350-401 Question #1246: Real Exam Question with Answer & Explanation

Question

Lab Simulation 28 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 the device console(s) and perform the tasks. - Console access is available for all required devices by clicking the device icon or using the tab(s) above the console window. - All necessary preconfigurations have been applied. - Do not change the enable password or hostname for any device. - Save your configurations to NVRAM before moving to the next item. - Click Next at the bottom of the screen to submit this lab and move to the next question. - When Next is clicked, the lab closes and cannot be reopened. Topology Tasks Answer: SW10 Int ran e02/-3 Channel-group 10 mode active Verification:

Options

• taskConfigure SW10 to achieve the following: 1. Enable immediate packet forwarding on interface e0/1 after link-state UP. 2. Establish a functional trunk link with SW30 on interface e0/0, ensuring PC2 can ping PC1. 3. Form an operational LACP port channel with SW20 across interfaces e0/2 and e0/3, ensuring PC2 can ping PC3.
• prerequisitesConsole access to virtual devices available; All necessary preconfigurations applied.

Explanation

Lab 28: SW10 Configuration Explanation

Overall Goal

This lab configures three distinct Layer 2 features on SW10, each solving a different network problem: eliminating STP delay on an access port, enabling inter-VLAN/trunk communication with SW30, and aggregating bandwidth via LACP with SW20.

Step-by-Step Reasoning

Step 1: spanning-tree portfast edge on e0/1

• What it does: Bypasses STP's Listening and Learning states (normally ~30 seconds) and puts the port directly into Forwarding.
• Why necessary: End devices (PCs, servers) don't create switching loops, so making them wait 30s for STP to converge is wasteful and causes problems like DHCP timeouts.
• If skipped: The port still works eventually, but there's a ~30-second delay after link-up before traffic flows — the task explicitly requires immediate forwarding.
• If misapplied: Never put PortFast on a port connected to another switch — it bypasses loop protection and can cause broadcast storms.

Step 2: switchport mode trunk on e0/0 (toward SW30)

• What it does: Configures the port to carry multiple VLANs using 802.1Q tagging, rather than being limited to a single access VLAN.
• Why necessary: PC2 and PC1 are likely in different VLANs or separated by this trunk link. Without trunk mode, only the native VLAN passes — tagged frames are dropped.
• If skipped: PC2 cannot ping PC1 because inter-switch VLAN traffic is blocked. The link may appear up but traffic from the wrong VLAN is silently discarded.
• Note: SW30 must also be configured as trunk on its side (preconfig handles this per the lab instructions).

Step 3: channel-group 10 mode active on e0/2-3 (toward SW20)

• What it does: Bundles both physical interfaces into EtherChannel (Port Channel 10) using LACP (mode active = actively sends LACP negotiation packets).
• Why necessary: Without EtherChannel, STP blocks one of the redundant links to prevent loops, cutting bandwidth in half. The channel-group aggregates them into a single logical link.
• mode active specifics: LACP has three modes — active (initiates negotiation), passive (responds but doesn't initiate), on (static, no LACP). Active works as long as the peer is active or passive. SW20 must be active or passive (preconfig handles this).
• If skipped: STP blocks e0/2 or e0/3. PC2 can still reach PC3 via one path, but the port channel is not operational — the task requires a functional LACP bundle.
• If interfaces are added to different channel groups: The port channel won't form.

Step 4: Save to NVRAM (write memory or copy run start)

• Configurations live in RAM by default. A reload without saving loses everything. Always required before moving to the next lab item.

What Would Break If Steps Are Out of Order

• PortFast can be applied at any time — it's interface-local with no dependencies.
• Trunk must be configured before you test PC2→PC1 pings; the VLAN traffic is dropped until both sides are in trunk mode.
• LACP channel-group must be applied to both interfaces in the range simultaneously (using interface range) — configuring them separately risks a brief STP topology change between commands, but functionally it still works.

Memory Tip

"PTA-S" — PortFast (end devices), Trunk (to other switches needing multi-VLAN), Active LACP (to aggregate redundant uplinks), Save.

Or remember the three problems being solved:

• Delay? → PortFast
• VLANs not passing? → Trunk
• Redundant links blocked? → EtherChannel (LACP active)

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