© Broadcom & © xFlow Research Inc

Multiple Spanning Tree Protocol

Revision History

Revision No.DescriptionAuthorDate
0.1Modified DesignDivya Kumaran Chandralekha , Rida Hanif , Wajahat RaziJuly 02, 2024

Table of Contents

Scope

This document describes the High Level Design of Multiple Spanning Tree Protocol.

Abbreviations

TermMeaning
BPDUBridge Protocol Data Unit
CISTCommon Internal Spanning Tree
CSTCommon Spanning Tree
ISTInternal Spanning Tree
MSTIMultiple Spanning Tree Instance
MSTIDMultiple Spanning Tree Identifier
MSTPMultiple Spanning Tree Protocol
STPSpanning Tree Protocol
VLANVirtual Local Area Network
VIDVLAN identifier

Overview

Multiple Spanning Tree Protocol (MSTP) enhances the Spanning Tree Protocol (STP) by enabling the creation of multiple spanning tree instances within a network. It provides a mechanism to map VLANs to specific spanning tree instances which offers network segmentation and improved control over traffic flow.

Introduction

Spanning Tree Protocol (STP) is a network protocol that operates at the Data Link layer (Layer 2) of the OSI model. Its primary purpose is to prevent loops in a Local Area Network (LAN). Network loops can cause broadcast storms and multiple frame copies, leading to network inefficiencies and failures. STP ensures a loop-free topology by creating a spanning tree that selectively blocks some network paths.

Multiple Spanning Tree Protocol (MSTP), standardized in IEEE 802.1s, is an extension of STP. MSTP enhances STP by providing better loop prevention, path redundancy, and optimal bandwidth utilization. MSTP allows network engineers to create multiple spanning trees, each associated with a set of VLANs, and assign them to different switch ports. This configuration optimizes bandwidth usage, reduces convergence time, and simplifies network management.

MSTP reduces convergence time compared to STP. When a network topology change occurs, only the affected MSTI needs to reconverge, minimizing the impact on the entire network.

Understanding MSTP: IST, CIST, CST, MST Regions, MSTIs, VLAN to MSTI Mapping

Before diving into the specific terms related to Multiple Spanning Tree Protocol (MSTP), it's important to first define the concept of an MST region. The following definitions outline the key components of MSTP, starting with the MST region.

  1. MST Regions: An MST region is a group of interconnected bridges that share the same MST configuration, including the MST configuration name, revision number, and VLAN-to-instance mappings.

  2. Default Internal Spanning Tree (IST)​: An internal spanning tree (IST) is a spanning tree that runs in an MST region. It is also called MSTI 0, a special MSTI to which all VLANs are mapped by Default.

  3. Common Spanning Tree (CST)​: The common spanning tree (CST) is a single spanning tree that connects all MST regions in a switched network.

  4. Common and Internal Spanning Tree (CIST)​: The common and internal spanning tree (CIST) is a single spanning tree that connects all devices in a switched network. It consists of the ISTs in all MST regions and the CST.

  5. MST Instances (MSTIs)​: MSTP divides the network into multiple regions, each containing several MSTIs. Each MSTI operates independently, allowing for efficient use of network resources and optimized load balancing across different VLANs.

MSTP Advantages

  1. VLAN-to-MSTI Mapping: MSTP maps VLANs to specific MSTIs using a VLAN mapping table. This mapping ensures that traffic within a VLAN follows the corresponding MSTI, optimizing the network path and improving performance.

  2. Traffic Load Sharing and Link Level Redundancy: By effectively utilizing VLAN-to-MSTI mapping, MSTP can facilitate traffic load sharing across multiple links and provide redundancy at the link level. This helps in balancing the traffic load and enhances overall network resilience.

  3. Optimized Bandwidth: MSTP ensures that network paths are used efficiently, reducing link blocking and optimizing bandwidth usage. This results in improved performance for data transmission across the network.

MSTP Operation

  1. MST BPDUs: MSTP uses Multiple Spanning Tree Bridge Protocol Data Units (MST BPDUs) to exchange information between switches. These BPDUs contain information about the MSTI and VLAN mappings, ensuring consistent spanning tree calculations across the network. MSTP calculates spanning trees based on MST BPDUs.

  2. MSTP BPDUs Characteristics: MSTP BPDUs, which utilize the IEEE Reserved Multicast MAC address, are untagged. This is in contrast to PVST BPDUs. Additionally, MSTP BPDUs are backward compatible with legacy STP, RSTP, and PVST+/RPVST+ BPDUs.

  3. Config Digest: A hash of MSTP-VLAN mapping knows as 'config digest' is carried in the BPDUs, coupled with region name and the revision number, determines the same/different MST region of the sending switch.

