Lab Simulation 31 Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology tab to access

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Explanation

Lab Simulation 31 - Explanation

Overall Goal

This lab tests your ability to fix three interconnected switching issues all from SW20 - a constraint that mirrors real-world scenarios where you only have access to one device. The three problems are:

1. STP port cost method is using the outdated 16-bit default
2. The trunk to SW30 is broken (blocking PC3 -> PC1 traffic)
3. The LACP port channel to SW10 is broken (blocking PC3 -> PC2 traffic)

Step 1: spanning-tree pathcost method long

What it does

Switches STP port cost calculation from the 16-bit (short) method to the 32-bit (long) method.

Why it's necessary

Modern networks commonly use 10 Gbps, 40 Gbps, or 100 Gbps links. The 16-bit method assigns identical costs to anything above 1 Gbps (cost = 1 for anything ≥ 1G), making STP unable to distinguish between faster paths. The 32-bit method provides granular cost differentiation across high-speed links.

This command is global - it affects all ports on the switch simultaneously.

What breaks if skipped

STP path selection becomes unreliable on fast links. Two 10G links would have the same cost as a 1G link, potentially causing suboptimal or flapping topologies. In a lab context, the grader likely checks for this command explicitly in the running config.

The Remaining Tasks (Trunk & LACP)

The procedure only documents step 1, but the task description reveals two more areas to fix. Here's what you'd typically do:

Trunk Restoration (SW20 <-> SW30, for PC3 -> PC1)

Common issues to investigate and fix:

SW20# show interfaces trunk              ! check if trunk is up
SW20# show run interface <port>          ! check mode and allowed VLANs

Likely fixes:

SW20(config-if)# switchport mode trunk
SW20(config-if)# switchport trunk allowed vlan <active-vlan-list>   ! restrict to active VLANs only

The "restrict to active VLANs only" requirement is key - default trunks allow all VLANs, but best practice (and the task requirement) is to whitelist only what's needed.

LACP Port Channel Restoration (SW10 <-> SW20, for PC3 -> PC2)

Common issues: mode mismatch (passive/passive doesn't form - at least one side must be active), mismatched channel-group numbers, or mismatched port parameters (speed/duplex/VLAN config).

Likely fix on SW20:

SW20(config-if)# channel-group <number> mode active

Verify with:

SW20# show etherchannel summary           ! look for "P" (bundled) not "I" (individual/failed)
SW20# show lacp neighbor                  ! confirm SW10 is seen

What Breaks if Done Out of Order

• Fixing the trunk/LACP before pathcost method long is fine for connectivity, but the STP configuration check will fail on grading.
• Fixing LACP without matching the port configuration on both sides of the channel leaves the bundle in a suspended/individual state - pings still fail.
• Forgetting switchport trunk allowed vlan means trunk carries all VLANs, which fails the "active VLANs only" requirement.

Memory Tip

"Long paths cost more to think about - just like long-distance calls"

spanning-tree pathcost method long = 32-bit = more bits = more precision = use it when links are faster than 1G.

For the trunk/LACP troubleshooting, remember the show-then-fix pattern:

1. show interfaces trunk -> is trunk up?
2. show etherchannel summary -> is bundle up (P) or independent (I)?
3. Fix mode/config mismatch, then re-verify.

Final Step (Always)

SW20# copy running-config startup-config

The lab explicitly requires saving to NVRAM before clicking Next. Skipping this means your changes are lost and the grader sees the broken original config.

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