Routing & Switching | STP, LACP & Inter-VLAN

I built a loop-free switching fabric using Rapid STP, aggregated uplinks with LACP, and enabled inter-VLAN routing on a Layer 3 switch. The design eliminated Layer 2 loops while providing redundant, load-shared uplinks with sub-second failover.

Objective & Context

Switching forwards within a broadcast domain; routing moves between them. This lab combines RSTP loop prevention, 802.3ad link aggregation for bandwidth and redundancy, and SVI-based inter-VLAN routing to connect the VLANs from the segmentation lab.

Environment & Prerequisites

Two managed switches with RSTP and LACP support.
Pre-configured VLANs (10/20/30/40) and a Layer 3 switch.
Redundant inter-switch links for the LACP bundle.

flowchart LR S1[Switch 1] ===|LACP bundle| S2[Switch 2] S1 -. RSTP blocked .- S2 S2 --> L3[L3 Switch
inter-VLAN SVIs]

Step-by-Step Execution

1. Enable Rapid STP

spanning-tree mode rapid-pvst

2. Bundle uplinks with LACP

interface range Gi0/1-2 ; channel-group 1 mode active

3. Create an inter-VLAN SVI

interface vlan 20 ; ip address 192.168.20.1 255.255.255.0 ; no shutdown

Port-channel1  LACP  active  Gi0/1 Gi0/2  bundled
Vlan20         up/up  192.168.20.1

Validation & Testing

Disconnect one bundle member and confirm traffic continues over the survivor; verify RSTP shows one forwarding path. Pass criteria: no loop, sub-second LACP failover, and successful inter-VLAN routing via the SVIs.

Advanced: Troubleshooting

LACP not bundling: both ends must be active/passive compatible with matching speed/duplex.
STP blocking wrong port: set bridge priority to elect the intended root.
No inter-VLAN routing: enable ip routing on the L3 switch.

Key Results

Eliminated Layer 2 loops with RSTP across redundant links.
Achieved sub-second failover on LACP member loss.
Doubled inter-switch bandwidth via a 2-link 802.3ad bundle.
Routed all 4 VLANs through SVIs on a single L3 switch.

Routing and Switching: STP, LACP, and Inter-VLAN Routing