MSTP BPDU Format

MSTI Configuration Messages

Refer toRFC IEEE 802.1s-2002for MSTP BPDU details.​

Requirements

Functional Requirements

  1. Support the creation of Multiple Spanning Tree Instances (MSTIs).

  2. Support the assignment of one or more VLANs to a specific MSTI within a region.

  3. Support the option to assign a region name and revision number to MSTP regions to achieve unique identification of VLAN to instance mapping across switches.

  4. Support path selection and forwarding behavior in MSTI to optimize network performance within each instance by configuring a distinct root bridge.

  5. Support the configuration of spanning tree parameters such as forward delay, hello timer, and hop count.

  6. The Destination MAC Address will be 01:80:C2:00:00:00 for MSTP BPDUs.

  7. Support backward compatibility with networks employing different spanning tree protocols, such as STP and RSTP.

  8. Support edge port functionality.

  9. Support BPDU guard functionality.

  10. Support root guard functionality.

  11. Support protocol operation on static breakout ports.

  12. Support protocol operation on Port-channel interfaces. Note: MSTP does not support aggregated path cost calculations.

Configuration & Management Requirements

  1. Support CLI configurations as mentioned in Configuration section

  2. Support show commands as mentioned in Configuration section

  3. Support debug commands as mentioned in Configuration section

  4. Support statistics commands as mentioned in Configuration section

  5. Support clear commands as mentioned in Configuration section

Architecture Design

Following diagram explains the architectural design and linkages for MSTP. MSTP uses multiple existing SONiC containers, configuration details of each is mentioned below as well.

STP Container

STP Container is responsible for actions taken for BPDU rx and BPDU tx. Following are the details for implementation:

STPMgr

Subscribes to CONFIG_DB and STATE_DB tables, parsing configurations and passes to STPd.

STPd

Responsible for all MST protocol related calculations. BPDUs are sent and received in STPd and states are updated accordingly.

STPSync

STPSync is a component integrated within STPd responsible for managing and updating all operational STP data to APP DB.

Stpd communicates with Linux Kernal for The BPDU reception/transmission, management of port or LAG changes via NetLink events, and synchronization of STP port states with the Linux Kernel.

SWSS Container

SWSS Container is responsible for passing on configurations to SAI as follows:

STPOrch

Updates SAI via following APIs:

  1. Creating/deleting instances

  2. Assigning VLAN to instance

  3. Creation of STP Port and assigning port state with respect to each instance

  4. Flushing FDB entries

Database Changes

MSTP design introduces some new tables for configuration along with slight modification in existing STP tables. Following are details of each individual table:

CONFIG DB

Existing Table

Following existing table of CONFIG_DB will be modified for MSTP implementation:

STP_GLOBAL_TABLE

A new value of mst for mode column

mode 		  = "pvst" / "mst"      ; a new option for mstp

Other fields of this table are not applicable for "mstp" mode.

New Tables

Following new tables will be added to CONFIG_DB:

STP_MST_TABLE

;Stores STP Global configuration
key                       = STP_MST| GLOBAL               ; GLOBAL MSTP table key 
name                      = 1*32CHAR                      ; MSTP region name(DEF:mac address of switch)
revision                  = 5*DIGIT                       ; MSTP revision number (0 o 65535 ,DEF:0)
max_hop                   = 2*DIGIT                       ; maximum hops (1 to 40, DEF:20)   
max_age                   = 2*DIGIT                       ; maximum age time in secs (6 to 40sec, DEF:20sec)
hello_time                = 2*DIGIT                       ; hello time in secs (1 to 10sec, DEF:2sec)
forward_delay             = 2*DIGIT                       ; forward_delay in secs (4 to 30sec, DEF:15 sec)

STP_MST_INST_TABLE

;Stores STP configurations per MSTI
key                 = STP_MST| "MST_INSTANCE":"Instance" id   ; MST with prefix "STP_MST"
bridge_priority     = 5*DIGIT                                 ; bridge priority (0 to 61440, DEF: 32768)
vlan_list           = "Vlans"                                 ; Lists of VLANs assigned to the MST instance

STP_MST_PORT_TABLE

;Stores STP interface details per MSTI
key                 = STP_MST_PORT| "MST_INSTANCE" id|ifname    ; MSTI+Intf with prefix "STP_MST_PORT"
path_cost           = 9*DIGIT                                   ; port path cost (1 to 20000000)
priority            = 3*DIGIT                                   ; port priority (0 to 240, DEF:128)

STP_PORT_TABLE

;Stores STP interface details
key                 = STP_PORT|ifname              ; ifname with prefix STP_PORT
edge_port           = BIT                          ; enabled or disabled
link_type           = "type"                       ; type can be of auto, point-to-point or shared
enabled             = BIT                          ; enabled or disabled
bpdu_guard          = BIT                          ; enabled or disabled
bpdu_guard_do       = BIT                          ; enabled or disabled 
root_guard          = BIT                          ; enabled or disabled 
path_cost           = 9*DIGIT                      ; port path cost (1 to 20000000)
priority            = 3*DIGIT                      ; port priority (0 to 240, DEF:128)

