DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet Switch User Manual
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This Switch supports three versions of the Spanning Tree Protocol; 802.1D-2004 ST P compatible, 802.11d-2004
Rapid STP and 802.1q-2005 MSTP. 802.1D STP will be familiar to most net working professionals. Howev er, since
802.1w RSTP has been recently introduced to D-Link managed Ethernet switches, a brief introduct ion to the
technology is provided below followed by a description of how to set up 80 2.1D STP and 802.1w RSTP.
802.1w Rapid Spanning Tree
The Switch implements two versions of the Spanning Tree Protocol, the Rapid Spanning T ree Protocol (RSTP) as
defined by the IEEE 802.1w specification and a version compatible with the IEEE 802.1D STP. RSTP can oper ate
with legacy equipment implementing IEEE 802.1D, however the advantag es of using RSTP will be lost.
The IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1D STP standard. R STP 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 b y Ethernet switches. The basic
function and much of the terminology is the same as STP. Most of the settings configured for ST P are also used for
RSTP. This section introduces some new Spanning Tree concepts and illustrates the main dif ferences between the
two protocols.
Port Transition States
An essential difference between the three protocols is in the way ports transition to a f orwarding stat e and in the wa y
this transition relates to the role of the port (forwarding or not forwarding) in the topo logy. RSTP combines the
transition states disabled, blocking and listening used in 802. 1D and creates a single state D iscardin g. In e ither cas e,
ports do not forward packets. In the STP port transition states disable d, block ing or lis teni ng or i n the RST P port s tat e
discarding, there is no functional difference, the port is not active in the net work topolog y. Table 6-2 below com pares
how the two protocols differ regarding the port state transition.
All three protocols calculate a stable topology in the same wa y. Every segment will have a sing le path to the root
bridge. All bridges listen for BPDU packets. However, BPDU pack ets are sent more frequently - with every Hello
packet. BPDU packets are sent even if a BPDU packet was not received. Therefor e, each link between bridges is
sensitive to the status of the link. Ultimately this difference results i n faster detection of failed links, and thus faster
topology adjustment. A drawback of 802.1D is this absence of immediate feed back 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 com pliant bridge l inks. Por ts do not need to wait for the
topology to stabilize before transitioning to a forwarding state. I n order to allow this rapid transition, the pr otocol
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 connec ted to a segment where a loop
cannot be created. An example would be a port connected directl y to a single w orkstation. Ports that are designated
as edge ports transition to a forwarding state immediately without going through the lis tening and lear ning states. An
edge port loses its status if it receives a BPDU packet, immediately becom ing a normal spanning tree port.