D-Link DES-3550 User Manual - Spanning Tree

DES-3550 Layer 2 

Fast Ethernet Switch User’s Guide

51

Spanning Tree 

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 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 two 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 5-7 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 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. 

802.1d STP 

802.1w 

RSTP 

Forwarding?

Learning? 

Disabled Discarding 

No 

No 

Blocking Discarding 

No 

No 

Listening Discarding 

No 

No 

Learning Learning 

No 

Yes 

Forwarding Forwarding 

Yes 

Yes 

Table 6- 1. Comparing Port States