D-Link Xstack DES-3528 Series Reference Manual page 100

xStack® DES-3528/DES-3552 Series Layer 2 Managed Stackable Fast Ethernet Switch Web UI Reference Guide
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.
BPDU Loop-back Prevention
When connected to other switches, STP is an important configuration in consistency for delivering packets to ports
and can greatly improve the throughput of your switch. Yet, even this function can malfunction with the emergence
of STP BPDU packets that occasionally loopback to the Switch, such as BPDU packets looped back from an
unmanaged switch connected to the Switch. To maintain the consistency of the throughput, the Switch now
implements the BPDU Loop-back prevention function.
When the BPDU Loop-back Detection function is enabled, the Switch will be protected against a loop occurring
between switches. Once a BPDU packet returns to the Switch, this function will detect that there is an anomaly
occurring and will place the receiving port in an error-disabled state. Consequentially, a message will be placed in
the Switch's Syslog and will be defined there as "BPDU Loop Back on Port #".
Setting the Loop-back Timer
802.1D-2004 RSTP 802.1D-1998 STP
Disabled Disabled
Discarding Blocking
Discarding Listening
Learning Listening
Forwarding Forwarding
Forwarding
No
No
No
No
Yes
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Learning
No
No
No
Yes
Yes