This document describes how to connect SG300 with Catalyst switch via STP.
Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on mainly on switches. The specification for STP is IEEE 802.1D. The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your layer 2 network.
STP stands for spanning tree protocol, RSTP stands for rapid STP, MSTP stands for multiple STP.
Catalyst switch default STP mode is PVST+, which runs STP for each VLAN, and encapsulates STP BPDU with VLAN tag (except VLAN 1).
For example, PVST+ BPDU for VLAN 2 is tagged with VLAN 2. While PVST+ BPDU for VLAN 1 is untagged.
Catalyst also supports rapid-PVST+ mode, which runs RSTP for each VLAN. STP BPDU is also encapsulated with VLAN tag.
PVST+ can setup different topology for different VLAN to achieve VLAN load balance.
SMB switch default STP mode is RSTP, and supports MSTP. SMB does not support PVST+ or rapid-PVST+.
Note: By default, Catalyst trunk allows all VLANs, while SMB switch allows no VLAN.
We need to remove extra VLANs from Catalyst and add required VLANs in SMB trunk to make VLANs match.
SMB switch can use default RSTP to connect to Catalyst switch in default PVST+ or rapid-PVST+ mode:
In this case, SMB switch and Catalyst switch are in different mode, there is boundary between them.
SMB STP communicates with Catalyst STP instance for VLAN 1. Catalyst STP BPDU for other VLANs pass through SMB switch as normal traffic.
If Catalyst in PVST+ mode is root for VLAN 1, then PVST+ load balance cannot be achieved since SMB switch blocks all VLAN tagged PVST+ BPDU.
But if we setup SMB switch as root, load balance can be achieved:
This is due to SMB switch ports are in forwading state and Catalyst tagged PVST+ BPDUs are passed through SMB STP region.
While in the diagram above, we need to use port priority to balance VLAN load, which is not a very trivial design.
So recommended connection mode is MSTP between SMB and Catalyst switch.
MSTP has multiple Spanning tree instances to build different topology for different vlan groups mapped to instance.
That can be used for VLAN load balance, as illustrated below:
vlan 1-10 are mapped to MSTP instance 1
vlan 11-20 are mapped to MSTP instance 2
D1 is root for vlan 1-10, D2 is root for vlan 11-20.
SG300/500 as access switch dual-homed to two distribution layer catalyst switches.
SG300/500 load balance vlan 1-10 to uplink to D1 and vlan 11-20 to uplink to D2.
In order for MSTP to work properly, please configure the name, revision number, and vlan to instance mapping to be the sam
across connected MSTP switches. These switches are said to be in the same MSTP region.
Connectivity for switches in different MSTP regions is beyond the scope of this document.
For more information regarding Catalyst MSTP, please refer to