STP - RSTP
Rapid Spanning Tree Protocol - Cisco's Version = Rapid (Per VLAN) Spanning Tree +
Last updated
Rapid Spanning Tree Protocol - Cisco's Version = Rapid (Per VLAN) Spanning Tree +
Last updated
As we know, classic SPT (IEEE 802.D) and Cisco's PVST+ can take up to 50 seconds for the toplogy to converge. It can take 20 seconds for the Max Age timer to timeout and another 30 seconds (15 seconds each for the listening and learning states) for the port to transition to the forwarding state
For large networks its best to use MSTP, however for small to medium networks without a huge number of VLAN's, Cisco's Rapid PVST+ is what we will run on the switches. All information for Cisco's RAPID PVST + applies to the IEEE standard 802.1w
RSTP servers the same purpose as STP, blocking specific ports to prevent loops
RSTP elects the ROOT BRIDGE with the same rules as STP
RSTP elects the ROOT PORTS with the same rules as STP [the interface with the lowest root cost becomes the root port with the same tie-breakers, neighbour bridge ID and then neighbour port ID
FInally RSTP elects designated ports with the same rules as STP, interface on switch with the lowest root cost will become designated and the interface on the other switch will become non-designated. If there is a tie, the switch with the lowest bridge ID will set its interface to 'designated'
Port costs were updated for RSTP. For speeds faster than 10Gbps, I belive are given a STP cost of 1
RSTP simplifies the STP port states. From the Spanning tree port states table at the beginning, RSTP has joined the "Blocking" and "Disabled" states and has now become the "DISCARDING STATE" and removed the "Listening" state.
If a port is administratively disabled (shutdown) - it will be in the discarding state. This was previously the "disabled" state
If a port is enabled but in a blocking state to prevent L2 loops, it also is in a discarding state. This was previously known as the "Blocking' state.
The original port roles were:
root port
designated port
non-designated port
The root port role remains unchanged in RSTP. The port that is closest (lowest root cost wise) to the ROOT BRIDGE becomes the root port of the switch. Also the ROOT BRIDGE is the only switch that does NOT have a root port
The designated port role remains unchanged in RSTP. The port on a collison domain that sends the best BPDU, is that segments designated port (only one per segment). The only other port is a root port or a non-designated port in classic spanning tree.
The non-designated port role is split into 2 separate roles in RSTP:
The alternate port role
The backup port role - Not one we will see as this comes about when 2 interfaces are connected to the same COLLISION domain - and this will only happen via a hub. The backup port is a discarding port and which receives a better BPDU from another interface on the same switch
Which ports are what ports?
By default on modern Cisco Switches, Rapid PVST is the default STP
Three different link types:
Edge - a port that is connected to an end host. Moves directly to forwarding without negotiation. Sounds like port fast - built into RSTP by default
Point-to-point - A direct connection between switches - the switch detects it automatically, but can configure manually
Shared ports connect to a hub and function in half-duplex - should be detected, but can configure manually. Doubt one will ever see this in a production network !!
SW3(config)# spanning-tree mode rstp
SW3(config-if)# spanning-tree portfast
SW3(config-if)# spanning-tree link-type point-to-point
SW3(config-if)# spanning-tree link-type shared
SW4 (config t)# int range fa0/1 - 2
SW4(config-if-range)# spanning-tree link-type point-to-point
SW4(config-if-range)# int fa0/24
SW4(config-int)# spanning tree portfast
SW4(config-int)# do sh spanning-tree
FROM THE ABOVE COMMAND SW4(config-int)# do sh spanning-tree
