D-Link xStack DGS-3620 Series Reference Manual page 131

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xStack DGS-3620 Series Layer 3 Managed Stackable Gigabit Switch Web UI Reference Guide
802.1D-2004 Rapid Spanning Tree
The Switch implements three versions of the Spanning Tree Protocol, the Multiple Spanning Tree Protocol (MSTP)
as defined by the IEEE 802.1Q-2005, the Rapid Spanning Tree Protocol (RSTP) as defined by the IEEE 802.1D-
2004 specification and a version compatible with the IEEE 802.1D-1998 STP. RSTP can operate with legacy
equipment implementing IEEE 802.1D-1998; however the advantages of using RSTP will be lost.
The IEEE 802.1D-2004 Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1D-1998 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. MSTP and RSTP
combine the transition states disabled, blocking and listening used in 802.1D-1998 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. Table 7-3 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-1998 is this absence of immediate feedback from adjacent bridges.
802.1Q-2005 MSTP
Disabled
Discarding
Discarding
Learning
Forwarding
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-1998/802.1D-2004/802.1Q-2005 Compatibility
802.1D-2004 RSTP 802.1D-1998 STP
Disabled Disabled
Discarding Blocking
Discarding Listening
Learning Learning
Forwarding Forwarding
Forwarding
No
No
No
No
Yes
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Learning
No
No
No
Yes
Yes