Spanning Tree Protocol; 802.1W Rapid Spanning Tree - D-Link DES-3326SR Manual

Spanning Tree Protocol

802.1w Rapid Spanning Tree

Configure STP
The Switch supports 802.1d Spanning Tree Protocol (STP) and 802.1w Rapid Spanning Tree Protocol (RSTP).
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 802.1 d 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 IEE 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 function 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 two protocols is in the way ports transition to a forwarding state and the 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. The Comparing Port States table below compares how the two protocols differ regarding the
port state transition.
Both 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 are sensitive to the status of the link. Ultimately this difference results faster detection of failed links, and
thus faster topology adjustment. A drawback of 802.1d is this absence of immediate feedback from adjacent
bridges.
802.1d STP
Disabled
Blocking
Listening
Learning
Forwarding
RSTP is capable of 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.
D-Link DES-3326SR Layer 3 Switch
Table 1. Comparing Port States
802.1w RSTP Forwarding?
Discarding
Discarding
Discarding
Learning
Forwarding
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Chapter 9
Learning?
No No
No No
No No
No Yes
Yes Yes