Spanning Tree and Rapid Spanning Tree - STP

Spanning tree protocol (STP) identifies links in the network and shuts down redundant links, preventing possible network loops and broadcast radiation on a bridged network. STP also provides redundant links for automatic failover when an active link fails. STP is enabled by default in Cumulus Linux for both VLAN-aware and traditional bridges.

Cumulus Linux supports RSTP, PVST, and PVRST modes:

  • Traditional bridges operate in both PVST and PVRST mode. The default is set to PVRST. Each traditional bridge has its own separate STP instance.
  • VLAN-aware bridges operate only in RSTP mode.

STP for a Traditional Mode Bridge

Per VLAN Spanning Tree (PVST) creates a spanning tree instance for a bridge. Rapid PVST (PVRST) supports RSTP enhancements for each spanning tree instance. To use PVRST with a traditional bridge, you must create a bridge corresponding to the untagged native VLAN and all the physical switch ports must be part of the same VLAN.

For maximum interoperability, when connected to a switch that has a native VLAN configuration, the native VLAN must be configured to be VLAN 1 only.

STP for a VLAN-aware Bridge

VLAN-aware bridges operate in RSTP mode only. RSTP on VLAN-aware bridges works with other modes in the following ways:

RSTP and STP

If a bridge running RSTP (802.1w) receives a common STP (802.1D) BPDU, it falls back to 802.1D automatically.

RSTP and PVST

The RSTP domain sends BPDUs on the native VLAN, whereas PVST sends BPDUs on a per VLAN basis. For both protocols to work together, you need to enable the native VLAN on the link between the RSTP to PVST domain; the spanning tree is built according to the native VLAN parameters.

The RSTP protocol does not send or parse BPDUs on other VLANs, but floods BPDUs across the network, enabling the PVST domain to maintain its spanning-tree topology and provide a loop-free network.

  • To enable proper BPDU exchange across the network, be sure to allow all VLANs participating in the PVST domain on the link between the RSTP and PVST domains.

  • When using RSTP together with an existing PVST network, you need to define the root bridge on the PVST domain. Either lower the priority on the PVST domain or change the priority of the RSTP switches to a higher number.

  • When connecting a VLAN-aware bridge to a proprietary PVST+ switch using STP, you must allow VLAN 1 on all 802.1Q trunks that interconnect them, regardless of the configured native VLAN. Only VLAN 1 enables the switches to address the BPDU frames to the IEEE multicast MAC address. The proprietary switch might be configured like this:

    switchport trunk allowed vlan 1-100
    

RSTP and MST

RSTP works with MST seamlessly, creating a single instance of spanning tree that transmits BPDUs on the native VLAN.

RSTP treats the MST domain as one giant switch, whereas MST treats the RSTP domain as a different region. To enable proper communication between the regions, MST creates a Common Spanning Tree (CST) that connects all the boundary switches and forms the overall view of the MST domain. Because changes in the CST need to be reflected in all regions, the RSTP tree is included in the CST to ensure that changes on the RSTP domain are reflected in the CST domain. This does cause topology changes on the RSTP domain to impact the rest of the network but keeps the MST domain informed of every change occurring in the RSTP domain, ensuring a loop-free network.

Configure the root bridge within the MST domain by changing the priority on the relevant MST switch. When MST detects an RSTP link, it falls back into RSTP mode. The MST domain choses the switch with the lowest cost to the CST root bridge as the CIST root bridge.

RSTP with MLAG

More than one spanning tree instance enables switches to load balance and use different links for different VLANs. With RSTP, there is only one instance of spanning tree. To better utilize the links, you can configure MLAG on the switches connected to the MST or PVST domain and set up these interfaces as an MLAG port. The PVST or MST domain thinks it is connected to a single switch and utilizes all the links connected to it. Load balancing is based on the port channel hashing mechanism instead of different spanning tree instances and uses all the links between the RSTP to the PVST or MST domains. For information about configuring MLAG, see Multi-Chassis Link Aggregation - MLAG.

Optional Configuration

There are a number of ways to customize STP in Cumulus Linux. Exercise caution when changing the settings below to prevent malfunctions in STP loop avoidance.

