Juniper JUNOS OS 10.3 - SOFTWARE Manual page 1666

Complete Software Guide for Junos
Overview and Topology
1570
®
OS for EX Series Ethernet Switches, Release 10.3
Before you configure the interface for loop protection, be sure you have:
RSTP operating on the switches.
NOTE: By default, RSTP is enabled on all EX Series switches.
A loop-free network in spanning-tree topologies is supported through the exchange of
a special type of frame called bridge protocol data unit (BPDU). Peer STP applications
running on the switch interfaces use BPDUs to communicate. Ultimately, the exchange
of BPDUs determines which interfaces block traffic (preventing loops) and which
interfaces become root ports and forward traffic.
A blocking interface can transition to the forwarding state in error if the interface stops
receiving BPDUs from its designated port on the segment. Such a transition error can
occur when there is a hardware error on the switch or software configuration error between
the switch and its neighbor. When this happens, a loop opens up in the spanning tree.
Loops in a Layer 2 topology cause broadcast, unicast, and multicast frames to
continuously circle the looped network. As a switch processes a flood of frames in a
looped network, its resources become depleted and the ultimate result is a network
outage.
CAUTION: An interface can be configured for either loop protection or root
protection, but not for both.
Three EX Series switches are displayed in Figure 39 on page 1571. In this example, they are
configured for RSTP and create a loop-free topology. Interface
traffic between Switch 3 and Switch 1; thus, traffic is forwarded through interface
on Switch 2. BPDUs are being sent from the root bridge on Switch 1 to both of these
interfaces.
This example shows how to configure loop protection on interface
it from transitioning from a blocking state to a forwarding state and creating a loop in
the spanning-tree topology.
is blocking
ge-0/0/6
ge-0/0/7
to prevent
ge-0/0/6
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