802.1W Rapid Spanning Tree; Port Transition States - D-Link DGS-3024 Manual

1. A configuration name defined by an alphanumeric string of up to 32 characters (defined in the Current MST
Configuration Identification window in the Configuration Name field).
2. A configuration revision number (named here as a Revision Level (0-65535) and found in the Current MST
Configuration Identification window) and;
3. A 4096-element table (defined here as a VID List in the Current MST Configuration Identification window)
that 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 MSTP (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 Table window when configuring the settings for an MSTI ID).
3. VLANs that will be shared must be added to the MSTP Instance ID (defined here as a VID List in the Current
MST Configuration Identification window when configuring the settings for an MSTI ID).

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 6-1 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 draw-
back of 802.1d is this absence of immediate feedback from adjacent bridges.
802.1d MSTP
Discarding
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-3024 Gigabit Ethernet Switch Manual
802.1w RSTP
Discarding
Discarding
Discarding
Learning
Forwarding
Table 7- 1. Comparing Port States
802.1d STP
Disabled
Blocking
Listening
Learning
Forwarding
35
Forwarding
No
No
No
No
Yes
Learning
No
No
No
Yes
Yes