W Rapid Spanning Tree; Port Transition States - D-Link DGS-3700-12 User Manual

1. A configuration name defined by an alphanumeric string of up to 32 characters (defined in the MST
Configuration Identification window in the Configuration Name field).
2. A configuration revision number (named here as a Revision Level and found in the MST Configuration
Identification window) and;
3. A 4096-element table (defined here as a VID List in the MST Configuration Identification window), which
will associate each of the possible 4096 VLANs supported by the Switch for a given instance.
To utilize the MSTP function on the Switch, three steps need to be taken:
1. The Switch must be set to the MSTP setting (found in the STP Bridge Global Settings window in the STP
Version field).
2. The correct spanning tree priority for the MSTP instance must be entered (defined here as a Priority in the
STP Instance Settings window when configuring an MSTI ID settings).
3. VLANs that will be shared must be added to the MSTP Instance ID (defined here as a VID List in the MST
Configuration Identification window when configuring an MSTI ID settings).

802.1w Rapid Spanning Tree

The Switch implements three versions of the Spanning Tree Protocol, the Multiple Spanning Tree Protocol (MSTP) as
defined by the IEEE 802.1s, 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. 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 discarding, there is no functional difference, the port is not active in the network topology.
Table 4-2 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.
802.1s 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.
DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet User Manual
802.1w RSTP
Disabled
Discarding
Discarding
Learning
Forwarding
Table 4 - 2. Comparing Port States
802.1D STP Forwarding
Disabled No
Blocking No
Listening No
Learning No
Forwarding Yes
86
Learning
No
No
No
Yes
Yes