D-Link DES-3326SR, User Manual

Page: 57 / 359

DES-3326SR Layer 3 Fast Ethernet Switch User’s Guide
45
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. 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 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 802.1w RSTP Forwarding? Learning?
Disabled Discarding No No
Blocking Discarding No No