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

STP prio nbr value


When implementing [Spanning Tree Protocol (STP)](/spanning-tree-protocol-stp), each interface on a device is assigned a port identification number.  All interfaces, including physical, virtual, and tunnel interfaces, are assigned such a number.  This number is assigned automatically and sequentially by the IOS based on the interfaces that exist on the device upon bootup.  As more interfaces are configured (i.e. virtual interfaces, [SVIs](/vlans-svi), [tunnels](/gre) and others), these numbers are assigned sequentially.  The number and how it is implemented is also dependent upon if the device in question is modular in nature (with cards that add more physical ports) or fixed.

The port identification value that has been assigned to each interface can be seen in various STP related commands.  The output of the show spanning-tree command can be seen below:

```
SW1#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID    Priority    32769
Address     000f.34ca.1000
Cost        19
Port        19 (FastEthernet0/17)
Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
Address     0011.bb0b.3600
Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/14              Desg FWD 19        128.16   P2p
Fa0/17              Root FWD 19        128.19   P2p
```

The value that appears after the "Port" label indicates the port identification number of the root port.  The port identification number is also part of the [Prio.Nbr](/stp-prio-nbr-value) value as well, and is the number that appears after the "." in the `128.16` and `128.19` designation.

The port identification number is also used as a tie-breaker of last resort to determine which port will be set to the blocked state.

## Links

https://networklessons.com/spanning-tree/introduction-to-spanning-tree

STP - port identification

SSH connectivity - troubleshooting

SSM Multicast Group Verification

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

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STP - MST region attributes

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STP - Max Age vs Message Age

STP - Multiple Spanning Tree (MST)

STP - Multiple types of STP on the same switch

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STP - Per VLAN Spanning Tree plus (PVST+)

STP - RSTP BPDU Flags and their usage

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STP - RSTP convergence process

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STP - RSTP proposal-agreement process

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STP - RSTP while timer

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STP - Shortest Path Bridging (SPB)

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STP - Topology Change Acknowledgement (TCA)

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

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STP - UplinkFast

STP - bridge priority

STP - determining the blocked port using port ID

STP - port roles

STP - port states

STP - spanning-tree root primary command

STP - what bridge ID does the MST region use

STP - what is a topology change

STP BPDU Filter

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STP BPDU Types

STP BPDU destination MAC address

STP Cost Calculation Methods

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STP Max Age

STP Message Age

STP PortFast Best Practice

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 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 topology change process

Security - 802.1x with WLC and AAA server

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Security - Best Practices for Repurposing Network Devices

Security - Cisco AnyConnect Secure Mobility Client

Security - Cisco Secure Client

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STP - port identification

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