RSTP Alternative Port vs Backup Port

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 RoleDescription
Root PortPort closest to the Root Bridge (Switches going to the Root switch)
Designated PortPorts going away from the Root Bridge (To clients)
Alternate PortA “backup” port for the Root port. If the Root Port fails, this port takes over
Backup PortEssentially a backup port for the designated port
RSTP Port Roles

RSTP Port States

A port can be in one of 3 states. Well technically 4 states if you include down/unplugged.

Port StateDescription
DiscardingPort discards packets (Alternate and Backup Ports)
LearningPort learns MACs and doesn’t forward data
ForwardingPort forwards data and learns MACs
RSTP Port States

RSTP Port Types

There are 3 port Types. Not to be confused with port States or Roles.

Port TypeDescription
Point to PointSwitch to Switch
Point to Point EdgeEdge of Network. Connected to a PC, Printer etc.
SharedHalf Duplex, Port connected to a Hub
STP/RSTP Port Types

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 NameDefault ValueDescription
Hello Timer2 SecondsTime between Hellos created by Root
MaxAge6 Seconds (Hello Timer * 3)How long a Switch should wait before trying to readjust the network
Forward Delay15 SecondsDelay 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 SpeedOld CostNew Cost
10 Mbps1002,000,000
100 Mbps19200,000
1 Gbps420,000
10 Gbps22000
100 GbpsN/A200
1 TbpsN/A20
STP/RSTP Path Cost

More information can be found at the following link.