Spanning Tree Priority

If you have a multiple spanning tree instance (MSTI 0, also known as a common spanning tree, or CST), you can set the tree priority for a bridge. The bridge with the lowest priority is elected the root bridge. The priority must be a number between 0 and 61440, and must be a multiple of 4096. The default is 32768.

To set the tree priority, run the following commands:

The following example command sets the tree priority to 8192:

cumulus@switch:~$ net add bridge stp treeprio 8192
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Configure the tree priority (mstpctl-treeprio) under the bridge stanza in the /etc/network/interfaces file, then run the ifreload -a command. The following example command sets the tree priority to 8192:

cumulus@switch:~$ sudo nano /etc/network/interfaces
...
auto bridge
iface bridge
    # bridge-ports includes all ports related to VxLAN and CLAG.
    # does not include the Peerlink.4094 subinterface
    bridge-ports bond01 bond02 peerlink vni13 vni24 vxlan4001
    bridge-pvid 1
    bridge-vids 13 24
    bridge-vlan-aware yes
    mstpctl-treeprio 8192
...
cumulus@switch:~$ ifreload -a

Cumulus Linux supports MSTI 0 only. It does not support MSTI 1 through 15.

PortAdminEdge (PortFast Mode)

PortAdminEdge is equivalent to the PortFast feature offered by other vendors. It enables or disables the initial edge state of a port in a bridge.

All ports configured with PortAdminEdge bypass the listening and learning states to move immediately to forwarding.

PortAdminEdge mode might cause loops if it is not used with the BPDU guard feature.

It is common for edge ports to be configured as access ports for a simple end host; however, this is not mandatory. In the data center, edge ports typically connect to servers, which might pass both tagged and untagged traffic.

To configure PortAdminEdge mode:

The following example commands configure PortAdminEdge and BPDU guard for swp5.

cumulus@switch:~$ net add interface swp5 stp bpduguard
cumulus@switch:~$ net add interface swp5 stp portadminedge
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Configure PortAdminEdge and BPDU guard under the switch port interface stanza in the /etc/network/interfaces file, then run the ifreload -a command. The following example configures PortAdminEdge and BPD guard on swp5.

cumulus@switch:~$ sudo nano /etc/netowrk/interfaces
...
auto swp5
iface swp5
    mstpctl-bpduguard yes
    mstpctl-portadminedge yes
...
cumulus@switch:~$ sudo ifreload -a

Runtime Configuration (Advanced)

A runtime configuration is non-persistent, which means the configuration you create here does not persist after you reboot the switch.

To configure PortAdminEdge and BPDU guard at runtime, run the following commands:

cumulus@switch:~$ sudo mstpctl setportadminedge br2 swp1 yes
cumulus@switch:~$ sudo mstpctl setbpduguard br2 swp1 yes

PortAutoEdge

PortAutoEdge is an enhancement to the standard PortAdminEdge (PortFast) mode, which allows for the automatic detection of edge ports. PortAutoEdge enables and disables the auto transition to and from the edge state of a port in a bridge.

Edge ports and access ports are not the same. Edge ports transition directly to the forwarding state and skip the listening and learning stages. Upstream topology change notifications are not generated when an edge port link changes state. Access ports only forward untagged traffic; however, there is no such restriction on edge ports, which can forward both tagged and untagged traffic.

When a BPDU is received on a port configured with PortAutoEdge, the port ceases to be in the edge port state and transitions into a normal STP port. When BPDUs are no longer received on the interface, the port becomes an edge port, and transitions through the discarding and learning states before resuming forwarding.

PortAutoEdge is enabled by default in Cumulus Linux.

To disable PortAutoEdge for an interface:

The following example commands disable PortAutoEdge on swp1:

cumulus@switch:~$ net add interface swp1 stp portautoedge no
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Edit the switch port interface stanza in the /etc/network/interfaces file to add the mstpctl-portautoedge no line, then run the ifreload -a command. The following example disables PortAutoEdge on swp1:

cumulus@switch:~$ sudo nano /etc/network/interfaces
...
auto swp1
iface swp1
    alias to Server01
    # Port to Server02
    mstpctl-portautoedge no
...
cumulus@switch:~$ sudo ifreload -a

To re-enable PortAutoEdge for an interface:

The following example commands re-enable PortAutoEdge on swp1:

cumulus@switch:~$ net del interface swp1 stp portautoedge no
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit
Edit the switch port interface stanza in the /etc/network/interfaces file to remove mstpctl-portautoedge no, then run the ifreload -a command.

