Port Transition States - D-Link DES-6500 User Manual

D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
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. 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
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.
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.
802.1w
Forwarding?
RSTP
Discarding
Discarding
Discarding
Learning
Forwarding Yes
Comparing Port States
34
Learning?
No No
No No
No No
No Yes
Yes