “tricky” technology, in my opinion the best way to understand the forwarding logic is create a diagram of the network and go through each device to see how and why a particular port was chosen.

In spanning-tree part 3 I will look at the topology form the DL-1’s view point.

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