Understanding How Etherchannel Guard Works; Understanding How Root Guard Works; Understanding How Loop Guard Works - Cisco 6500 Series Software Configuration Manual

Understanding How EtherChannel Guard Works

Figure 16-5 Adding a Network Device in a Shared-Medium Topology
Switch C
Understanding How EtherChannel Guard Works
EtherChannel guard detects a misconfigured EtherChannel where interfaces on the Catalyst 6500 series
switch are configured as an EtherChannel while interfaces on the other device are not or not all the
interfaces on the other device are in the same EtherChannel.
In response to misconfiguration detected on the other device, EtherChannel guard puts interfaces on the
Catalyst 6500 series switch into the errdisabled state.

Understanding How Root Guard Works

The STP root guard feature prevents a port from becoming root port or blocked port. If a port configured
for root guard receives a superior BPDU, the port immediately goes to the root-inconsistent (blocked)
state.

Understanding How Loop Guard Works

Loop guard helps prevent bridging loops that could occur because of a uni-directional link failure on a
point-to-point link. When enabled globally, the loop guard applies to all point-to-point ports on the
system. Loop guard detects root ports and blocked ports and ensures that they keep receiving BPDUs
from their designated port on the segment. If a loop guard enabled root or blocked port stop a receiving
BPDUs from its designated port, it transitions to the loop-inconsistent blocking state, assuming there is
a physical link error on this port. The port recovers from this loop-inconsistent state as soon as it receives
a BPDU.
You can enable loop guard on a per-port basis. When you enable loop guard, it is automatically applied
to all of the active instances or VLANs to which that port belongs. When you disable loop guard, it is
disabled for the specified ports. Disabling loop guard moves all loop-inconsistent ports to the listening
state.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
16-6
Switch A
(Root)
Blocked port
Added switch
Chapter 16 Configuring Optional STP Features
Switch B
(Designated Bridge)
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