Spanning Tree and Rapid Spanning Tree - STP
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. Cumulus Linux enables STP by default 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 PVRST. Each traditional bridge has its own separate STP instance.
- VLAN-aware bridges operate only in RSTP mode.
STP for a Traditional Mode Bridge
PVST creates a spanning tree instance for a bridge. 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, you must configure the native VLAN to VLAN 1.
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 builds 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.
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 ensure proper communication between the regions, MST creates a CST that connects all the boundary switches and forms the overall view of the MST domain. Because changes in the CST must reflect in all regions, the RSTP tree exists is in the CST to ensure that changes on the RSTP domain are in the CST domain. Topology changes on the RSTP domain impact the rest of the network but inform the MST domain 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 chooses 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 connects to a single switch and utilizes all the links connected to it. Load balancing depends 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.
Configure STP
There several 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 the root bridge. The priority must be a number between 0 and 61440, and must be a multiple of 4096. The default is 32768.
If you are running MLAG and have multiple bridges, the STP priority must be the same on all bridges on both peer switches.
The following example command sets the tree priority to 8192:
cumulus@switch:~$ nv set bridge domain br_default stp priority 8192
cumulus@switch:~$ nv config apply
Configure the tree priority (mstpctl-treeprio) under the bridge stanza in the /etc/network/interfaces file, then run the ifreload -a command.
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 with PortAdminEdge bypass the listening and learning states and go straight to forwarding.
PortAdminEdge mode causes loops if you do not use it with BPDU guard.
You typically configure edge ports as access ports for a simple end host. In the data center, edge ports connect to servers, which pass both tagged and untagged traffic.
The following example commands configure PortAdminEdge and BPDU guard for swp5:
cumulus@switch:~$ nv set interface swp5 bridge domain br_default stp admin-edge on
cumulus@switch:~$ nv set interface swp5 bridge domain br_default stp bpdu-guard on
cumulus@switch:~$ nv config apply
Configure PortAdminEdge and BPDU guard under the switch port interface stanza in the /etc/network/interfaces file, then run the ifreload -a command.
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 port with PortAutoEdge receives a BPDU, the port stops being in the edge port state and transitions into a normal STP port. When the interface no longer receives BPDUs, the port becomes an edge port, and transitions through the discarding and learning states before it resumes forwarding.
Cumulus Linux enables PortAutoEdge by default.
The following example commands disable PortAutoEdge on swp1:
cumulus@switch:~$ nv set interface swp1 bridge domain br_default stp auto-edge off
cumulus@switch:~$ nv config apply
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.
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
The following example commands reenable PortAutoEdge on swp1:
cumulus@switch:~$ nv set interface swp1 bridge domain br_default stp auto-edge on
cumulus@switch:~$ nv config apply
/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 an unauthorized device affecting the forwarding path. For example, if you add a new host to an access port off a leaf switch and the host sends an STP BPDU, BPDU guard protects against undesirable topology changes in the environment.
The following example commands set BPDU guard for swp5:
cumulus@switch:~$ nv set interface swp5 bridge domain br_default stp bpdu-guard on
cumulus@switch:~$ nv config apply
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.
cumulus@switch:~$ sudo nano /etc/network/interfaces
...
auto swp5
iface swp5
mstpctl-bpduguard yes
...
cumulus@switch:~$ sudo ifreload -a
To see if a port has BPDU guard on or if the port receives a BPDU:
cumulus@switch:~$ nv show bridge domain br_default stp
cumulus@switch:~$ mstpctl showportdetail br_default
bridge:swp5 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
If a port receives a BPDU, it goes into a protodown state, which results in a local OPER DOWN (carrier down) on the interface. Cumulus Linux also sets the protodown reason as bpduguard and records a log message in /var/log/syslog.
To show the reason for the port protodown, run the ip -p -j link show <interface> command.
cumulus@switch:~$ ip -p -j link show swp5
To recover from the protodown state, remove the protodown reason and protodown from the interface with the mstpctl clearbpduguardviolation <bridge> <interface> command.
cumulus@switch:~$ mstpctl clearbpduguardviolation bridge swp5
Bringing up the disabled port does not correct the problem if the configuration on the connected end station does not resolve.
