Multisource Detection - Enterasys C3G124-24 Configuration Manual

listening if it fails to get a response. This protects against misconfiguration and protocol failure by
the connected bridge.
The Disputed BPDU mechanism protects against looping in situations where there is one way
communication. A disputed BPDU is one in which the flags field indicates a designated role and
learning and the priority vector is worse than that already held by the port. If a disputed BPDU is
received, the port is forced to the listening state. When an inferior designated BPDU with the
learning bit set is received on a designated port, its state is set to discarding to prevent loop
formation. Note that the Dispute mechanism is always active regardless of the configuration
setting of Loop Protection.
Loop Protect operates as a per port, per MST instance feature. It should be set on inter-switch
links. It is comprised of several related functions:
• Control of port forwarding state based on reception of agreement BPDUs
• Control of port forwarding state based on reception of disputed BPDUs
• Communicating port non-forwarding status through traps and syslog messages
• Disabling a port based on frequency of failure events
Port forwarding state in the designated port is gated by a timer that is set upon BPDU reception. It
is analogous to the rcvdInfoWhile timer the port uses when receiving root information in the root/
alternate/backup role.
There are two operational modes for Loop Protect on a port. If the port is connected to a device
known to implement Loop Protect, it uses full functional mode. Otherwise the port operates in
limited functional mode.
Connection to a Loop Protect switch guarantees that the alternate agreement mechanism is
implemented. This means the designated port can rely on receiving a response to its proposal
regardless of the role of the connected port, which has two important implications. First, the
designated port connected to a non-root port may transition to forwarding. Second, there is no
ambiguity when a timeout happens; a Loop Protect event has occurred.
In full functional mode, when a type 2 BPDU is received and the port is designated and point-to-
point, the timer is set to 3 times helloTime. In limited functional mode there is the additional
requirement that the flags field indicate a root role. If the port is a boundary port the MSTIs for
that port follow the CIST, that is, the MSTI port timers are set according to the CIST port timer. If
the port is internal to the region then the MSTI port timers are set independently using the
particular MSTI message.
Message age expiration and the expiration of the Loop Protect timer are both Loop Protect events.
A notice level syslog message is produced for each such event. Traps may be configured to report
these events as well. A syslog message and trap may be configured for disputed BPDUs.
It is also configurable to force the locking of a SID/port for the occurrence of one or more events.
When the configured number of events happen within a given window of time, the port is forced
into blocking and held there until it is manually unlocked via management.

Multisource Detection

Multisource detection is a feature that prevents network disruption due to excessive topology
changes caused by a full duplex port transmitting multiple BPDUs with different source MAC
addresses, and hence different BPDU information.
When a port is point-to-point, the received priority information comes from the most recently
received BPDU. When a port is non-point-to-point, the received information reflects the best
priority information out of all the received BPDUs. Typical scenarios for multisource detection are
when a switch is connected to a device which
Spanning Tree Configuration Summary
Enterasys C3 Configuration Guide 13-3