APP DB

New Tables

Following new tables are introduced as part of MSTP Feature:

STP_MST_INST_TABLE

;Stores the STP per  MSTI operational details
key                       = STP_MST_INST_TABLE:"MST" id  
vlan_list                 = vlan_id-or-range[,vlan_id-or-range]       ; list of VLAN IDs assigned to MST instance
bridge_address            = 16HEXDIG                                  ; bridge id
regional_root_address     = 16HEXDIG                                  ; regional root bridge id
root_address              = 16HEXDIG                                  ; root bridge id
root_path_cost            = 1*9DIGIT                                  ; root path cost
regional_root_cost        = 1*9DIGIT                                  ; regional root path cost
root_max_age              = 1*2DIGIT                                  ; Max age of root bridge (6 to 40 sec, DEF:20sec)
root_hello_time           = 1*2DIGIT                                  ; Hello time of root bridge (1 to 10 sec, DEF:2sec)
root_forward_delay        = 1*2DIGIT                                  ; forward delay of root bridge (4 to 30sec , DEF:15sec)
remining_hops             = 1*2DIGIT                                  ; Remaining Max-hops
root_port                 = ifName                                    ; Root port name

STP_MST_PORT_TABLE

;Stores STP MST instance interface details
key                 = STP_MST_PORT_TABLE:"MST":id:ifname     ; MSTI+Intf with prefix "STP_MST_PORT
port_number         = 1*3DIGIT                               ; port number or bridge port 
path_cost           = 1*9DIGIT                               ; port path cost (1 to 200000000)
priority            = 3*DIGIT                                ; port priority (0 to 240, DEF:128)
port_state          = "state"                                ; DISABLED/DISCARDED/LISTENING/LEARNING/FORWARDING
role                = "role"                                 ; DESIGNATED/ROOT/ALTERNATIVE/MASTER 
desig_cost          = 1*9DIGIT                               ; designated cost
external_cost       = 1*9DIGIT                               ; designated cost
desig_root          = 16HEXDIG                               ; designated root
desig_reg_cost      = 16HEXDIG                               ; designated root
desig_bridge  	    = 16HEXDIG                               ; designated bridge
desig_port    	    = 1*9DIGIT                               ; designated port
fwd_transitions     = 1*5DIGIT                               ; number of forward transitions
bpdu_sent           = 1*10DIGIT                              ; bpdu transmitted
bpdu_received       = 1*10DIGIT                              ; bpdu received

STP_PORT_TABLE

;Stores STP interface detail
key                 = STP_PORT_TABLE:ifname    ; ifname with prefix STP_INTF
edge_port           = BIT                       ; edge port  enabled or disabled
link_type           = "type"                    ; point-to-point or shared link type
mst_boundary        = BIT                       ; enabled or disabled
mst_boundary_proto  = BIT                       ; enabled or disabled

STP_INST_PORT_FLUSH_TABLE

;Defines instance and port for which FDB Flush needs to be performed
key                 = STP_INST_PORT_FLUSH_TABLE:instane:ifname         ; FDB Flush instance id and port
name           
state               = "true"

Existing Tables

Following already present APP_DB tables are also used for implementation of MSTP:

STP_PORT_STATE_TABLE

The table holds the state of a port i.e forwarding, learning, blocking with respect to each instance.

STP_VLAN_INSTANCE_TABLE

The table holds the VLAN to instance mapping.

SAI

Existing SAI Attributes

Following table shows the existing SAI Attributes that will be used:

ComponentSAI Attribute
STP InstanceSAI_STP_ATTR_VLAN_LIST
SAI_STP_ATTR_BRIDGE_ID
SAI_STP_ATTR_PORT_LIST
STP PortSAI_STP_PORT_ATTR_STP
SAI_STP_PORT_ATTR_BRIDGE_PORT
SAI_STP_PORT_ATTR_STATE
STP Port StatesSAI_STP_PORT_STATE_LEARNING
SAI_STP_PORT_STATE_FORWARDING
SAI_STP_PORT_STATE_BLOCKING
VLAN STP InstanceSAI_VLAN_ATTR_STP_INSTANCE
Switch STP AttributesSAI_SWITCH_ATTR_Default_STP_INST_ID
SAI_SWITCH_ATTR_MAX_STP_INSTANCE

New SAI Attributes

MSTP design will not require any new SAI Attributes for Control Packet trap. We will use the existing traps as DMAC and LLC fields are identical for STP and MSTP

Additional Features

BPDU Guard

BPDU guard feature will also be supported by MSTP. BPDU Guard feature disables the connected device ability to initiate or participate in STP on edge ports. When STP BPDUs are received on the port where BPDU guard is enabled the port will be shutdown. User can re-enable the port administratively after ensuring the BPDUs have stopped coming on the port.

