Connectivity Guide

Root assignment
RPVST+ assigns the root bridge according to the lowest bridge ID. Assign one bridge as the root bridge and the other as a
secondary root bridge.
● Configure the device as the root or secondary root in CONFIGURATION mode.
spanning-tree vlan vlan-id root {primary | secondary}
○ vlan-id — Enter the VLAN ID number, from 1 to 4093.
○ primary — Enter the bridge as primary or root bridge. The primary bridge value is 24576.
○ secondary — Enter the bridge as secondary or secondary root bridge. The secondary bridge value is 28672.
Configure root bridge as primary
OS10(config)# spanning-tree vlan 1 root primary
Verify root bridge information
OS10# show spanning-tree active
Spanning tree enabled protocol rapid-pvst with force-version rstp
VLAN 1
Executing IEEE compatible Spanning Tree Protocol
Root ID Priority 24577, Address 90b1.1cf4.a523
Root Bridge hello time 2, max age 20, forward delay 15
Bridge ID Priority 24577, Address 90b1.1cf4.a523
We are the root of VLAN 1
Configured hello time 2, max age 20, forward delay 15
Interface Designated
Name PortID Prio Cost Sts Cost Bridge ID PortID
----------------------------------------------------------------------
ethernet1/1/5 128.276 128 500 FWD 0 24577 90b1.1cf4.a523 128.276
ethernet1/1/6 128.280 128 500 LRN 0 24577 90b1.1cf4.a523 128.280
Interface
Name Role PortID Prio Cost Sts Cost Link-type Edge
-------------------------------------------------------------
ethernet1/1/5 Desg 128.276 128 500 FWD 0 AUTO No
ethernet1/1/6 Desg 128.280 128 500 LRN 0 AUTO No
Loop guard
To configure loop guard on an interface:
● Enable loop guard on a per-port or port-channel interface in INTERFACE mode.
spanning-tree guard {loop | root | none}
○ loop — Enables loop guard on an interface.
○ root — Enables root on an interface.
○ none — Enables the guard mode to none.
● Disable loop guard on a port or port-channel interface in INTERFACE mode.
no spanning-tree guard loop
Port enabled with loop guard conditions
● Loop guard is supported on any STP-enabled port or port-channel interface in RPVST+ mode.
● You cannot enable root guard and loop guard at the same time on an STP port. The loop guard configuration overwrites an
existing root guard configuration and vice versa.
● Enabling BPDU guard and loop guard at the same time on a port results in a port that remains in the Blocking state and
prevents traffic from flowing through it. For example, when you configure both Portfast BPDU guard and loop guard:
○ If a BPDU is received from a remote device, BPDU guard places the port in the Err-Disabled Blocking state and no traffic
forwards on the port.
Layer 2
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