MST and RSTP with vPC and EtherChannel

MST (Multiple Spanning Tree) and RSTP (Rapid Spanning Tree Protocol) are compatible Spanning Tree Protocol (STP) variants if properly configured.

When you use vPC (Virtual PortChannel) and EtherChannel, there are a couple of things to keep in mind.

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

The primary switch sends and processes BPDUs on the vPC interfaces. The secondary switch only relays these BPDUs and doesn't create any BPDUs on its own.

Misc

  1. Root Bridge Elections: It's important 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.

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