Loop Guard - Cisco Catalyst 2960 series Configuration Manual

Information About Optional Spanning-Tree Features
LAN, the designated switch of which is either an IEEE 802.1D switch or a switch with a different MST region
configuration.
Root guard enabled on an interface applies to all the VLANs to which the interface belongs. VLANs can be
grouped and mapped to an MST instance.
Misuse of the root guard feature can cause a loss of connectivity.
Caution
Related Topics
Enabling Root Guard , on page 312

Loop Guard

You can use loop guard to prevent alternate or root ports from becoming designated ports because of a failure
that leads to a unidirectional link. This feature is most effective when it is enabled on the entire switched
network. Loop guard prevents alternate and root ports from becoming designated ports, and spanning tree
does not send BPDUs on root or alternate ports.
When the switch is operating in PVST+ or rapid-PVST+ mode, loop guard prevents alternate and root ports
from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.
When the switch is operating in MST mode, BPDUs are not sent on nonboundary ports only if the interface
is blocked by loop guard in all MST instances. On a boundary port, loop guard blocks the interface in all MST
instances.
Related Topics
Enabling Loop Guard , on page 313
STP PortFast Port Types
You can configure a spanning tree port as an edge port, a network port, or a normal port. A port can be in
only one of these states at a given time. The default spanning tree port type is normal. You can configure the
port type either globally or per interface.
Depending on the type of device to which the interface is connected, you can configure a spanning tree port
as one of these port types:
• A PortFast edge port—is connected to a Layer 2 host. This can be either an access port or an edge trunk
port (portfast edge trunk). This type of port interface immediately transitions to the forwarding state,
bypassing the listening and learning states. Use PortFast edge on Layer 2 access ports connected to a
single workstation or server to allow those devices to connect to the network immediately, rather than
waiting for spanning tree to converge.
Even if the interface receives a bridge protocol data unit (BPDU), spanning tree does not place the port
into the blocking state. Spanning tree sets the port's operating state to non-port fast even if the configured
state remains port fast edge and starts participating in the topology change.
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