BPDU Differences in RSTP
In regular STP, BPDUs are originated by the root and relayed by each
switch. In RSTP, each switch originates BPDUs, whether or not it receives a
BPDU on its root port. All eight bits of the BPDU type field are used by
RSTP. The TC and TC Ack bits are still used. The other six bits specify the
port’s role and its RSTP state, and are used in the port handshake. The RSTP
BPDU is set to Type 2, Version 2. PVST is done by Rapid PVST+ on
Catalyst switches.
RSTP Fast Convergence
The Rapid Spanning tree process understands and incorporates topology
changes much quicker than the previous version.
■ RSTP uses a mechanism similar to BackboneFast—When an inferior
BPDU is received, the switch accepts it. If the switch has another path
the root, it uses that and informs its downstream switch of the alternate
path.
■ Edge ports work the same as Portfast ports—They automatically
transition directly to forwarding.
■ Link type—If you connect two switches through a point-to-point link
and the local port becomes a designated port, it exchanges a handshake
with the other port to quickly transition to forwarding. Full-duplex
links are assumed to be point-to-point, half-duplex links are assumed
to be shared.
■ Backup and alternate ports—Ports that can transition to forwarding
when no BPDUs are received from a neighbor switch (similar to
UplinkFast).
If an RSTP switch detects a topology change, it sets a TC timer to twice the
hello time and sets the TC bit on all BPDUs sent out to its designated and
root ports until the timer expires. It also clears the MAC addresses learned
on these ports.
If an RSTP switch receives a TC BPDU, it clears the MAC addresses on that
port and sets the TC bit on all BPDUs sent out its designated and root ports
until the TC timer expires.