Spanning Tree; 802.1W Rapid Spanning Tree; Port Transition States; Edge Port - D-Link DES-3010F 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 draw-
back 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.
802.1d STP
Disabled
Blocking
Listening
Learning
Forwarding
Table 6- 2. Comparing Port States
86
Forwarding
No
No
No
No
Yes
Learning
No
No
No
Yes
Yes