Quiz: Cisco CLI

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4 questions

L0 (1 questions)

1. What do the two parts of 'show interface' status mean: 'is up, line protocol is down' vs 'is up, line protocol is up'?

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The first part is Layer 1 (physical): is the link electrically/optically active? The second part is Layer 2 (data link): is the protocol negotiation successful? 'Up/down' means the physical link is good but L2 failed — usually a speed/duplex mismatch or encapsulation mismatch. 'Up/up' means both layers are working. 'Administratively down' means someone shut the port.

L1 (1 questions)

1. A server gets DHCP from the wrong subnet after being moved to a new rack. What Cisco command sequence diagnoses this?

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1. 'show vlan brief' — verify which VLAN the server's port is assigned to.
2. 'show interface switchport' — confirm access vs trunk mode and the assigned VLAN.
3. 'show interface trunk' — if it is a trunk, check which VLANs are allowed. The server port is likely still in the old VLAN. Fix: 'interface Gi0/1; switchport access vlan '.

L2 (1 questions)

1. What is PortFast and BPDU Guard, and why do server-facing ports need both?

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PortFast skips the STP Listening->Learning stages (30 seconds on classic STP), moving the port directly to Forwarding. Without it, servers time out on DHCP or PXE boot. BPDU Guard protects PortFast ports — if a BPDU is received (someone plugged in a switch), the port goes err-disabled immediately, preventing an accidental loop. Both should be enabled on every server-facing access port.

L3 (1 questions)

1. Explain how an unexpected root bridge election can cause a network-wide outage and how to prevent it.

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STP elects one root bridge per VLAN based on lowest bridge priority (then lowest MAC as tiebreaker). If someone connects a switch with a lower priority, it becomes root and all traffic paths recalculate globally, causing 30+ seconds of packet loss during reconvergence. Prevention: set explicit low priority on intended root bridges (spanning-tree vlan X priority 4096), enable root guard on non-root ports, and use BPDU guard on access ports.