Port Priority; Root Guard; Loop Guard - Lenovo CN4093 Application Manual

Port Priority

Root Guard

Loop Guard

© Copyright Lenovo 2015
The port priority helps determine which bridge port becomes the root port or the
designated port. The case for the root port is when two switches are connected
using a minimum of two links with the same path-cost. The case for the designated
port is in a network topology that has multiple bridge ports with the same
path-cost connected to a single segment, the port with the lowest port priority
becomes the designated port for the segment.
Use the following commands to configure the port priority:
CN 4093(config)# interface port <port alias or number>
CN 4093(config-if)# spanning-tree stp <1-128> priority <0-65535>
where priority value is a number from 0 to 240, in increments of 16 (such as 0, 16, 32,
and so on). If the specified priority value is not evenly divisible by 16, the value will
be automatically rounded down to the nearest valid increment whenever manually
changed in the configuration.
The root guard feature provides a way to enforce the root bridge placement in the
network. It keeps a new device from becoming root and thereby forcing STP
re-convergence. If a root-guard enabled port detects a root device, that port will be
placed in a blocked state.
You can configure the root guard at the port level using the following commands:
CN 4093(config)# interface port <port alias or number>
CN 4093(config-if)# spanning-tree guard root
The default state is none (disabled).
In general, STP resolves redundant network topologies into loop-free topologies.
The loop guard feature performs additional checking to detect loops that might not
be found using Spanning Tree. STP loop guard ensures that a non-designated port
does not become a designated port.
To globally enable loop guard, enter the following command:
CN 4093(config)# spanning-tree loopguard
Note: The global loop guard command will be effective on a port only if the
port-level loop guard command is set to default as shown below:
CN 4093(config-if)# spanning-tree guard loop none
To enable loop guard at the port level, enter the following command:
CN 4093(config)# interface port <port alias or number>
CN 4093(config-if)# spanning-tree guard loop
The default state is "none", i.e. disabled.
Chapter 10: Spanning Tree Protocols
155