How do I fix 'Bundle members show suspended or standalone'?

Likely cause: LACP mode mismatch (one side active, other side on) or no LACP at all. Fix: Set both sides to active (or active/passive pair) and check show etherchannel summary.

How do I fix 'RSTP still takes ~30s to converge'?

Likely cause: A legacy 802.1D neighbor forces fallback timers. Fix: Identify the old switch with show spanning-tree and isolate or upgrade it.

How do I fix 'Inter-VLAN routing fails'?

Likely cause: SVI down because no access port in that VLAN is up, or missing IP routing. Fix: Confirm the SVI is up/up and ip routing is enabled on the L3 switch.

How do I fix 'Traffic uses only one bundle member'?

Likely cause: Hashing algorithm sends the test flows down the same link. Fix: This is normal: LACP balances flows, not packets; test with diverse source/destination pairs.

How do I fix 'Loop despite STP'?

Likely cause: A device with STP disabled bridging two VLANs, or a unidirectional link. Fix: Enable loop guard and UDLD on fiber links; audit for unmanaged switches.

Routing & Switching Explained: Networking Lab

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.

Learning Objectives

Enable Rapid STP and interpret port roles and states.
Bundle redundant links with LACP (802.3ad) for bandwidth and failover.
Create SVIs for inter-VLAN routing on a Layer 3 switch.
Verify loop-free topology and bundle membership.
Explain when to route on an SVI vs a dedicated router interface.

Environment & Prerequisites

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.
Two managed switches with RSTP and LACP (802.3ad) support.
Pre-configured VLANs 10/20/30/40 from the VLAN lab.
A Layer 3 switch or router for the inter-VLAN SVIs.
Redundant inter-switch links for the LACP bundle.
Console or management access to both switches.

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

Expected output:

SW1(config)# spanning-tree mode rapid-pvst
SW1(config)#
(verify with: show spanning-tree vlan 20)

What this teaches: RSTP blocks redundant ports logically so the physical loop never becomes a broadcast storm.

2. Bundle uplinks with LACP

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

Expected output:

SW1(config)# interface range Gi0/1-2
SW1(config-if-range)# channel-group 1 mode active
Creating a port-channel interface Port-channel 1
SW1(config-if-range)# exit
SW1(config)#

What this teaches: LACP negotiates the bundle dynamically, so both ends must agree on active or passive mode.

3. Create an inter-VLAN SVI

interface vlan 20 ; ip address 192.168.20.1 255.255.255.0 ; no shutdown

Expected output:

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

What this teaches: an SVI is a virtual Layer 3 interface that lets the switch route between VLANs directly.

How It Works

Switching forwards frames within a broadcast domain using MAC tables; routing forwards packets between domains using IP tables. A Layer 3 switch does both: it switches at wire speed in hardware and routes through SVIs, which are virtual interfaces bound to VLANs with their own IP addresses.

Redundant links create loops, and loops create broadcast storms, so STP exists to block redundant paths logically while keeping them physically connected. RSTP converges in seconds rather than the 30 to 50 seconds of classic STP by using handshake-based proposals between switches instead of timer-based aging.

LACP bundles multiple physical links into one logical channel. Traffic is distributed across member links by a hashing algorithm (usually source/destination MAC or IP), and if a member fails, its traffic moves to the survivors in under a second. Both ends must run LACP in compatible modes (active/passive); a static bundle with no negotiation is the brittle alternative.

flowchart LR S1["Switch 1"] <==>|"LACP bundle"| S2["Switch 2"] S1 -.->|"RSTP blocked"| S2 S2 --> L3["L3 switch: SVIs"] L3 --> V20["VLAN 20"] L3 --> V30["VLAN 30"]

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.

show etherchannel summary ; show spanning-tree vlan 20 brief

Expected output:

SW1# show etherchannel summary
Group  Port-channel  Protocol    Ports
------+-------------+-----------+----------------------------
1      Po1(RU)         LACP      Gi0/1(P)        Gi0/2(P)

SW1# show spanning-tree vlan 20 brief
VLAN20
  Spanning tree enabled protocol rstp
  Root ID    Priority    4096
             Address     aabb.cc00.0100
             This bridge is the root

RU means the bundle is in use, P means each member is bundled in the port-channel. Confirm the RSTP root is the intended switch before calling the topology loop-free.

Troubleshooting

Symptom	Likely cause	Fix
Bundle members show suspended or standalone	LACP mode mismatch (one side active, other side on) or no LACP at all	Set both sides to active (or active/passive pair) and check show etherchannel summary
RSTP still takes ~30s to converge	A legacy 802.1D neighbor forces fallback timers	Identify the old switch with show spanning-tree and isolate or upgrade it
Inter-VLAN routing fails	SVI down because no access port in that VLAN is up, or missing IP routing	Confirm the SVI is up/up and ip routing is enabled on the L3 switch
Traffic uses only one bundle member	Hashing algorithm sends the test flows down the same link	This is normal: LACP balances flows, not packets; test with diverse source/destination pairs
Loop despite STP	A device with STP disabled bridging two VLANs, or a unidirectional link	Enable loop guard and UDLD on fiber links; audit for unmanaged switches
channel-group accepted but member stays suspended	Member interfaces differ in speed, duplex, or trunk allowed list, so LACP cannot bundle them	Match speed/duplex and trunk config on all members, then shut/no shut the port-channel

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.

Key Takeaways

RSTP gives fast convergence, but only if every switch in the domain speaks it.
LACP needs matching modes on both ends; verify the bundle, do not assume it.
An SVI needs at least one up port in its VLAN to stay up.
Hashing balances flows across members; single-flow tests will not show aggregation.

Practice Challenges

Challenge 1: Measure failover time

Goal: Run a continuous ping across the bundle, pull one member link, and measure the outage duration.

Hint: Use ping with 100 ms intervals and timestamp the gap when the member goes down.

Challenge 2: Tune STP costs

Goal: Force a specific switch to become root for VLAN 20 by tuning priority, then verify with show spanning-tree.

Hint: Lower the bridge priority with spanning-tree vlan 20 priority 4096 on the desired root.

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

Objective & Context

Learning Objectives

Environment & Prerequisites

Prerequisites

Step-by-Step Execution

1. Enable Rapid STP

2. Bundle uplinks with LACP

3. Create an inter-VLAN SVI

How It Works

Validation & Testing

Troubleshooting

Key Results

Key Takeaways

Practice Challenges

Challenge 1: Measure failover time

Challenge 2: Tune STP costs

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