H3C Technologies H3C SecPath F1000-E - Manual (Page 127)

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NOTE:
BPDU guard does not take effect on loopback-testing-enabled ports. For more information about
loopback testing, see
Network Management Configuration Guide.
Enabling root guard
The root bridge and secondary root bridge of a spanning tree should be located in the same MST region.
Especially for the CIST, the root bridge and secondary root bridge are put in a high-bandwidth core
region during network design. However, due to possible configuration errors or malicious attacks in the
network, the legal root bridge may receive a configuration BPDU with a higher priority. The current legal
root bridge will be superseded by another device, causing an undesired change of the network topology.
As a result, the traffic that should go over high-speed links is switched to low-speed links, resulting in
network congestion.
To prevent this situation, MSTP provides the root guard function. If the root guard function is enabled on
a port of a root bridge, this port plays the role of designated port on all MSTIs. Once this port receives
a configuration BPDU with a higher priority from an MSTI, it immediately sets that port to the listening
state in the MSTI, without forwarding the packet (this is equivalent to disconnecting the link connected
with this port in the MSTI). If the port receives no BPDUs with a higher priority within twice the forwarding
delay, it will revert to its original state.
Configure root guard on a designated port.
To enable root guard:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enter Ethernet interface view or Layer 2
aggregate interface view. interface interface-type
interface-number
N/A
3. Enable the root guard function for the
ports.
stp root-protection Disabled by default.
NOTE:
Among loop guard, root guard and edge port settings, only one function (whichever is configured the
earliest) can take effect on a port at the same time.
Enabling loop guard
A device that keeps receiving BPDUs from the upstream device can maintain the state of the root port and
blocked ports. However, link congestion or unidirectional link failures may cause these ports to fail to
receive BPDUs from the upstream devices. The device will reselect the port roles: Those ports in
forwarding state that failed to receive upstream BPDUs will become designated ports, and the blocked
ports will transition to the forwarding state, resulting in loops in the switched network. The loop guard
function can suppress the occurrence of such loops.
The initial state of a loop guard-enabled port is discarding in every MSTI. When the port receives BPDUs,
its state transitions normally. Otherwise, it stays in the discarding state to prevent temporary loops.
Configure loop guard on the root port and alternate ports of a device.
To enable loop guard: