Spanning Tree Notes

This is part of the Cisco Notes series on Mike’s World News.

Spanning-Tree Protocol or STP is IEEE standard 802.1d.
Rapid Spanning-Tree Protocol or RSTP is IEEE standard 802.1w.

The first manufactured switch will be, by default, the root switch in a Spanning Tree.  This is because it has the lowest bridge ID (lowest priority plus mac address) is the root switch.  If all switches are using the default priority of 32768, then the switch with the lowest MAC address is the root switch.

With Spanning-Tree, LOWER IS ALWAYS BETTER.

The Root port is used on all non-root switches to reach root.  There is one root port per switch.  This is the best/fast way to get back to root according to the cost to get there.

Designated port

The Alternate port (in RSTP) or the Blocking Port (STP) is a backup Root port.  If the path used by the Root port goes down, then STP will switch to use the Alternate/Blocking port.

Different bandwidths have different default costs associated with them

Bandwith STP Default Cost
 10 MB  100
 100 MB  19
1 GB 4
10 GB 2

VLAN = Broadcast domain

BPDU are sent out every 2 seconds.

Watch Improving Performance with Spanning Tree on Cisco CCNA TV

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