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As a best practice, it is always recommended to manually set up each bridge's priority, port priority, and port path cost to ensure proper Layer2 functionality at all times. Leaving STP related values to defaults are acceptable for a network that consists of 1 to 2 bridges running with (R/M)STP enabled, but it is highly recommended to manually set these values for larger networks. Since STP elects a root bridge and root ports by checking STP related values from bridges over the network, then leaving STP settings to automatic may elect an undesired root bridge and root ports and in case of a hardware failure can result in an inaccessible network.
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RouterOS bridge does not work with PVST and its variants. The PVST BPDUs (with a MAC destination 01:00:0C:CC:CC:CD) are treated by RouterOS bridges as typical multicast packets. In simpler terms, they undergo RouterOS bridge/switch forwarding logic and may get tagged or untagged. |
Monitoring
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You can check the STP status of a bridge by using the /interface bridge monitor command, for example:
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/interface bridge monitor bridgebridge1 state: enabled current-mac-address: 64B8:D169:54F4:D930:2719:E6FE root-bridge: yesno root-bridge-id: 0x30000x1000.64B8:D169:54F4:D930:2719:E6FD root-path-cost: 04000 root-port: nonesfp-sfpplus2 port-count: 52 designated-port-count: 1 fast-forward: 5yes |
Note that the root bridge doesn't have any root ports, only designated ports.
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/interface bridge port monitor 2 [find interface=sfp-sfpplus2] interface: ether3sfp-sfpplus2 status: in-bridge port-number: 31 role: root-port edge-port: no edge-port-discovery: yes point-to-point-port: yes external-fdb: no sending-rstp: yes learning: yes forwarding: yes path-cost: 2000 root-path-cost: 104000 designated-bridge: 0x30000x8000.64DC:D12C:546E:D99E:2711:E61C designated-cost: 02000 designated-port-number: 4 hw-offload-group: switch12 |
Note that root-Note that root-bridge-id consists of the bridge priority and the bridge's MAC address, for non-root bridges the root bridge will be shown as designated-bridge. One port can have one role in an STP enabled network, below is a list of possible port roles:
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When creating a bridge or adding a port to the bridge the following are the default values that are assigned by RouterOS:
- Default bridge priority: 32768 / 0x8000
- Default bridge port path cost: 10
- Default bridge port priority: 0x80
- BPDU message age increment: 1
- HelloTime: 2
- Default max message age: 20
a port to the bridge the following are the default values that are assigned by RouterOS:
- Default bridge priority: 32768 / 0x8000
- Default bridge port path cost: based on interface speed
- Default bridge port priority: 0x80
- BPDU message age increment: 1
- HelloTime: 2
- Default max message age: 20
The bridge interface setting port-cost-mode changes the port path-cost and internal-path-cost mode for bridged ports, utilizing automatic values based on interface speed. This setting does not impact bridged ports with manually configured path-cost or internal-path-cost properties. Below are examples illustrating the path-costs corresponding to specific data rates (with proportionate calculations for intermediate rates):
| Data rate | Long | Short |
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| 10 Mbps | 2,000,000 | 100 |
| 100 Mbps | 200,000 | 19 |
| 1 Gbps | 20,000 | 4 |
| 10 Gbps | 2,000 | 2 |
| 100 Gbps | 200 | 1 |
For bonded interfaces, the highest path-cost among all bonded member ports is applied, this value remains unaffected by the total link speed of the bonding. For virtual interfaces (such as VLAN, EoIP, VXLAN), as well as wifi, wireless, and 60GHz interfaces, a path-cost of 20,000 is assigned for long mode, and 10 for short mode. For dynamically bridged interfaces (e.g. wifi, wireless, PPP, VPLS), the path-cost defaults to 20,000 for long mode and 10 for short mode. However, this can be manually overridden by the service that dynamically adds interfaces to bridge, for instance, by using the CAPsMAN datapath.bridge-cost setting. RouterOS versions prior to 7.13 RouterOS does not change port path cost based on the link speed, for 10M, 100M, 1000M, and 10000M link speeds the default path cost value when a port is added to a bridge are was always 10.
The age of a BPDU is determined by how many bridges have the BPDU passed times the message age since RouterOS uses 1 as the message age increment, then the BPDU packet can pass as many bridges as specified in the max-message-age parameter. By default this value is set to 20, this means that after the 20th bridge the BPDU packet will be discarded and the next bridge will become a root bridge, note that if max-message-age=20on is set, then it is hard to predict which ports will be the designated port on the 21st bridge and may result in traffic not being able to be forwarded properly.
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| Property | Description |
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| priority (integer: 0..65535 decimal format or 0x0000-0xffff hex format; Default: 32768 / 0x8000) | /interface bridge msti, MST Instance priority, used to elect a regional root inside a MSTP region. |
| internal-path-cost (integer: 1..4294967295200000000; Default: 10) | /interface bridge port, path cost to the regional root for unknown VLAN IDs (MSTI0), used on a root port inside a MSTP region. |
| priority (integer: 0..240; Default: 128) | /interface bridge port mst-override, MST port priority for a defined MST Instance, used on a bridge port on the regional root bridge. |
| internal-path-cost (integer: 1..200000000; Default: 10) | /interface bridge port mst-override, MST port path cost for a defined MST Instance, used on a non-root bridge port inside a MSTP region. |
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| Property | Description |
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| priority (integer: 0..65535 decimal format or 0x0000-0xffff hex format; Default: 32768 / 0x8000) | /interface bridge, CIST bridge priority, used to elect a CIST root bridge. |
| priority (integer: 0..240; Default: 128) | /interface bridge port, CIST port priority, used on a CIST root bridge to elect CIST root ports. |
| path-cost (integer: 1..4294967295200000000; Default: 10) | /interface bridge port, CIST port path cost, used on a CIST non-root bridge port to elect CIST root ports. |
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| Property | Description |
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| disabled (yes | no; Default: no) | Whether entry is disabled. |
| internal-path-cost (integer: 1..200000000; Default: 10) | Path cost for an MST instance's VLAN mapping, used on VLANs that are facing towards the root bridge to manipulate path selection, lower path cost is preferred. |
| identifier (integer: 1..31; Default: ) | MST instance identifier. |
| priority (integer: 0..240; Default: 128) | The priority an MST instance's VLAN, used on VLANs that are facing away from the root bridge to manipulate path selection, lower priority is preferred. |
| interface (name; Default: ) | Name of the port on which use configured MST instance's VLAN mappings and defined path cost and priority. |
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