Enterasys Matrix 2G4072-52 Configuration Manual page 455

Both upstream and downstream facing ports are protected. When a root or alternate port loses its
path to the root bridge due to a message age expiration it takes on the role of designated port. It will
not forward traffic until a BPDU is received. When a port is intended to be the designated port in
an ISL it constantly proposes and will not forward until a BPDU is received, and will revert to
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.
Matrix DFE-Platinum and Diamond Series Configuration Guide
Spanning Tree Configuration Summary
Loop Protect
6-3