Root Guard

Root guard feature will also be supported by MSTP. The Root Guard feature provides a way to enforce the root bridge placement in the network and allows STP to interoperate with user network bridges while still maintaining the bridged network topology that the administrator requires. When BPDUs are received on a root guard enabled port, the STP state will be moved to "Root inconsistent" state to indicate this condition. Once the port stops receiving superior BPDUs, Root Guard will automatically set the port back to a FORWARDING state.

Edge Port

Edge ports immediately transition to the forwarding state upon activation and do not participate in STP topology calculations.

MSTP standard does not support uplink fast so uplink fast functionality will be disable for MSTP.

Sequence Diagrams

MSTP global enable

MSTP global disable

MSTP region name/version change

MSTIs Instance Creation

MSTIs Instance Deletion

Add VLAN to Existing Instance

Del VLAN from Exisiting Instance

Configuration Commands

Following configuration commands will be provided for configuration of MSTP:

Global Level

  • config spanning_tree {enable|disable} {mst}​

    • Enables or disables mstp at global level on all ports of the switch.

    • Only one mode of STP can be enabled at a time.

    • By Default disabled.

  • config spanning_tree max_hops <max-hops-value>​

    • Specify the number of maximum hops before the BPDU is discarded inside a region.

    • max-hops-value: Default: 20, range: 1-255

  • config spanning_tree hello <hello-value>​

    • Specify configuring hello interval in sec for transmission of BPDUs.

    • Default: 2, range 1-10

  • config spanning_tree max_age <max_age-value>​

    • Specify configuring maximum time to listen for root bridge in seconds.

    • Default: 2, range 6-40

Region Level

Below commands allow configuring on region basis:

  • config spanning_tree mst region-name <region-name>​

    • Edit the name of region

    • region-name: Case sensitive, characters should be less than or equal to 32, Default: mac-address of bridge

  • config spanning-tree mst revision <revision-number>​

    • Revision number is used to track changes in the configuration and to synchronize the configuration across the switches in the same region.

    • revision-number: Default: 0, range: 0-65535

Instance Level

Below commands allow configuration of an instance:

  • config spanning_tree mst instance <instance-id> priority <priority-value>​

    • Configure priority of bridge for an instance.

    • instance-id: id of the instance for which bridge priority is to be defined. If the provided instance id is not created yet, an error message is displayed.

    • priority-value: Default: 32768, range: 0-61440 (should be multiple of 4096)

  • config spanning_tree mst instance <instance-id> vlan (add|del) <vlan-id>​

    • VLAN to instance mapping.

    • instance-id: id of the instance to which VLAN is to be mapped. If the provided instance id is not created yet, an error message is displayed.

    • vlan-id: Range: 1-4094. If the provided VLAN is not created yet, an error message is displayed.

    • Instance is only active when there is at least one VLAN member port configured for one of the mapped VLANs.

Instance, Interface Level

Following commands are used for spanning-tree configurations on per instance, per interface basis:

  • config spanning_tree mst instance <instance-id> interface <ifname> priority <priority-value>​

    • Configure priority of an interface for an instance.

    • priority-value: Default: 128, range: 0-240

    • Supported Instances : 64

  • config spanning_tree mst instance <instance-id> interface <ifname> cost <cost-value>​

    • Configure path cost of an interface for an instance.

    • cost-value: Range: 1-200000000

Interface Level

Following new commands will be added:

  • config spanning_tree interface {enable|disable} <ifname>​

    • Configure an interface for MSTP.

  • config spanning_tree interface edgeport {enable|disable} <ifname>​

    • This command allow enabling or disabling of edge port on an interface.

  • config spanning_tree interface bpdu_guard {enable|disable} <ifname>​

    • This command allow enabling or disabling of bpdu_guard on an interface.

  • config spanning_tree interface root_guard {enable|disable} <ifname>​

    • This command allow enabling or disabling of root_guard on an interface.

  • config spanning_tree interface priority <ifname> <port_priority-value>​

    • Specify configuring the port level priority for root bridge in seconds.

    • Default: 128, range 0-240

  • config spanning_tree interface cost <ifname> <cost-value>​

    • Specify configuring the port level priority for root bridge in seconds.

    • Default: 0, range 1-200000000

  • config spanning_tree interface link-type {P2P|Shared-Lan|Auto} <ifname>​

    • Specify configuring the interface at different link types.

