Spanning Tree; 802.1W Rapid Spanning Tree; Port Transition States; Edge Port - D-Link des-3010f User Manual

DES-3010F/DES-3010FL/DES-3010G/DES-3018/DES-3026 Fast Ethernet Switch Manual

Spanning Tree

This Switch supports two versions of the Spanning Tree Protocol; 802.1d STP and 802.1w Rapid STP. 802.1d STP will be
familiar to most networking professionals. However, since 802.1w RSTP has been recently introduced to D-Link managed
Ethernet switches, a brief introduction to the technology is provided below followed by a description of how to set up 802.1d STP
and 802.1w RSTP.

802.1w Rapid Spanning Tree

The Switch implements two versions of the Spanning Tree Protocol, 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. RSTP combines 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 port state discarding, there is no functional
difference, the port is not active in the network topology. Table 6-2 below compares how the two 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.
802.1w RSTP
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.

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, all ports operating
in full-duplex mode are considered to be P2P ports, unless manually overridden through configuration.
802.1d STP Forwarding
Disabled No
Blocking No
Listening No
Learning No
Forwarding Yes
Table 6- 2. Comparing Port States
84
Learning
No
No
No
Yes
Yes