BPDU Guard

You can configure BPDU guard to protect the spanning tree topology from unauthorized switches affecting the forwarding path. For example, if you add a new switch to an access port off a leaf switch and this new switch is configured with a low priority, it might become the new root switch and affect the forwarding path for the entire layer 2 topology.

To configure BPDU guard:

The following example commands set BPDU guard for swp5:

cumulus@switch:~$ net add interface swp5 stp bpduguard
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Edit the switch port interface stanza in the /etc/network/interfaces file to add the mstpctl-bpduguard yes line, then run the ifreload -a command. The following example sets BPDU guard for interface swp5:

cumulus@switch:~$ sudo nano /etc/network/interfaces
...
auto swp5
iface swp5
    mstpctl-bpduguard yes
...
cumulus@switch:~$ sudo ifreload -a

If a BPDU is received on the port, STP brings down the port and logs an error in /var/log/syslog. The following is a sample error:

mstpd: error, MSTP_IN_rx_bpdu: bridge:bond0 Recvd BPDU on BPDU Guard Port - Port Down

To determine whether BPDU guard is configured, or if a BPDU has been received:

cumulus@switch:~$ net show bridge spanning-tree | grep bpdu
  bpdu guard port    yes                bpdu guard error     yes
cumulus@switch:~$ mstpctl showportdetail bridge bond0
bridge:bond0 CIST info
  enabled            no                      role                 Disabled
  port id            8.001                   state                discarding
  external port cost 305                     admin external cost  0
  internal port cost 305                     admin internal cost  0
  designated root    8.000.6C:64:1A:00:4F:9C dsgn external cost   0
  dsgn regional root 8.000.6C:64:1A:00:4F:9C dsgn internal cost   0
  designated bridge  8.000.6C:64:1A:00:4F:9C designated port      8.001
  admin edge port    no                      auto edge port       yes
  oper edge port     no                      topology change ack  no
  point-to-point     yes                     admin point-to-point auto
  restricted role    no                      restricted TCN       no
  port hello time    10                      disputed             no
  bpdu guard port    yes                      bpdu guard error     yes
  network port       no                      BA inconsistent      no
  Num TX BPDU        3                       Num TX TCN           2
  Num RX BPDU        488                     Num RX TCN           2
  Num Transition FWD 1                       Num Transition BLK   2
  bpdufilter port    no
  clag ISL           no                      clag ISL Oper UP     no
  clag role          unknown                 clag dual conn mac   0:0:0:0:0:0
  clag remote portID F.FFF                   clag system mac      0:0:0:0:0:0

The only way to recover a port that has been placed in the disabled state is to manually bring up the port with the sudo ifup <interface> command. See Interface Configuration and Management for more information about ifupdown.

Bringing up the disabled port does not correct the problem if the configuration on the connected end-station has not been resolved.

Bridge Assurance

On a point-to-point link where RSTP is running, if you want to detect unidirectional links and put the port in a discarding state, you can enable bridge assurance on the port by enabling a port type network. The port is then in a bridge assurance inconsistent state until a BPDU is received from the peer. You need to configure the port type network on both ends of the link for bridge assurance to operate properly.

Bridge assurance is disabled by default.

To enable bridge assurance on an interface:

The following example commands enable bridge assurance on swp1:

cumulus@switch:~$ net add interface swp1 stp portnetwork
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Edit the switch port interface stanza in the /etc/network/interfaces file to add the mstpctl-portnetwork yes line, then run the ifreload -a command. The following example enables bridge assurance on swp5:

cumulus@switch:~$ sudo nano /etc/network/interfaces
...
auto swp5
iface swp5
    mstpctl-portnetwork yes
...
cumulus@switch:~$ sudo ifreload -a

Runtime Configuration (Advanced)

A runtime configuration is non-persistent, which means the configuration you create here does not persist after you reboot the switch.

