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Rapid Spanning Tree Protocol
- Rapid Spanning Tree Protocol (RSTP) is introduced to reduce the convergence time upon
network topology changes. RSTP uses the same process as STP in selecting the root bridge, root
ports, and designated ports, as well as the same rules applied when determining the blocking and
forwarding states of switch ports. RSTP only works in switches that support it and is
backward-compatible with switches that only support traditional STP, just that the RSTP fast
convergence ability is sacrificed or lost when it interoperates with STP.
- Link ports are connections between 2 switches.
Edge ports are connections to end systems, eg: PC, printer. No bridging loop can ever occur.
Switch Switch (full-duplex) Link-type Point-to-point (P2p Peer)
Switch Hub (half-duplex) Link-type Shared (Shr Peer))
Switch PC (full-duplex) Edge-type Point-to-point (Edge P2p)
Switch PC (half-duplex) Edge-type Shared (Edge Shr)
- RSTP works same as STP, but does not work in hub-based networks – link-type shared; it only
works in switched networks – link-type point-to-point and edge-type point-to-point.
- STP and RSTP port states:
Operational State STP State (802.1D) RSTP State (802.1w)
Disabled Disabled Disabled
Enabled Blocking Discarding
Enabled Listening Discarding
Enabled Learning Learning
Enabled Forwarding Forwarding
- RSTP added 2 more port roles. Both types of port do not forward traffic (discarding state).
Alternate Port Alternate path to the root bridge (backup of the root port). The port that
received the second best BPDU from another switch. Can be immediately
placed in forwarding state and act as the root port if the current root port fails.
Backup Port Backup of the designated port for a segment. Exists only when a switch
has 2 or more connections to a shared LAN segment (connect using a hub).
The port that received the same BPDU from the same switch. Only either
one of the links will be placed in forwarding state. Can be immediately
placed in forwarding state if the other link fails.
SW1
Designated
Port
Fa0/1
Fa0/2
Designated
Port
Root
Port
Root
Port
Fa0/1
Hub
Fa0/1
SW2
Fa0/2
Designated
Port
SW3
Fa0/3
Alternate
Fa0/2 Port
Backup
Port
Figure 5-4: RSTP Port Designations
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