Stp Bridge Global Settings - D-Link DGS-3120-24TC Product Manual

xStack® DGS-3120 Series Managed Switch Web UI Reference Guide
Port Transition States
An essential difference between the three 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. MSTP and RSTP
combine the transition states disabled, blocking and listening used in 802.1D-1998 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/MSTP port state discarding, there is no functional difference, the port is not active in the
network topology. Table 7-3 below compares how the three protocols differ regarding the port state transition.
All three 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-1998 is this absence of immediate feedback from adjacent bridges.
802.1Q-2005 MSTP
Disabled
Discarding
Discarding
Learning
Forwarding
RSTP is capable of a 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/MSTP, all ports operating in full-duplex mode are considered to be P2P ports, unless manually overridden
through configuration.
802.1D-1998/802.1D-2004/802.1Q-2005 Compatibility
MSTP or RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to
802.1D-1998 format when necessary. However, any segment using 802.1D-1998 STP will not benefit from the
rapid transition and rapid topology change detection of MSTP or 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 or MSTP.
The Spanning Tree Protocol (STP) operates on two levels:
1. On the switch level, the settings are globally implemented.
2. On the port level, the settings are implemented on a per-user-defined group of ports basis.

STP Bridge Global Settings

On this page the user can configure the STP bridge global parameters.
To view the following window, click L2 Features > Spanning Tree > STP Bridge Global Settings, as show below:
802.1D-2004 RSTP
Disabled
Discarding
Discarding
Learning
Forwarding
802.1D-1998 STP
Disabled
Blocking
Listening
Learning
Forwarding
78
Forwarding
No
No
No
No
Yes
Learning
No
No
No
Yes
Yes