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DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet Switch User Manual
107
Spanning Tree
This Switch supports three versions of the Spanning Tree Protocol; 802.1D-2004 STP compatible, 802.11d-2004
Rapid STP and 802.1q-2005 MSTP. 802.1D STP will be familiar to most networking professionals. However, since
802.1w RSTP has been recently introduced to D-Link managed Ethernet switches, a brief introduction to the
technology is provided below followed by a description of how to set up 802.1D STP and 802.1w RSTP.
802.1w Rapid Spanning Tree
The Switch implements two versions of the Spanning Tree Protocol, the Rapid Spanning Tree Protocol (RSTP) as
defined by the IEEE 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 functions 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 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. 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. Table 6-2 below compares
how the two 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.
802.1w RSTP 802.1D STP Forwarding Learning
Discarding Disabled No No
Discarding Blocking No No
Discarding Listening No No
Learning Learning No Yes
Forwarding Forwarding Yes Yes
Table 3 - 2 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.