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DES-3200-10/18/28/28F Layer 2 Ethernet Managed Switch User Manual
Port Transition States
An essential difference between the three protocols is in the way ports transition to a forwarding state and in the way
this transition relates to the role of the port (forwarding or not forwarding) in the topology. MSTP and RSTP combine
the transition states disabled, blocking and listening used in STP and creates a single state Discarding. In either case,
ports do not forward packets. In the STP port transition states disabled, blocking or listening or in the RSTP/MSTP
port state discarding, there is no functional difference, the port is not active in the network topology. The table below
compares how the three protocols differ regarding the port state transition.
All three protocols calculate a stable topology in the same way. Every segment will have a single path to the root
bridge. All bridges listen for BPDU packets. However, BPDU packets are sent more frequently - with every Hello
packet. BPDU packets are sent even if a BPDU packet was not received. Therefore, each link between bridges is
sensitive to the status of the link. Ultimately this difference results in faster detection of failed links, and thus faster
topology adjustment. A drawback of STP is this absence of immediate feedback from adjacent bridges.
MSTP RSTP STP Forwarding Learning
Disabled Disabled Disabled No No
Discarding Discarding Blocking
No No
Discarding Discarding Listening
No No
Learning Learning Learning
No
Yes
Forwarding Forwarding Forwarding Yes Yes
Table 3 - 1. Comparing Port States
RSTP is capable of a more rapid transition to a forwarding state - it no longer relies on timer configurations - RSTP
compliant bridges are sensitive to feedback from other RSTP compliant bridge links. Ports do not need to wait for the
topology to stabilize before transitioning to a forwarding state. In order to allow this rapid transition, the protocol
introduces two new variables: the edge port and the point-to-point (P2P) port.
Edge Port
The edge port is a configurable designation used for a port that is directly connected to a segment where a loop
cannot be created. An example would be a port connected directly to a single workstation. Ports that are designated
as edge ports transition to a forwarding state immediately without going through the listening and learning states. An
edge port loses its status if it receives a BPDU packet, immediately becoming a normal spanning tree port.
P2P Port
A P2P port is also capable of rapid transition. P2P ports may be used to connect to other bridges. Under RSTP/MSTP,
all ports operating in full-duplex mode are considered to be P2P ports, unless manually overridden through
configuration.
STP/RSTP/MSTP Compatibility
MSTP or RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to
STP format when necessary. However, any segment using STP will not benefit from the rapid transition and rapid
topology change detection of MSTP or RSTP. The protocol also provides for a variable used for migration in the event
that legacy equipment on a segment is updated to use RSTP or MSTP.
The Spanning Tree Protocol (STP) operates on two levels:
1. On the switch level, the settings are globally implemented.
2. On the port level, the settings are implemented on a per user-defined group of ports basis.
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