    • Default : Auto

    Note : These functionalities need to be explicitly enabled.​

Show Commands

  • show spanning_tree mst

    The output of this command will be as follows for mstp:

admin@sonic: show spanning_tree detail

Spanning-tree Mode: MSTP
#######  MST0 (CIST)       Vlans mapped : 1, 4-8, 202-4094
Bridge               Address 8000.80a2.3526.0c5e
Root                 Address 8000.80a2.3526.0c5e
                     Port     Root                  Path cost 0 
Regional Root        Address  8000.80a2.3526.0c5e
                     Internal cost 0                Rem hops 20 
Operational          Hello Time  2, Forward Delay 15, Max Age 20, Txholdcount 6
Configured           Hello Time  2, Forward Delay 15, Max Age 20, Max Hops 20

Interface           Role        State           Cost       Prio.Nbr    Type
---------------    --------     ----------      -------    ---------   -----------
Ethernet20         DESIGNATED   FORWARDING     2000        128.25       P2P
Ethernet46         DESIGNATED   FORWARDING     800         128.86       P2P
PortChannel1001    DESIGNATED   FORWARDING     1000        128.45       P2P


#######  MST1   (CIST)     Vlans mapped : 2, 300, 400
Bridge               Address 8000.80a2.3526.0c5e
Root                 Address 8000.80a2.3526.0c5e
                     Port    Root            Path cost 0    Rem Hops 20

Interface           Role        State           Cost       Prio.Nbr     Type
---------------    --------     ----------      -------    ---------    -----------
Ethernet46         DESIGNATED   FORWARDING      800         128.86       P2P
PortChannel1001    DESIGNATED   FORWARDING      1000        128.45       P2P
  • show spanning_tree mst detail
admin@sonic: show spanning_tree detail


#######  MST0 (CIST)     Vlans mapped : 1, 4-8, 202-4094
Bridge                Address 8000.80a2.3526.0c5e
 Root                 Address 8000.80a2.3526.0c5e
                      Port    Root            Path cost 0    Rem Hops 20

Ethernet20 is DESIGNATED FORWARDING
Port info              port id 86 priority 128          cost 800
Designated             Address 8000.80a2.3526.0c5e      cost 0
Designated bridge      Address 8000.80a2.3526.0c5e      port id 25
Timers:  forward transitions 0
Bpdu send 80, received 0

Ethernet46 is DESIGNATED FORWARDING
Port info              port id 25 priority 128          cost 1000
Designated             Address 8000.80a2.3526.0c5e      cost 0
Designated bridge      Address 8000.80a2.3526.0c5e      port id 86
Timers:  forward transitions 0
Bpdu send 40, received 0

PortChannel1001 is DESIGNATED FORWARDING
Port info              port id 25 priority 128          cost 2000
Designated             Address 8000.80a2.3526.0c5e      cost 0
Designated bridge      Address 8000.80a2.3526.0c5e      port id 45
Timers:  forward transitions 0
Bpdu send 30, received 0

#######  MST1 (CIST)   Vlans mapped : 2, 300, 400
Bridge                Address 8000.80a2.3526.0c5e
 Root                 Address 8000.80a2.3526.0c5e
                      Port    Root            Path cost 0    Rem Hops 20

Ethernet46 is DESIGNATED FORWARDING
Port info              port id 85 priority 128          cost 2000
Designated             Address 8000.80a2.3526.0c5e      cost 0
Designated bridge      Address 8000.80a2.3526.0c5e      port id 86
Timers:  forward transitions 0
Bpdu send 80, received 0


PortChannel1001 is DESIGNATED FORWARDING
Port info              port id 25 priority 128          cost 2000
Designated             Address 8000.80a2.3526.0c5e      cost 0
Designated bridge      Address 8000.80a2.3526.0c5e      port id 45
Timers:  forward transitions 0
Bpdu send 80, received 0
  • show spanning_tree mst instance 
admin@sonic: show spanning_tree mst instance 0

#######  MST0 (CIST)  Vlans mapped : 1, 4-8, 202-4094
Bridge                Address 8000.80a2.3526.0c5e
Root                  Address 8000.80a2.3526.0c5e
                      Port      Root                    Path cost 0 
Regional Root         Address   8000.80a2.3526.0c5e
                      Internal cost 0         Rem hops 20 
Operational           Hello Time  2, Forward Delay 15, Max Age 20, Txholdcount 6
Configured            Hello Time  2, Forward Delay 15, Max Age 20, Max Hops 20

Interface           Role        State           Cost       Prio.Nbr    Type
---------------    --------     ----------      -------    ---------   -----------
Ethernet20         DESIGNATED   FORWARDING     2000        128.25       P2P
Ethernet46         DESIGNATED   FORWARDING     800         128.86       P2P
PortChannel1001    DESIGNATED   FORWARDING     1000        128.45       P2P
  • show spanning_tree mst interface
admin@sonic: show spanning_tree mst interface  Ethernet46

Link Type: P2P        Bpdu filter: False
Boundary : internal   Bpdu guard:  False

Instance           Role        State           Cost       Prio.Nbr     Vlans 
---------------    --------     ----------      -------    ---------    -----------
0                  DESIGNATED   FORWARDING      800         128.86       1, 4-8, 202-4094
1                  DESIGNATED   FORWARDING      1000        128.45       2, 300, 400
  • show spanning_tree bpdu_guard
admin@sonic:  show spanning_tree bpdu_guard


PortNum               Shutdown Configured             Port Shut due to BPDU Guard
---------             --------------------           -----------------------------
Ethernet46             Yes                            Yes             
PortChannel1001        No                             NA
  • show spanning_tree root_guard
admin@sonic:  show spanning_tree root_guard


