Spanning Tree; 802.1W Rapid Spanning Tree - D-Link DES-3550 User Manual

DES-3550 Layer 2

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
Disabled
Blocking
Listening
Learning
Forwarding
Fast Ethernet Switch User's Guide
802.1w
Forwarding?
RSTP
Discarding No
Discarding No
Discarding No
Learning No
Forwarding Yes
Table 6- 1. Comparing Port States
51
Learning?
No
No
No
Yes
Yes