To enable bridge assurance at runtime, run mstpctl:

cumulus@switch:~$ sudo mstpctl setportnetwork br1007 swp1.1007 yes

cumulus@switch:~$ sudo mstpctl showportdetail br1007 swp1.1007 | grep network
  network port       yes                     BA inconsistent      yes

To monitor logs for bridge assurance messages, run the following command:

cumulus@switch:~$ sudo grep -in assurance /var/log/syslog | grep mstp
  1365:Jun 25 18:03:17 mstpd: br1007:swp1.1007 Bridge assurance inconsistent

BPDU Filter

You can enable bpdufilter on a switch port, which filters BPDUs in both directions. This disables STP on the port as no BPDUs are transiting.

Using BDPU filter might cause layer 2 loops. Use this feature deliberately and with extreme caution.

To configure the BPDU filter on an interface:

The following example commands configure the BPDU filter on swp6:

cumulus@switch:~$ net add interface swp6 stp portbpdufilter
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Edit the switch port interface stanza in the /etc/network/interfaces file to add the mstpctl-portbpdufilter yes line, then run the ifreload -a command. The following example configures BPDU filter on swp6:

cumulus@switch:~$ sudo nano /etc/network/interfaces
...
auto swp6
iface swp6
    mstpctl-portbpdufilter yes
...
cumulus@switch:~$ sudo ifreload -a

Runtime Configuration (Advanced)

A runtime configuration is non-persistent, which means the configuration you create here does not persist after you reboot the switch.

To enable BPDU filter at runtime, run mstpctl. For example:

cumulus@switch:~$ sudo mstpctl setportbpdufilter br100 swp1.100=yes swp2.100=yes

Parameter List

Spanning tree parameters are defined in the IEEE 802.1D and 802.1Q specifications.

The table below describes the STP configuration parameters available in Cumulus Linux. For a comparison of STP parameter configuration between mstpctl and other vendors, read this knowledge base article.

Most of these parameters are blacklisted in the ifupdown_blacklist section of the /etc/netd.conf file. Before you configure these parameters, you must edit the file to remove them from the blacklist.

Parameter
NCLU Command
Description
mstpctl-maxagenet add bridge stp maxage <seconds>Sets the maximum age of the bridge in seconds. The default is 20. The maximum age must meet the condition 2 * (Bridge Forward Delay - 1 second) >= Bridge Max Age.
mstpctl-ageingnet add bridge stp ageing <seconds>Sets the Ethernet (MAC) address ageing time for the bridge in seconds when the running version is STP, but not RSTP/MSTP. The default is 1800.
mstpctl-fdelaynet add bridge stp fdelay <seconds>Sets the bridge forward delay time in seconds. The default value is 15. The bridge forward delay must meet the condition 2 * (Bridge Forward Delay - 1 second) >= Bridge Max Age.
mstpctl-maxhopsnet add bridge stp maxhops <max-hops>Sets the maximum hops for the bridge. The default is 20.
mstpctl-txholdcountnet add bridge stp txholdcount <hold-count>Sets the bridge transmit hold count. The default value is 6.
mstpctl-forceversnet add bridge stp forcevers RSTP|STPSets the force STP version of the bridge to either RSTP/STP. The default is RSTP.
mstpctl-treeprionet add bridge stp treeprio <priority>Sets the tree priority of the bridge for an MSTI (multiple spanning tree instance). The priority value is a number between 0 and 61440 and must be a multiple of 4096. The bridge with the lowest priority is elected the root bridge. The default is 32768. See Spanning Tree Priority above.
Note: Cumulus Linux supports MSTI 0 only. It does not support MSTI 1 through 15.
mstpctl-hellonet add bridge stp hello <seconds>Sets the bridge hello time in seconds. The default is 2.
mstpctl-portpathcostnet add interface <interface> stp portpathcost <cost>Sets the port cost of the interface. The default is 0.
mstpd supports only long mode; 32 bits for the path cost.
mstpctl-treeportprionet add interface <interface> stp treeportprio <priority>Sets the priority of the interface for the MSTI. The priority value is a number between 0 and 240 and must be a multiple of 16. The default is 128.
Note: Cumulus Linux supports MSTI 0 only. It does not support MSTI 1 through 15.
mstpctl-portadminedgenet add interface <interface> stp portadminedgeEnables or disables the initial edge state of the interface in the bridge. The default is no.
In NCLU, to use a setting other than the default, you must specify this attribute without setting an option. See PortAdminEdge above.
mstpctl-portautoedgenet add interface <interface> stp portautoedgeEnables or disables the auto transition to and from the edge state of the interface in the bridge. PortAutoEdge is enabled by default. See PortAutoEdge above.
mstpctl-portp2pnet add interface <interface> stp portp2p yes|noEnables or disables the point-to-point detection mode of the interface in the bridge.
mstpctl-portrestrrolenet add interface <interface> stp portrestrroleEnables or disables the ability of the interface in the bridge to take the root role. The default is no.
To enable this feature with the NCLU command, you specify this attribute without an option (portrestrrole). To enable this feature by editing the /etc/network/interfaces file, you specify mstpctl-portrestrrole yes.
mstpctl-portrestrtcnnet add interface <interface> stp portrestrtcnEnables or disables the ability of the interface in the bridge to propagate received topology change notifications. The default is no.
mstpctl-portnetworknet add interface <interface> stp portnetworkEnables or disables the bridge assurance capability for a network interface. The default is no. See Bridge Assurance above.
mstpctl-bpduguardnet add interface <interface> stp bpduguardEnables or disables the BPDU guard configuration of the interface in the bridge. The default is no. See BPDU Guard above.
mstpctl-portbpdufilternet add interface <interface> stp portbpdufilterEnables or disables the BPDU filter functionality for an interface in the bridge. The default is no. See BPDU Filter above.
mstpctl-treeportcostnet add interface <interface> stp treeportcost <port-cost>Sets the spanning tree port cost to a value from 0 to 255. The default is 0.

