RSTP provides rapid connectivity following the failure of a switch, a switch port, or a LAN.
A new root port and the designated port on the other side of the bridge transition to
forwarding through an explicit handshake between them. RSTP allows switch port
con?¬?guration so that the ports can transition to forwarding directly when the switch
reinitializes. Figure 2-30 shows an RSTP topology.
Figure 2-30 RSTP Topology
Per VLAN RSTP
The RSTP (802.1w) standard uses CST, which assumes only one spanning-tree instance for
the entire switched network, regardless of the number of VLANs. Per VLAN Rapid
Spanning Tree Plus (PVRST+) de?¬?nes a spanning-tree protocol that has one instance of
RSTP per VLAN.
Multiple Spanning Tree Protocol
Multiple Spanning Tree Protocol (MSTP), originally de?¬?ned in IEEE 802.1s and later
merged into IEEE 802.1Q-2003, de?¬?nes a spanning-tree protocol that has several spanningtree
instances running for the network. But unlike PVRST+, which has one instance of
RSTP per VLAN, MSTP reduces the switch load by allowing a single instance of spanning
tree to run for multiple VLANs.
100Base-T
10Base-T
Designated
Port (F)
Designated
Port (F)
Root Bridge
Port 1
Designated
Port (F)
Port 1
Alternate
Port (DIS)
Root Port (F)
Port 0
Root Port (F)
Port 0
Switch Z
Switch X Switch Y
Port 1 Port 0
60 Chapter 2: Medium-Sized Switched Network Construction
RSTP Port Roles
RSTP de?¬?nes the port roles as follows:
?– Root: A forwarding port elected for the spanning-tree topology.
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