W Rapid Spanning Tree; Port Transition States - D-Link DES-3526 Manual

one virtual bridge that runs a single spanning tree. Consequentially, frames assigned to different
VLANs will follow different data routes within administratively established regions on the network,
continuing to allow simple and full processing of frames, regardless of administrative errors in
defining VLANs and their respective spanning trees.
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
STP Bridge Global Settings window in the Configuration Name field).
2. A configuration revision number (named here as a Revision Level and found in the STP
Bridge Global Settings window) and;
3. A 4096-element table (defined here as a VID List in the MST Configuration Table 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 MST Configuration Table 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 Table 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 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 drawback of 802.1d is
this absence of immediate feedback from adjacent bridges.
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