Port Transition States; 802.1D/802.1W Compatibility; Stp Switch Settings - D-Link DGS-3324SRi User Manual

D-Link DGS-3324SRi Intelligent Stackable Gigabit Ethernet Switch

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.
P2P Port
A P2P port is also capable of rapid transition. P2P ports may be used to connect to other bridges. Under RSTP,
all ports operating in full-duplex mode are considered to be P2P ports, unless manually overridden through
configuration.

802.1d/802.1w Compatibility

RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to 802.1d
format when necessary. However, any segment using 802.1 STP will not benefit from the rapid transition and
rapid topology change detection of RSTP. The protocol also provides for a variable used for migration in the
event that legacy equipment on a segment is updated to use RSTP.

STP Switch Settings

The Spanning Tree Protocol (STP) operates on two levels: on the switch level, the settings are globally
implemented. On the port level, the settings are implemented on a per user-defined group of ports basis. To open
the following window, open the Spanning Tree folder in the Configuration menu and click the STP Switch
Settings link.
802.1w RSTP Forwarding?
Discarding No
Discarding No
Discarding No
Learning No
Forwarding Yes
Comparing Port States
35
Learning?
No
No
No
Yes
Yes