MST and RSTP with vPC and EtherChannel

MST (Multiple Spanning Tree) and RSTP (Rapid Spanning Tree Protocol) are compatible SSpanning Tree Protocol (STP) variants that are compatible if properly configured. In networking environments utilizing vPC (Virtual PortChannel) and EtherChannel, certain considerations need to be accounted for to ensure smooth interoperability:

  • vPC Architecture: In vPC configurations, the two switches (with MST enabled) act as a single logical STP device to the broader network. This helps in maintaining consistent STP operations and prevents loops due to dual-homed devices. MST BPDUs are synchronized between vPC peers, making the MST domain appear as a single logical switch to an RSTP-enabled access switch.

  • EtherChannel Functionality: EtherChannel presents itself as a single logical link for STP, meaning STP calculations occur on the aggregated link rather than the individual connections. RSTP on access switches and MST on vPC peers both perceive EtherChannel links in this manner.

  • STP BPDU Handling in vPC: Within a vPC setup, only one switch sends STP BPDUs for the port channel to prevent duplicate BPDUs from reaching the access switch, thus preserving correct STP operations.

Configuration Considerations

  1. Root Bridge Elections: It's crucial to set the root bridge for the CIST (Common and Internal Spanning Tree) proactively to one of the vPC peers. If an access switch inadvertently becomes the root, it might disrupt the intended network topology. Explicit root bridge priority settings can control this.

  2. Instance Mismatches: The access switch running RSTP may only participate in the CIST and remains unaware of other MST instances. This might lead to suboptimal path decisions if VLANs in non-CIST instances need to route through the access switch.

  3. Configuration Consistency: Ensure that configurations within the vPC domain are consistent, especially with VLAN assignments and STP configurations, to prevent potential network loops or blocked ports.

Overall, this setup is a valid network design. Thorough attention to potential issues should allow for effective implementation and operation.

https://networklessons.com/cisco/ccie-routing-switching-written/mst-pvst-interoperability