PortNum               VLAN/MST                      Current State
---------             ---------------------         ---------------------------------------------
Ethernet46             1, 4-8, 202-4094             Inconsistent state 
PortChannel1001                                     Consistent state
  • show spanning_tree mst instance <instance-id> interface <ifname>
admin@sonic:  show spanning_tree  mst instance 0 interface Ethernet46


Port        Prio   Path    Edge     State         Role     Designated    Designated          Designated
Num         rity   Cost    Port                            Cost          Root                Bridge
Ethernet46  128    800     N        FORWARDING    Root     0             32768002438eefbc3   32768002438eefbc3

Statistics Commands

  • show spanning_tree mst statistics instance <instance-id>

admin@sonic:  show spanning_tree  mst  statistics instance 0 


MSTP instance 0 (CIST) -  VLANs 1, 4-8, 202-4094
--------------------------------------------------------------------
PortNum           BPDU Tx     BPDU Rx     TCN Tx     TCN Rx             
Ethernet20        80	      4           3          4
Ethernet46        40	      6           3          4
PortChannel15     30	      6           4          1

Clear Commands

  • sonic-clear spanning_tree statistics

  • sonic-clear spanning_tree mst statistics instance <instance-id>

Debug Commands

Following debug commands will be supported for enabling additional logging which can be viewed in /var/log/stpd.log, orchagent related logs can be viewed in /var/log/syslog.

  • debug spanning_tree mst instance <instance-id>

  • debug spanning_tree bpdu [tx|rx]

  • debug spanning_tree event

  • debug spanning_tree verbose

  • debug spanning_tree interface <ifname>

Following debug commands will be supported for displaying internal data structures

  • debug spanning_tree dump global

  • debug spanning_tree dump mst instance <instance-id>

YANG Model

Model will be extended as follows for MSTP:

module sonic-stp {

    yang-version 1.1;

    namespace "http://github.com/sonic-net/sonic-stp";
    prefix "stp";

    import sonic-port {
        prefix port;
        revision-date 2019-07-01;
    }

    import sonic-portchannel {
        prefix lag;
        revision-date 2021-06-13;
    }

    import sonic-vlan {
        prefix vlan;
        revision-date 2021-04-22;
    }

    import sonic-device_metadata { 
        prefix device_metadata;
        revision-date 2021-02-27;
    }

    description "STP yang Module for SONiC OS";

    revision 2023-04-18 {
        description "First Revision";
    }

    grouping interfaceAttr {
        leaf path_cost {
            type uint64 {
                range "1..200000000" {
                    error-message "Invalid Port Path Cost value.";
                }
        }
            Default 200;
            description
                "The port's contribution, when it is the Root Port,
                to the Root Path Cost for the Bridge";
        }

        leaf priority {
            type uint8 {
                range "0..240" {
                    error-message "Invalid Port Priority value.";
                }
        }
            Default 128;
            description
                "The manageable component of the Port Identifier,
                also known as the Port Priority";
        }

        leaf link_type {
            type enumeration {
                    enum "P2P" {
                    description "Point-to-Point link type.";
                }
                enum "Shared-Lan" {
                description "Shared LAN link type.";
                }
                enum "Auto" {
                description "Automatically determine link type.";
                }
        }
            Default "Auto";
            description
                "The type of link for the interface. Options include 
                P2P, Shared-Lan, or Auto.";
        }
    }
    container sonic-stp {

        container STP_GLOBAL {
            description "Global STP table";

            leaf mode {
                type enumeration {
                    enum "pvst";
                    enum "mstp";
                }
                description "STP mode";
            }

            leaf forward_delay {
		must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode!='mst'" {
			error-message "Configuration not allowed in MST mode";
			error-app-tag stp-invalid;
		}
                type uint8 {
                    range "4..30" {
                        error-message "forward_delay value out of range";
                    }
                }
                Default 15;
                description "Forward delay in sec";
            }

            leaf hello_time {
		must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode!='mst'" {
			error-message "Configuration not allowed in MST mode";
			error-app-tag stp-invalid;
		}
                type uint8 {
                    range "1..10" {
                        error-message "hello_time value out of range";
                    }
                }
                Default 2;
                description "Hello time in sec";
            }

            leaf max_age {
		must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode!='mst'" {
			error-message "Configuration not allowed in MST mode";
			error-app-tag stp-invalid;
		}
                type uint8 {
                    range "6..40" {
                        error-message "max_age value out of range";
                    }
                }
                Default 20;
                description "Max age";
            }

            leaf rootguard_timeout {
		must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode!='mst'" {
			error-message "Configuration not allowed in MST mode";
			error-app-tag stp-invalid;
		}
                type uint16 {
                    range "5..600" {
                        error-message "rootguard_timeout value out of range";
                    }
                }
                Default 30;
                description "Root guard timeout in sec";
            }

