Which Command Should the Administrator Use Practice Questions

MTU mismatch between the interfaces

Why wrong: MTU mismatch causes OSPF packets to be silently dropped, but the adjacency may still reach the EXCHANGE state before failing.

Passive interface configuration on one router

Why wrong: Passive interface prevents sending hellos, so the neighbor would not be discovered at all.

Area ID mismatch on the interfaces

Why wrong: Area ID mismatch would result in a different area and the adjacency would not form due to area mismatch.

The core routers use LDP labels and ignore DSCP

Why wrong: LDP doesn't process DSCP; DSCP remarking must be due to a policy.

The ingress PE did not set MPLS EXP bits

Why wrong: The problem is DSCP being cleared, not MPLS EXP.

The MPLS EXP bits are not copied from DSCP

Why wrong: MPLS EXP bits are often copied from DSCP, but this does not cause DSCP to become 0.

Spanning Tree Protocol (STP)

Why wrong: STP runs on CE devices, not PEs.

Rapid Spanning Tree Protocol (RSTP)

Why wrong: Same as above.

MAC address aging

Why wrong: MAC aging removes stale entries but does not prevent loops.

Anycast SID for the group containing the router.

Why wrong: Not a strict hop.

Binding SID (BSID) of another policy.

Why wrong: Not for strict hop.

Node SID (Prefix SID) of the router.

Why wrong: Allows any path to the node.

Customer routes are not installed in the global table.

Why wrong: Routes are installed in the VRF table, not the global table.

BGP sessions between PEs flap.

Why wrong: Route-target settings do not affect BGP session stability between PEs.

The PE-CE routing protocol fails.

Why wrong: PE-CE routing operates independently of route-target configuration.

Enter interface VLAN mode, then assign IP address, then enable the interface, then create the VLAN.

Why wrong: This is incorrect because you cannot enter interface VLAN mode or configure it before the VLAN is created; the interface does not exist yet.

Create the VLAN, then assign IP address, then enter interface VLAN mode, then enable the interface.

Why wrong: This is incorrect because the IP address must be assigned within the interface VLAN configuration mode, not before entering it.

Create the VLAN, then enable the interface, then enter interface VLAN mode, then assign IP address.

Why wrong: This is incorrect because you cannot enable the interface before entering its configuration mode; the no shutdown command is issued inside interface config mode.

1. Enter global configuration mode (configure terminal) 2. Verify the configuration (e.g., show ip route) 3. Configure the static route using the ip route command 4. Exit configuration mode (end or exit)

Why wrong: This is incorrect because verifying a route before it has been configured is illogical; verification should occur after configuration.

1. Configure the static route using the ip route command 2. Enter global configuration mode (configure terminal) 3. Verify the configuration (e.g., show ip route) 4. Exit configuration mode (end or exit)

Why wrong: This is incorrect because the ip route command must be executed in global configuration mode; attempting to configure a route before entering that mode will result in an error.

1. Exit configuration mode (end or exit) 2. Enter global configuration mode (configure terminal) 3. Configure the static route using the ip route command 4. Verify the configuration (e.g., show ip route)

Why wrong: This is incorrect because exiting configuration mode first is pointless and will place you in user EXEC or privileged EXEC mode, preventing further configuration steps.

Configure BGP prefix-independent convergence (PIC) on all PE routers.

Why wrong: BGP PIC improves convergence speed after a failure but does not minimize the impact of an RR failure causing route withdrawals.

Implement BGP add-paths capability on RRs to advertise multiple paths to clients.

Why wrong: Add-paths allows advertising multiple paths for the same prefix, but if the RR fails, those paths are lost anyway.

Deploy redundant RRs with the same cluster ID and use the 'bgp cluster-id' command to ensure clients only accept routes from one RR at a time.

Why wrong: Same cluster ID causes clients to prefer one RR; if that RR fails, routes from the other may not be accepted until the failing RR withdraws.

The interface speed is not configured.

Why wrong: Interface speed is auto-negotiated; error would be different.

The interface is not part of a bridge domain.

Why wrong: QoS does not require bridge domain.

The shape average command in the parent policy uses an unsupported value.

Why wrong: Shape average values are checked at commit; this error message is specific.

RSVP is not enabled on the headend

Why wrong: Without RSVP, tunnel would not come up at all.

The tunnel destination is not reachable via IGP

Why wrong: That would prevent tunnel setup regardless of path.

The tunnel is configured with 'autoroute announce'

Why wrong: That affects routing, not path selection.

show mpls interface

Why wrong: Shows interfaces enabled for MPLS, not forwarding entries.

show mpls ldp neighbor

Why wrong: Shows LDP neighbors, not forwarding details.

show ip route 10.1.1.0

Why wrong: Shows IP routing table, not MPLS forwarding.

VRF not properly configured.

Why wrong: The VRF is configured, but import target mismatch prevents installation.

Route distinguisher missing.

Why wrong: RD is present (implicitly in the route); it does not affect import filtering.

Next-hop unreachable.

Why wrong: There is no indication that the next-hop is unreachable.

ERO in RSVP PATH message

Why wrong: That is for RSVP-TE.

Label stack with PHP

Why wrong: PHP is for MPLS LDP.

IGP shortest path only

Why wrong: SR-TE can override IGP paths.

Local Path Protection

Why wrong: Refers to segment routing protection.

Global Repair

Why wrong: Not a standard FRR method.

1:1 detour

Why wrong: Each LSP gets its own detour.

fair-queue

Why wrong: Fair-queue is default for class-default.

priority percent 30

Why wrong: Priority creates LLQ, not CBWFQ.

bandwidth 30

Why wrong: Without 'percent', it's in kbps.