
One of the fundamental mechanisms of [Spanning Tree Protocol (STP)](/spanning-tree-protocol-stp) is the election of a [root bridge](/stp-root-bridge-selection).  Using the `spanning-tree vlan x root primary` command, it is possible to designate the local switch as the root bridge for the specified [VLAN](/vlan).  Now what this command actually does is a little more involved than simply designating it the root bridge.

The `spanning-tree root primary` command in Cisco IOS operates by running a hardwired script that adjusts the switch's [priority](/stp-bridge-priority) to make it the root bridge for a specified VLAN. The behavior of this command affects the [priority values](/stp-bridge-priority-values) in various ways, depending on the IOS version and platform.  According to the linked Cisco documentation below:

1. **IOS 15.X**: The switch sets its priority to 24576 to become the root for the specified VLAN. If another switch has a lower priority, it sets its priority to 4096 less than the lowest priority.
2. **IOS 12.X**: The command sets the bridge priority to 8192. If the switch does not become the root, the priority is adjusted to 100 less than the current root's priority. If it still does not become the root, an error occurs.
3. **Other Documentation**: The `primary` keyword sets the priority to 24576, and the `secondary` keyword sets it to 28672.

In all cases, the command runs a script that examines the current root bridge and adjusts the local switch's priority to make it the root. This script runs only when the command is executed and does not automatically re-run if the network topology changes, meaning the command is not preemptive.

This behavior ensures that the local switch becomes or remains the root bridge when the command is executed.  Depending upon the configuration found on other switches in the STP topology, this command may actually fail to fulfil its requirement, therefore it should be used cautiously, with a clear knowledge of the configuration found on other switches in the topology.

## Links

https://forum.networklessons.com/t/spanning-tree-root-bridge-configuration/13949/8?u=lagapidis.

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/15-2_2_e/command/reference/cr_2960/cli3.html#pgfId-1947142

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/lanswitch/command/lsw-cr-book.pdf

https://www.ciscopress.com/articles/article.asp?p=2995351&seqNum=2

https://networklessons.com/spanning-tree/spanning-tree-root-bridge-configuration

STP - spanning-tree root primary command

STP - Aging time reduction with a TCN

STP - Aging time return to default

STP - BPDU Filter Use Cases

STP - BPDU generation and port roles

STP - BPDUGuard, BPDUFilter, and Portfast

STP - BPDUs on a switch where STP is disabled

STP - BPDUs, VLAN IDs, and PVST+

STP - Cost of last link added by receiving switch

STP - Disabling spanning-tree

STP - Dispute

STP - Factors to Consider When Choosing a Root Bridge

STP - Learning State

STP - MST CIST Root

STP - MST CST and CIST

STP - MST Virtual Bridge and its role in interfacing with other STP topologies

STP - MST and PVST+ selecting root port

STP - MST maps groups of VLANs

STP - MST region attributes

STP - MST viewed as a single switch by PVST+

STP - Max Age vs Message Age

STP - Multiple Spanning Tree (MST)

STP - Multiple types of STP on the same switch

STP - PVST+ and Rapid PVST+ interaction

STP - Per VLAN Spanning Tree plus (PVST+)

STP - RSTP BPDU Flags and their usage

STP - RSTP EdgePort

STP - RSTP Proposal Timer Operation

STP - RSTP convergence process

STP - RSTP port roles

STP - RSTP prerequisites for immediate move to forward state

STP - RSTP proposal-agreement process

STP - RSTP while timer cannot be observed

STP - RSTP while timer

STP - Root Inconsistency State

STP - Root Link Query (RLQ) BPDU

STP - Shortest Path Bridging (SPB)

STP - Time to select root bridge

STP - Topology Change Acknowledgement (TCA)

STP - UDLD and Fiber Links

STP - Understanding MST regional root identifier and proposal-agreement process

STP - UplinkFast and reconvergence

STP - UplinkFast and root port delay timer

STP - UplinkFast

STP - bridge priority

STP - connecting one port to another port on the same switch

STP - determining the blocked port using port ID

STP - port identification

STP - port roles

STP - port states

STP - what bridge ID does the MST region use

STP - what is a topology change

STP BPDU Filter

STP BPDU Guard

STP BPDU Types

STP BPDU destination MAC address

STP Cost Calculation Methods

STP Loop Guard

STP Max Age

STP Message Age

STP PortFast

STP Portfast Options

STP Root Bridge selection

STP Root Guard

STP Topology Change (TC) flag

STP Topology Change Timer

STP UDLD and Autonegotiation

STP UDLD

STP bridge priority values

STP changing link cost

STP configuring on EtherChannel

STP configuring portfast

STP contents of a BPDU

STP cost long mode

STP cost short mode

STP determining blocked port using cost

STP direct vs indirect link failure

STP fine tuning the spanning tree protocol

STP how a switch identifies a BPDU

STP how to find the root bridge

STP interface type

STP prio nbr value

STP topology change process

Security - 802.1x with WLC and AAA server

Security - AAA for RESTCONF

Security - Best Practices for Repurposing Network Devices

Security - Cisco AnyConnect Secure Mobility Client

Security - Cisco Secure Client

Security - Cisco VPN Client

Security - Cisco VPN client software

Security - Diffie-Hellman groups

Security - Extensible Authentication Protocol over LAN (EAPOL)

Security - GETVPN

Security - IEEE 802.1X

Security - IOS Usernames and Passwords

Security - IPsec nonce

Security - Kerberos

Security - MAC Authentication Bypass

Security - Network Access Control (NAC)

Security - Next Generation Firewall