            leaf priority {
		must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode!='mst'" {
			error-message "Configuration not allowed in MST mode";
			error-app-tag stp-invalid;
		}
                must ". mod 4096 = 0" {
                    error-message "bridge priority must be a multiple of 4096";
                }      

                type uint16 {
                    range "0..61440" {
                        error-message "priority value out of range";
                    }
                }
                Default 32768;
                description "Bridge priority";
            }
        }

	container STP_MST {
	     	max-elements 1;
		key "keyleaf";
                sonic-ext:dependent-on "STP_LIST";

                leaf keyleaf {
                    type enumeration {
                        enum GLOBAL;
                    }
                    description
                        "Key node identifier. It's value is always GLOBAL";
                }

                leaf name {
                    must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode='mst'" {
                        error-message "Configuration allowed in MST mode";
                        error-app-tag stp-invalid;
                    }
                    type string {
			length "1..32";
		    }
                    description
                        "MST Region name";
                }

                leaf revision {
                    must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode='mst'" {
                        error-message "Configuration allowed in MST mode";
                        error-app-tag stp-invalid;
                    }
                    type uint16 {
                    range "0..65535" {
                        error-message "revision value out of range";
                    }
                }
                    description
                        "MST Revision number";
                }

                leaf max_hops {
                    must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode='mst'" {
                        error-message "Configuration allowed in MST mode";
                        error-app-tag stp-invalid;
                    }
                    type uint8 {
                    	range "1..255" {
                        	error-message "max-hops value out of range";
                    	}
		    }
                    Default 20;
                    description
                        "MST Max hops";
                }

                leaf hello_time {
                    must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode='mst'" {
                        error-message "Configuration allowed in MST mode";
                        error-app-tag stp-invalid;
                    }
                    type uint8 {
                    	range "1..10" {
                        	error-message "hello_time value out of range";
                    	}
		    }
		    Default 2;
                    description
                        "MST hello time";
                }

                leaf max_age {
                    must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode='mst'" {
                        error-message "Configuration allowed in MST mode";
                        error-app-tag stp-invalid;
                    }
                     type uint8 {
                    	range "6..40" {
                        	error-message "max_age value out of range";
                    	}
	            }
                    Default 20;
                    description
                        "MST max age";
                }

                leaf forward_delay {
                    must "../../../STP/STP_LIST[keyleaf='GLOBAL']/mode='mst'" {
                        error-message "Configuration allowed in MST mode";
                        error-app-tag stp-invalid;
                    }
	            type uint8 {
                    	range "4..30" {
                        	error-message "forward_delay value out of range";
			}
                    }
                    Default 15;
                    description
                        "MST forward delay";
                }
	 }
	 container STP_MST_INST {
            list STP_MST_INST_LIST {
                key "instance";

                leaf instance {
                    type uint16;
                    description
                        "Instance identifier";
                }

                leaf-list vlan {
                    type string;
                    description
                        "Vlan list";
                }

                leaf bridge_priority {
                    type uint16 {
                        range "0..61440" {
                            error-message "Invalid Bridge Priority value.";
                        }
                    }
                    Default 32768;
                    description
                        "The manageable component of the Bridge Identifier";
                }
            }
        }

        container STP_MST_PORT {
            list STP_MST_PORT_LIST {
                key "inst_id ifname";

                leaf inst_id {
                    type leafref {
                        path "../../../STP_MST_INST/STP_MST_INST_LIST/instance";
                    }
                    description
                        "Reference to MST Instance";
                }

                leaf ifname {
                    type leafref {
                        path "../../../STP_PORT/STP_PORT_LIST/ifname";
                    }
                    description
                        "Reference to Ethernet interface or PortChannel in the STP database";
                }
                uses interfaceAttr;
            }
        }

	list STP_MST_INST_TABLE_LIST {
            sonic-ext:db-name "APPL_DB";
            key "inst_id";

            leaf inst_id {
                type leafref {
                     path "../../../STP_MST_INST/STP_MST_INST_LIST/instance";
                }
                description
                     "Reference to MST Instance";
            }

            leaf-list vlan {
                type string;
                description
                     "Vlan list";
            }

            leaf bridge_priority {
                type uint16;
                description
                      "The manageable component of the Bridge Identifier";
            }

            leaf root_priority {
                 type string;
                 description
                        "The manageable component of the Port Identifier";
            }

            leaf root_address {
                type string;
                description
                        "";
            }

            leaf root_cost {
                type string;
                description
                        "";
            }

            leaf root_port {
                type string;
                description
                        "";
            }

            leaf bridge_address {
                type string;
                description
                        "";
            }
        }
    }
}

Rest API Support

Rest API is out of scope for this HLD.

Warm Boot

Warm boot will not be supported. The IEEE 802.1s standard of MSTP does not define any potential way to support it as this might cause loops in the network.

