350-401 Question #1236: Real Exam Question with Answer & Explanation

Question

Lab Simulation 18 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 OSPF is preconfigured on all devices except R20. Configure R20 to complete these tasks. Task 1: Configure OSPF according to the topology using these requirements: - Use Process ID 20. - Use Loopback0 for the Router ID. - Advertise all networks into OSPF. o Do not use network statements under the OSPF process to accomplish this task. Task 2: Configure a /18 summary route for Area 40. - Advertise only Type 3 LSAs into Area 0. Answer: Task 1: Configure OSPF on R20 We need to configure R20 to use OSPF Process ID 20, set Loopback0 as the Router ID, and advertise all connected networks without using network statements under the OSPF process. Instead, we will use the interface configuration to enable OSPF on the relevant interfaces. R20(config)# interface loopback 0 R20(config-if)# ip address 20.20.20.20 255.255.255.255 R20(config-if)# exit R20(config)# router ospf 20 R20(config-router)# router-id 20.20.20.20 R20(config-router)# exit R20(config)# interface e0/0 R20(config-if)# ip ospf 20 area 40 R20(config-if)# exit R20(config)# interface e0/1 R20(config-if)# ip ospf 20 area 0 R20(config-if)# exit R20(config)# interface e0/2 R20(config-if)# ip ospf 20 area 40 Step 1: Set up the Loopback0 interface to be used for the Router ID with IP address 20.20.20.20/32. Step 2: Configure OSPF Process ID 20 and set the router ID using the Loopback0 IP address. Step 3: Apply OSPF to the relevant interfaces using the interface configuration (ip ospf 20 area <area>), which automatically advertises the connected networks. Task 2: Configure a /18 Summary Route for Area 40 To summarize the Area 40 routes into a /18 network, we need to configure R20 to aggregate all the networks in Area 40 into a single summary route. The networks in Area 40 are 10.40.50.0/24 (connected to R40). R20(config)# router ospf 20 R20(config-router)# area 40 range 10.40.0.0 255.255.192.0 Step 1: Under the OSPF process, we use the area 40 range command to configure a summary route for Area 40 with the address 10.40.0.0 and subnet mask 255.255.192.0 (which is equivalent to a /18).

Options

• taskResolve EtherChannel negotiation issues on Sw10 (using LACP) with Sw20 and modify the spanning tree configuration on Sw10 to ensure it is always the root for VLAN 10 and VLAN 30.
• prerequisitesConsole access to Sw10. Changes must be made only on Sw10, as Sw20 or Sw30 are inaccessible. All necessary preconfigurations have been applied.

Explanation

Lab Simulation: EtherChannel (LACP) + Spanning Tree Root on Sw10

Overall Goal

Sw10 needs to form a Layer 2 EtherChannel trunk with Sw20 using LACP (802.3ad), then become the guaranteed STP root bridge for VLAN 10 and VLAN 30. Since you can only touch Sw10, every decision must account for what Sw20 is likely already configured with.

Step-by-Step Breakdown

1. interface range e0/0 - 1

Selects both physical interfaces simultaneously. EtherChannel bundles ports together — you must configure them identically or the channel won't form. Doing them one at a time risks mismatched settings.

2. switchport trunk encapsulation dot1q

Sets the trunking encapsulation to 802.1Q (the industry standard). On some IOS platforms, you must explicitly set this before changing the port mode. If skipped, the switchport mode trunk command will fail with an error on switches that support both ISL and dot1q.

3. switchport mode trunk

Forces the port into unconditional trunk mode. This ensures VLAN traffic is tagged across the link regardless of negotiation. Required for EtherChannel links — access mode ports cannot carry multiple VLANs across a bundle.

4. channel-group 2 mode passive

Assigns both interfaces to EtherChannel group 2 using LACP in passive mode. Passive means "I'll respond to LACP if the other side initiates." Since Sw20 is preconfigured and likely set to active, this is the correct pairing — active + passive = channel forms. Two passive ends will never negotiate (both wait). Two active ends also work, but you can't change Sw20.

LACP modes: active = always sends LACP PDUs. passive = only responds. At least one side must be active.

5. no shutdown

Brings the interfaces up. Interfaces may be administratively down, and a shutdown port cannot participate in EtherChannel negotiation. Always explicitly bring them up after configuration.

6. spanning-tree vlan 10 priority 0

Sets the STP bridge priority for VLAN 10 to 0 — the lowest possible value. The switch with the lowest bridge priority wins the root bridge election. Using 0 (vs. the default 32768 or even 4096 used by spanning-tree vlan 10 root primary) guarantees root status regardless of what other switches are configured with.

7. spanning-tree vlan 30 priority 0

Same as above, but for VLAN 30. STP elections are per-VLAN in PVST+, so each VLAN requires its own priority configuration.

What Goes Wrong If Steps Are Skipped or Reordered

Memory Tips

• LACP pairing rule: At least one side must be active. Think of it as a conversation — someone has to start talking.
• Priority = lower wins: Bridge priority is like a golf score. 0 is a hole-in-one — unbeatable.
• EtherChannel config order: Encapsulation → Mode → Bundle → Up. Think: "Encode, Mode, Group, Go."
• channel-group vs channel-protocol: You don't need channel-protocol lacp explicitly — specifying mode active/passive implicitly uses LACP. mode on uses neither PAgP nor LACP (static).

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