W Rapid Spanning Tree; Port Transition States - D-Link xStack DES-3800 Series User Manual

Each switch utilizing the MSTP on a network will have a single MSTP configuration that will have the following three attributes:
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 7-1 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 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.
xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
802.1w RSTP
Disabled
Discarding
Discarding
Learning
Forwarding
Figure 7- 31. Comparing Port States
802.1d STP Forwarding
Disabled No
Blocking No
Listening No
Learning No
Forwarding Yes
98
Learning
No
No
No
Yes
Yes