Enabling Loop Guard - 3Com MSR 50 Series Configuration Manual

480 C 23: MSTP C
HAPTER

Enabling Loop Guard

n
ONFIGURATION
The root bridge and secondary root bridge of a panning tree should be located in
the same MST region. Especially for the CIST, the root bridge and secondary root
bridge are generally 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. In
this case, the current legal root bridge will be superseded by another device,
causing undesired change of the network topology. As a result of this kind of
illegal topology change, the traffic that should go over high-speed links is drawn
to low-speed links, resulting in network congestion.
To prevent this situation from happening, MSTP provides the root guard function
to protect the root bridge. If the root guard function is enabled on a port, this port
will keep playing the role of designated port on all MST instances. Once this port
receives a configuration BPDU with a higher priority from an MST instance, it
immediate sets that instance port to the listening state, without forwarding the
packet (this is equivalent to disconnecting the link connected with this port). If the
port receives no BPDUs with a higher priority within twice the forwarding delay,
the port will revert to its original state.
Follow these steps to enable root guard:
To do...
Enter system view
Enter Ethernet Enter Ethernet
interface view interface view
or port group
Enter port
view
group view
Enable the root guard function
for the ports(s)
We recommend that you enable loop guard on your device.
By keeping receiving BPDUs from the upstream device, a device can maintain the
state of the root port and other blocked ports. However, due to link congestion or
unidirectional link failures, these ports may fail to receive BPDUs from the
upstream device. In this case, the downstream device will reselect the port roles:
those ports 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.
If a loop guard-enabled port fails to receive BPDUs from the upstream device, and
if the port took part in STP computing, all the instances on the port, no matter
what roles they play, will be set to, and stay in, the Discarding state.
Follow these steps to enable loop guard:
Use the command...
system-view
interface interface-type
interface-number
port-group { manual
port-group-name |
aggregation agg-id }
stp root-protection
Remarks
-
User either command
Configured in Ethernet
interface view, the setting is
effective on the current port
only; configured in port
group view, the setting is
effective on all ports in the
port group.
Required
Disabled by default