xStack DES-3500 Series Layer 2 Stackable Fast Ethernet Managed Switch User Manual
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802.1d MSTP
802.1w RSTP
802.1d STP
Forwarding
Learning
Discarding Discarding Disabled No
No
Discarding Discarding Blocking No
No
Discarding Discarding Listening No
No
Learning Learning Learning No
Yes
Forwarding Forwarding Forwarding Yes
Yes
Table 6- 1. Comparing Port States
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/802.1w/802.1s Compatibility
MSTP or RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to 802.1d format
when necessary. However, any segment using 802.1d 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 Loopback Detection
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 loop back to the Switch, such as BPDU packets looped back from an unmanaged switch connected to the DES-
3500 Series switches. To maintain the consistency of the throughput, the DES-3500 Series switches now implement the STP
Loopback Detection function.
When the STP Loopback 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 Loopback on Port #”.
Setting the Loopback Timer
The Loopback timer plays a key role in the next step the switch will take to resolve this problem. Choosing a non-zero value on
the timer will enable the Auto-Recovery Mechanism. When the timer expires, the switch will again look for its returning BPDU
packet on the same port. If no returning packet is received, the switch will recover the port as a Designated Port in the Discarding
State. If another returning BPDU packet is received, the port will remain in a blocked state, the timer will reset to the specified
value, restart, and the process will begin again.
For those who choose not to employ this function, the Loopback Recovery time must be set to zero. In this case, when a BPDU
packet is returned to the Switch, the port will be placed in a blocking state and a message will be sent to the Syslog of the switch.
To recover the port, the administrator must disable the state of the problematic port and enable it again. This is the only method
available to recover the port when the Loopback Recover Time is set to 0.