User is expected to do a cold reboot when MSTP is running. If a user tries to perform a warm boot while MSTP is enabled, an error message will be displayed. User will first need to disable MSTP so the topology converges and reevaluates the paths.

Testing Requirements

Unit Test Cases

CLI Test Cases

  1. Verify CLI to enable MSTP globally.

  2. Verify CLI to disable MSTP globally.

  3. Verify CLI to set MSTP region name.

  4. Verify CLI to set MSTP region revision.

  5. Verify CLI to set MSTP port priority.

  6. Verify CLI to set MSTP hello-time.

  7. Verify CLI to set MSTP cost-value.

  8. Verify CLI to set MSTP max-hops.

  9. Verify CLI to set MSTP root guard.

  10. Verify CLI to set MSTP BPDU guard.

  11. Verify CLI to set edge port.

  12. Verify CLI to create an instance.

  13. Verify CLI to delete an instance.

  14. Verify CLI to clear MSTP region name.

  15. Verify CLI to clear MSTP region revision.

  16. Verify CLI to clear MSTP port priority.

  17. Verify CLI to clear MSTP hello-time.

  18. Verify CLI to clear MSTP cost-value.

  19. Verify CLI to clear MSTP max-hops.

  20. Verify CLI to clear MSTP root guard.

  21. Verify CLI to clear MSTP BPDU guard.

  22. Verify CLI to clear bridge priority in an instance.

  23. Verify CLI to map VLAN to an instance.

  24. Verify CLI to delete VLAN from an instance.

  25. Verify CLI to set interface priority on a per-instance basis.

  26. Verify CLI to set interface cost on a per-instance basis.

  27. Verify CLI to display all information related to the region.

  28. Verify CLI to display information related to a specific instance.

  29. Verify CLI to display information about a specific interface in a specific instance.

  30. Verify CLI to display statistics.

  31. Verify CLI to display statistics on a per-instance basis.

  32. Verify CLI to clear statistics.

  33. Verify CLI to clear statistics on a per-instance basis.

  34. Verify CLI to clear statistics on a per-interface per-instance basis.

  35. Verify the commands that are disabled for MSTP mode.

Functional Test Cases

  1. Verify CONFIG DB is populated with configured MSTP parameters.

  2. Verify MSTP instances running on multiple VLANs.

  3. Verify multiple VLANs can be mapped to a single instance.

  4. Verify VLAN to STP instance mapping is populated correctly in APP DB, ASIC DB, and Hardware.

  5. Verify instance STP ports are created for its VLAN members.

  6. Verify correct format of MSTP BPDU.

  7. Verify MSTP traps are created.

  8. Verify correct operation of CIST and MSTIs.

  9. Verify two bridges are in the same region only if they have the same region name, revision, and instance to VLAN mapping.

  10. Verify the loop-free topology inside and between regions.

  11. Verify the operational values of forward delay, max age, hello timer, and max hops are of the CIST root.

  12. Verify FDB flush as a result of topology change.

  13. Verify instance is active only when there is at least one VLAN member of any of the VLANs that are mapped to it.

  14. Verify altering bridge priority can alter the selection of CIST root, regional root, and MSTI root.

  15. Verify altering port priority can alter the selection of designated port.

  16. Verify max-hops by changing value.

  17. Verify hello-timer by changing timer values.

  18. Verify max-age by changing intervals.

  19. Verify altering bridge priority will alter Root Bridge selection.

  20. Verify altering port priority will alter Root port selection.

  21. Verify altering port priority will alter Designated port selection.

  22. Verify altering port cost results in the path with the lowest root path cost being selected as the root port.

  23. Verify port states on the same physical interface.

  24. Verify MSTP interoperability with STP and RSTP.

  25. Verify MSTP operational data is synced to APP DB and ASIC DB correctly.

  26. Verify MSTP over LAG.

  27. Verify MSTP over static breakout ports.

  28. Verify BPDU Guard Activation on Ports.

  29. Verify BPDU Guard Port State Transition.

  30. Verify BPDU Guard Recovery Mechanism.

  31. Verify BPDU Guard and MSTP Interaction.

  32. Verify MSTP Topology Changes Detection.

  33. Verify MSTP and VLAN Integration.

  34. Verify MSTP Path Cost Calculation.

  35. Verify MSTP Failover.

Logging and Debugging Test Cases

  1. Verify debugging logs for a region.

  2. Verify debugging logs for an instance.

  3. Verify debugging of internal data structure of a region.

  4. Verify debugging of internal data structure of an instance.

  5. Verify debugging of internal data structure of a specific interface in an instance.

SAI

  1. Verify creation of STP instance.

  2. Verify adding VLAN to an instance.

  3. Verify deleting VLAN from an instance.

  4. Verify adding a port to an instance.

  5. Verify deleting a port from an instance.

  6. Verify port state for different instances.

L3

  1. Verify normal flow of L3 traffic with the MSTP topology.