The following is some basic info on STP and RSTP. This list is not comprehensive. Refer to the link at the bottom of the page for more in depth details.
RSTP Priority
Default Priority is 32768 + VLAN ID
For example, if we are using vlan 10, then our default priority is 32768 + 10 = 32778
RSTP Priority can be set from 0 – 61440 in increments of 4096.
RSTP Port Roles
Ports can fill 1 of 4 roles.
| Port Role | Description |
| Root Port | Port closest to the Root Bridge (Switches going to the Root switch) |
| Designated Port | Ports going away from the Root Bridge (To clients) |
| Alternate Port | A “backup” port for the Root port. If the Root Port fails, this port takes over |
| Backup Port | Essentially a backup port for the designated port |
RSTP Port States
A port can be in one of 3 states. Well technically 4 states if you include down/unplugged.
| Port State | Description |
| Discarding | Port discards packets (Alternate and Backup Ports) |
| Learning | Port learns MACs and doesn’t forward data |
| Forwarding | Port forwards data and learns MACs |
RSTP Port Types
There are 3 port Types. Not to be confused with port States or Roles.
| Port Type | Description |
| Point to Point | Switch to Switch |
| Point to Point Edge | Edge of Network. Connected to a PC, Printer etc. |
| Shared | Half Duplex, Port connected to a Hub |
RSTP Timers
There are three RSTP timers. STP has the same timers, but the MaxAge is 10 seconds, and the Forward Delay is used for both Learning and Listening states which takes a total of 30 seconds to complete.
| Timer Name | Default Value | Description |
| Hello Timer | 2 Seconds | Time between Hellos created by Root |
| MaxAge | 6 Seconds (Hello Timer * 3) | How long a Switch should wait before trying to readjust the network |
| Forward Delay | 15 Seconds | Delay used for Learning/Listening in STP. Shouldn’t be necessary if legacy bridges are not used. |
Port Cost
There are technically two types of cost. The newer cost values were introduced so we could use faster ethernet speeds.
By default Cisco switches use the old cost values, but they can be changed to use the new ones.
spanning-tree pathcost method long
| Ethernet Speed | Old Cost | New Cost |
| 10 Mbps | 100 | 2,000,000 |
| 100 Mbps | 19 | 200,000 |
| 1 Gbps | 4 | 20,000 |
| 10 Gbps | 2 | 2000 |
| 100 Gbps | N/A | 200 |
| 1 Tbps | N/A | 20 |
More information can be found at the following link.
https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html