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DGS-3024 Gigabit Ethernet Switch 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 802.1d 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 discard-
ing, there is no functional difference, the port is not active in the network topology. Table 6-1 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 802.1d is this absence of immediate feedback from adjacent bridges.
802.1d MSTP 802.1w RSTP 802.1d STP Forwarding Learning
Discarding Discarding Disabled
No
No
Discarding Discarding Blocking No No
Discarding Discarding Listening No No
Learning Learning Learning No Yes
Forwarding Forwarding Forwarding Yes Yes
Table 7- 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.
802.1d/802.1w/802.1s Compatibility
MSTP or RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to 802.1d
format when necessary. However, any segment using 802.1d 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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