Router R1 Cannot Reach R3 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.

OSPF database overload protection

Why wrong: Protects against excessive LSAs, does not reduce SPF time.

Bidirectional Forwarding Detection (BFD)

Why wrong: Provides fast failure detection, but does not affect SPF calc.

LSA pacing timers (pacing-interval)

Why wrong: Controls LSA generation rate, not SPF calculation.

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.

OSPFv3 authentication configured

Why wrong: Authentication is optional and not required for basic operation.

OSPFv3 LSA type 9 for link-local addresses

Why wrong: LSA type 9 is automatically generated, not a configuration requirement.

OSPFv3 TE requires the use of wide metrics.

Why wrong: TE metrics can be separate from OSPF cost; wide metrics are not mandatory.

OSPFv3 TE only supports point-to-point links.

Why wrong: OSPFv3 TE supports both broadcast and point-to-point links.

The BGP route is suppressed due to route update delay

Why wrong: BGP update delay is not a common cause for routes not being installed.

The VRF name does not match

Why wrong: VRF name misconfiguration would prevent session establishment, but session is up.

The RD is not configured

Why wrong: RD is required for VRF, but its absence would cause other issues; session might not establish.

The OSPF network type is incorrectly set to point-to-point

Why wrong: OSPF network type does not affect BGP next-hop tracking.

The BGP timers are set too low, causing premature route withdrawal

Why wrong: Low timers would cause faster convergence, not slower.

The LDP session between the PEs is down

Why wrong: LDP is used for label distribution, not for BGP next-hop resolution.

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 BGP neighbor is not activated under the VPLS address-family

Why wrong: Incorrect. If BGP L2VPN routes are exchanged, the neighbor is activated under the VPLS address-family.

The MTU mismatch between PEs

Why wrong: Incorrect. MTU mismatch would cause pseudowire status to show 'MTU mismatch' or similar, but the pseudowire may still be up.

The VFI name is not consistent across PEs

Why wrong: Incorrect. VFI names are locally significant; they do not need to match across PEs.

Static default route from VRF to global

Why wrong: A static route could direct traffic but without NAT the return traffic won't know VRF.

Route leaking between VRF and global

Why wrong: Leaking routes would expose the global table to the customer.

VRF-Aware Firewall

Why wrong: Not a standard Cisco feature for this purpose.

Queuing

Why wrong: Queuing manages packet order and drop decisions, not rate limiting.

Marking

Why wrong: Marking sets QoS marks, doesn't limit rate.

Policing

Why wrong: Policing drops exceeding traffic, not buffering.

OSPF uses the Bellman-Ford algorithm to compute routes.

Why wrong: OSPF uses Dijkstra's SPF algorithm.

OSPF is a distance-vector routing protocol.

Why wrong: OSPF is a link-state routing protocol.

OSPF uses TLVs to encode routing information.

Why wrong: TLVs are used by IS-IS, not OSPF.

Router Information TLV

Why wrong: Used for capabilities.

Traffic Engineering TLV

Why wrong: Used for TE information.

Extended Link TLV

Why wrong: That carries Adjacency SID.

Downstream on demand with conservative label retention

Why wrong: Labels are requested only when needed and only kept if from the next hop.

Downstream on demand with liberal label retention

Why wrong: Labels are requested but all are kept.

Downstream unsolicited with conservative label retention

Why wrong: Labels are sent unsolicited but only kept if from the next hop.