Understanding Loop Protection For Spanning-Tree Instance Interfaces - Juniper ACX1000 Configuration Manual

ACX Series Universal Access Router Configuration Guide
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Understanding Loop Protection for Spanning-Tree Instance Interfaces

426
Understanding Root Protection for Spanning-Tree Instance Interfaces in a Layer 2
Switched Network on page 429
Understanding BPDU Protection for Spanning-Tree Instance Interfaces on page 423
BPDU Protection on All Edge Ports of the Bridge
Spanning-tree protocol loop protection enhances the normal checks that spanning-tree
protocols perform on interfaces. Loop protection performs a specified action when BPDUs
are not received on a nondesignated port interface. You can choose to block the interface
or issue an alarm when bridge protocol data units (BPDUs) are not received on the port.
The spanning-tree protocol family is responsible for breaking loops in a network of bridges
with redundant links. However, hardware failures can create forwarding loops (STP
loops) and cause major network outages. Spanning-tree protocols break loops by blocking
ports (interfaces). However, errors occur when a blocked port transitions erroneously to
a forwarding state.
Ideally, a spanning-tree protocol bridge port remains blocked as long as a superior
alternate path to the root bridge exists for a connected LAN segment. This designated
port is determined by receiving superior BPDUs from a peer on that port. When other
ports no longer receive BPDUs, the spanning-tree protocol considers the topology to be
loop free. However, if a blocked or alternate port moves into a forwarding state, this
creates a loop.
By default (that is, without spanning-tree protocol loop protection configured), an
interface that stops receiving BPDUs will assume the designated port role and possibly
result in a spanning-tree protocol loop.
By default, a spanning-tree protocol interface that stops receiving bridge protocol data
unit (BPDU) data frames will transition to the designated port (forwarding) state, creating
a potential loop. To prevent a spanning-tree instance interface from interpreting a lack
of received BPDUs as a "false positive" condition for assuming the designated port role,
you can configure one of the following loop protection options:
Configure the router to raise an alarm condition if the spanning-tree instance interface
has not received BPDUs during the timeout interval.
Configure the router to block the spanning-tree instance interface if the interface has
not received BPDUs during the timeout interval.
NOTE: Spanning-tree instance interface loop protection is enabled for all
spanning-tree instances on the interface, but blocks or alarms only those
instances that stop receiving BPDUs.
You can configure spanning-tree protocol loop protection to improve the stability of
Layer 2 networks. We recommend you configure loop protection only on non-designated
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