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Summary
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The purpose of spanning tree protocol is to provide the ability to create loop-free Layer 2 topologies while having redundant links. While connecting multiple bridges or just cross-connecting bridge ports, it's possible to create network loops that can severely impact the stability of the network. Spanning tree protocol aims to resolve this problem by introducing the concept of the root bridge, all bridges in the same Layer 2 domain will exchange information about the shortest path to the root bridge. Afterward, each bridge will negotiate which ports to use to reach the root bridge. This information exchange is done with the help of Bridge Protocol Data Units (BPDUs). STP will disable certain ports for each bridge in order to avoid loops, while still ensuring that all bridges can communicate with each other. For an in-depth description of protocol please refer to 802.1Q.
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.
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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| Note |
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When using bridges that are set to use 802.1Q as EtherType, they will send out BPDUs to 01:80:C2:00:00:00, which are used by MSTP, RSTP, and STP. When using 802.1ad as bridge VLAN protocol, the BPDUs are not compatible with 802.1Q bridges and they are sent to 01:80:C2:00:00:08. (R/M)STP will not function properly if there are different bridge VLAN protocols across the Layer2 network. |
STP and RSTP
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STP and Rapid STP are used widely across many networks, but almost all networks have switched over using only RSTP since of its benefits. STP is a very old protocol and has a convergence time (the time needed to fully learn network topology changes and to continue properly forwarding traffic) of up to 50 seconds. RSTP has a lot of smaller convergence time, a few seconds or even a few milliseconds. It is recommended to use RSTP instead of STP since it is a lot faster and is also backward compatible with STP. One of the reasons why RSTP is faster is because of reduced possible port states, below is a list of possible STP port states:
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