DGS-3224TGR Gigabit Ethernet Switch User’s Guide
15
Spanning Tree
802.1w Rapid Spanning Tree
The DGS-3224TGR implements two versions of the Spanning Tree Protocol, the Rapid Spanning Tree Protocol (RSTP) as
defined by the IEE 802.1w specification and a version compatible with the IEEE 802.1d STP. RSTP can operate with
legacy equipment implementing IEEE 802.1d, however the advantages of using RSTP will be lost.
The IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1d STP standard. RSTP was developed in
order to overcome some limitations of STP that impede the function of some recent switching innovations, in particular,
certain Layer 3 function that are increasingly handled by Ethernet switches. The basic function and much of the
terminology is the same as STP. Most of the settings configured for STP are also used for RSTP. This section introduces
some new Spanning Tree concepts and illustrates the main differences between the two protocols.
Port Transition States
An essential difference between the two protocols is in the way ports transition to a forwarding state and the in the way this
transition relates to the role of the port (forwarding or not forwarding) in the topology. RSTP combines 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 port state discarding there is no
functional difference, the port is not active in the network topology. The table below compares how the two protocols differ
regarding the port state transition.
Both 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 faster detection of failed links, and thus faster topology adjustment. A drawback
of 802.1d is this absence of immediate feedback from adjacent bridges.
STP/RSTP Comparison
802.1d STP 802.1w RSTP Forwarding? Learning?
Disabled Discarding No No
Blocking Discarding No No
Listening Discarding No No
Learning Learning No Yes
Forwarding Forwarding Yes Yes
Comparing Port States
RSTP is capable of 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, all ports
operating in full-duplex mode are considered to be P2P ports, unless manually overridden through configuration.