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 it receives a BPDU from the peer. You need to configure the port type network on both ends of the link for bridge assurance.
Cumulus Linux disables bridge assurance by default.
The following example commands enable bridge assurance on swp1:
cumulus@switch:~$ nv set interface swp5 bridge domain br_default stp network on
cumulus@switch:~$ nv config apply
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.
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 sometimes causes layer 2 loops. Use this feature with caution.
The following example commands configure the BPDU filter on swp6:
cumulus@switch:~$ nv set interface swp6 bridge domain br_default stp bpdu-filter on
cumulus@switch:~$ nv config apply
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.
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
Restricted Role
To enable the interface in the bridge to take the restricted role:
cumulus@switch:~$ nv set interface swp1 bridge domain br_default stp restrrole on
cumulus@switch:~$ nv config apply
Edit the switch port interface stanza in the /etc/network/interfaces file to add the mstpctl-portrestrrole yes line, then run the ifreload -a command.
cumulus@switch:~$ sudo nano /etc/network/interfaces
...
auto swp6
iface swp6
mstpctl-portrestrrole yes
...
cumulus@switch:~$ sudo ifreload -a
Additional STP Parameters
The table below describes additional STP configuration parameters available in Cumulus Linux. You can set these optional parameters manually by editing the /etc/network/interfaces file. Cumulus Linux does not provide NVUE commands for these parameters.
The IEEE 802.1D and 802.1Q specifications describe STP parameters. For a comparison of STP parameter configuration between mstpctl and other vendors, read this knowledge base article.
| Parameter | Description |
|---|---|
mstpctl-maxage | 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. Add this parameter to the bridge stanza of the /etc/network/interfaces file. |
mstpctl-ageing | Sets the MAC address ageing time for the bridge in seconds when the running version is STP, but not RSTP or MSTP. The default is 1800. Add this parameter to the bridge stanza of the /etc/network/interfaces file. |
mstpctl-fdelay | 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. Add this parameter to the bridge stanza of the /etc/network/interfaces file. |
mstpctl-maxhops | Sets the maximum hops for the bridge. The default is 20. Add this parameter to the bridge stanza of the /etc/network/interfaces file. |
mstpctl-txholdcount | Sets the bridge transmit hold count. The default value is 6 seconds. Add this parameter to the bridge stanza of the /etc/network/interfaces file. |
mstpctl-forcevers | Sets the force STP version of the bridge to either RSTP/STP. The default is RSTP. Add this parameter to the bridge stanza of the /etc/network/interfaces file. |
mstpctl-hello | Sets the bridge hello time in seconds. The default is 2. Add this parameter to the bridge stanza of the /etc/network/interfaces file. |
mstpctl-portpathcost | Sets the port cost of the interface in the bridge. The default is 0.mstpd supports only long mode; 32 bits for the path cost.Add this parameter to the interface stanza of the /etc/network/interfaces file. |
mstpctl-portp2p | Enables or disables point-to-point detection mode of the interface in the bridge. Add this parameter to the interface stanza of the /etc/network/interfaces file. |
mstpctl-portrestrtcn | Enables or disables the interface in the bridge to propagate received topology change notifications. The default is no. Add this parameter to the interface stanza of the /etc/network/interfaces file. |
mstpctl-treeportcost | Sets the spanning tree port cost to a value from 0 to 255. The default is 0. Add this parameter to the interface stanza of the /etc/network/interfaces file. |
Be sure to run the sudo ifreload -a command after you set the STP parameter in the /etc/network/interfaces file.
Troubleshooting
To check STP status for a bridge:
cumulus@switch:~$ nv show bridge domain br_default stp
operational applied description
-------- ----------- ------- ---------------------------------------------------------------------
priority 32768 32768 stp priority. The priority value must be a number between 4096 and...
state up up The state of STP on the bridge
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 is sometimes 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
Related Information
- GitHub - mstpd project
- brctl(8)
- bridge-utils-interfaces(5)
- ifupdown-addons-interfaces(5)
- mstpctl(8)
- mstpctl-utils-interfaces(5)