300-320 · Question #441
An engineer is designing a network using RSTP. Several devices on the network support only legacy STP. Which outcome occurs?
The correct answer is D. RSTP and STP interoperate, but the fast convergence is not used. RSTP (802.1w) is backward-compatible with legacy STP (802.1D). When an RSTP switch detects a legacy STP BPDU on a port, it automatically falls back to standard 802.1D STP behavior on that port. This means the two protocols can interoperate in the same network. However, the…
Question
An engineer is designing a network using RSTP. Several devices on the network support only legacy STP. Which outcome occurs?
Options
- ARSTP and STP choose the protocol with the best performance.
- BRSTP and STP interoperate and fast convergence is achieved.
- CRSTP and STP are not compatible and legacy ports error disable.
- DRSTP and STP interoperate, but the fast convergence is not used.
How the community answered
(32 responses)- A6% (2)
- B3% (1)
- D91% (29)
Explanation
RSTP (802.1w) is backward-compatible with legacy STP (802.1D). When an RSTP switch detects a legacy STP BPDU on a port, it automatically falls back to standard 802.1D STP behavior on that port. This means the two protocols can interoperate in the same network. However, the rapid convergence features of RSTP (such as the Proposal/Agreement handshake and edge port transitions) cannot be used on ports that have fallen back to legacy STP mode. The network will converge, but at the slower 802.1D speed (typically 30–50 seconds) for those segments. Fast convergence is therefore not fully achieved wherever legacy STP devices are present.
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