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Configuring RSTP for interoperability with 3rd party switches

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It may become necessary in your deployment to configure your MS switches for compatibility with another vendor's core or distribution layer switches.

In order to ensure that Spanning Tree is properly configured, it is important to understand the following terms and concepts:

  • What is Spanning Tree? - Spanning tree is a protocol that prevents loops within your LAN
  • Root Bridge - This is the switch that becomes the root bridge of the spanning tree. It is typically the bridge with the smallest (lowest) bridge ID. Each bridge has a configurable priority number and a MAC Address; the bridge ID contains both numbers combined together - Bridge priority + MAC (32768.0200.0000.1111). The Bridge priority default is 32768 and can only be configured in multiples of 4096(Spanning tree uses the 12 bits extended system ID).
  • BPDU (Bridge Protocol Data Unit) - To ensure that each bridge has enough information, bridges use special data frames called Bridge Protocol Data Units (BPDUs) to exchange information about bridge IDs and root path costs.
  • Port States - STP/RSTP has several different port states:
    • Blocking - A port that would cause a switching loop if it were active. No user data is sent or received over a blocking port, but it may go into forwarding mode if the other links in use fail and the spanning tree algorithm determines the port may transition to the forwarding state. BPDU data is still received in blocking state. Prevents the use of looped paths.
    • Listening - The switch processes BPDUs and awaits possible new information that would cause it to return to the blocking state. It does not populate the MAC address table and it does not forward frames.
    • Learning - While the port does not yet forward frames it does learn source addresses from frames received and adds them to the filtering database (switching database). It populates the MAC Address table, but does not forward frames.
    • Forwarding - A port receiving and sending data, normal operation. STP still monitors incoming BPDUs that would indicate it should return to the blocking state to prevent a loop.
    • Disabled - Not strictly part of STP, a network administrator can manually disable a port
    • Note: To prevent the delay when connecting hosts to a switch and during some topology changes, Rapid STP was developed and standardized by IEEE 802.1w, which allows a switch port to rapidly transition into the forwarding state during these situations

Setup

When connecting your MS series to an existing infrastructure, it is key to note what type of spanning tree you are currently running. Please note that the MS series is compatible with industry standard STP (IEEE 802.1D and RSTP (IEEE 802.1w) and runs RSTP by default. MS switches do not currently run MSTP (IEEE 802.1s), and neighboring MSTP topologies will need to be configured accordingly. For best practices, be sure that your existing switch fabric is running a protocol that is compatible with that of the MS series.

Configuration

Before connecting your MS series switches, be sure that you have configured STP/RSTP for your environment. This can be done globally for all of your switches in the network by navigating to Configure > Switch settings. From here you can enable/disable the protocol as well as select priority for each of your switches. Once you have configured RSTP, you should be able to see the reported root bridge when viewing the status for any of your switches (Monitor > Switches).

It is recommended to configure all LAN ports used for client devices (e.g. laptops, workstations, etc) as access as this will cause the ports to transion directly into a forwarding state, thus reducing the time it takes for the port to begin functioning.

For more compatibility information, Meraki recommends checking documentation for your respective switch model.

Sources: Wikipedia, Spanning Tree

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Last modified
13:51, 2 Nov 2015

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ID: 1177

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