Troubleshooting

To check STP status for a bridge:

Run the net show bridge spanning-tree command:

cumulus@switch:~$ net show bridge spanning-tree
Bridge info
  enabled         yes
  bridge id       8.000.44:38:39:FF:40:94
    Priority:     32768
    Address:      44:38:39:FF:40:94
  This bridge is root.

  designated root 8.000.44:38:39:FF:40:94
    Priority:     32768
    Address:      44:38:39:FF:40:94

  root port       none
  path cost     0          internal path cost   0
  max age       20         bridge max age       20
  forward delay 15         bridge forward delay 15
  tx hold count 6          max hops             20
  hello time    2          ageing time          300
  force protocol version     rstp

INTERFACE  STATE  ROLE  EDGE
---------  -----  ----  ----
peerlink   forw   Desg  Yes
vni13      forw   Desg  Yes
vni24      forw   Desg  Yes
vxlan4001  forw   Desg  Yes

The mstpctl utility provided by the mstpd service configures STP. The mstpd daemon is an open source project used by Cumulus Linux to implement IEEE802.1D 2004 and IEEE802.1Q 2011.

The mstpd daemon starts by default when the switch boots and logs errors to /var/log/syslog.

mstpd is the preferred utility for interacting with STP on Cumulus Linux. brctl also provides certain tools for configuring STP; however, they are not as complete and output from brctl might be misleading.

To show the bridge state, run the brctl show command:

cumulus@switch:~$ sudo brctl show
  bridge name     bridge id               STP enabled     interfaces
  bridge          8000.001401010100       yes             swp1
                                                          swp4
                                                          swp5

To show the mstpd bridge port state, run the mstpctl showport bridge command:

cumulus@switch:~$ sudo mstpctl showport bridge
  E swp1 8.001 forw F.000.00:14:01:01:01:00 F.000.00:14:01:01:01:00 8.001 Desg
    swp4 8.002 forw F.000.00:14:01:01:01:00 F.000.00:14:01:01:01:00 8.002 Desg
  E swp5 8.003 forw F.000.00:14:01:01:01:00 F.000.00:14:01:01:01:00 8.003 Desg

The source code for mstpd and mstpctl was written by Vitalii Demianets and is hosted at the URL below.

  • GitHub - mstpd project
  • brctl(8)
  • bridge-utils-interfaces(5)
  • ifupdown-addons-interfaces(5)
  • mstpctl(8)
  • mstpctl-utils-interfaces(5)