Free 500-240 sample questions

Correct answer: The absence of a configured clock source or an inadequate clock recovery mechanism on the ASR 920.

Clock slips in a CESoPSN environment are almost always related to timing recovery. The egress router (ASR 920) must accurately reconstruct the TDM clock from the arriving packets. This requires a stable clock source (like SyncE or PTP) and a properly configured clock recovery mechanism (adaptive or differential). Without this, packet delay variation (PDV) in the network will cause the recovered clock to drift, leading to slips.

Correct answer: To advertise the loopback addresses of PEs with a transport label, creating a BGP-free core.

In a Unified or Seamless MPLS architecture, BGP Labeled Unicast (BGP-LU) is used to stitch together different IGP domains (e.g., aggregation and core). It advertises the PE router loopbacks across the ASBRs with a transport label. This allows the core P routers to forward traffic towards the PEs without needing to run BGP themselves, thus creating a 'BGP-free core' and enhancing scalability.

Correct answer: Clearance of the first Fresnel Zone.

While all factors are part of a link budget calculation, the most critical factor for reliability over a wooded area is ensuring adequate clearance of the first Fresnel Zone. This elliptical zone around the line-of-sight path must be kept largely free of obstructions (like trees) to avoid signal reflection, diffraction, and attenuation, which would severely degrade the link performance. It is recommended to have at least 60% of the first Fresnel Zone clear.

Correct answer: Low Latency Queuing (LLQ)

Low Latency Queuing (LLQ) provides a strict priority queue within a Class-Based Weighted Fair Queuing (CBWFQ) policy. This is ideal for delay-sensitive traffic like PTP or voice. Packets in the LLQ are serviced first, up to a configured bandwidth limit, ensuring they experience minimal delay and are protected from drops, which is critical for maintaining synchronization accuracy.

Correct answer: To ensure that after a failure is repaired, the ring's Alternate Port reverts to its original, deterministic location.

In REP, one port in the ring is logically blocked (the Alternate Port) to prevent loops. A failure causes this port to unblock and traffic to reconverge. By designating a preferred Primary Edge Port, the network administrator defines a deterministic blocking point. When the failure is repaired, REP will preemptively revert the blocking to the designated Alternate Port, ensuring predictable traffic paths and preventing the blocking point from remaining in a non-optimal location.

Correct answer: The Service Layer with YANG models.

The UMMT architecture includes a Service Layer that utilizes YANG data models. These models define network services (like L3VPN, EVPN, etc.) in a standardized, programmable way. Orchestration systems like Cisco NSO (Network Services Orchestrator) consume these YANG models to automate the provisioning of complex services across the network without needing to manage device-specific CLI commands.

Correct answer: Ethernet Virtual Private Network (EVPN)

EVPN is the modern standard for scalable L2VPN services in service provider networks. It uses BGP as the control plane to advertise MAC addresses and VLAN information, which overcomes the scalability limitations of traditional VPLS (which relies on flooding and learning). The ASR 9000 running IOS XR has robust support for EVPN, making it the ideal choice for multi-tenant L2 services in an aggregation role.

Correct answers: Circuit Emulation (CESoPSN/SAToP); MPLS Layer 2 VPN (Pseudowires)

Correct answer: True

This statement is true. A Boundary Clock (BC) is a PTP-aware device that terminates PTP messages on one port (acting as a slave) and regenerates them on other ports (acting as a master). This helps to remove accumulated packet delay variation from the network and improves timing accuracy, making it a critical component in large-scale PTP deployments.

Correct answer: The MPLS MTU on the transport path is too small to accommodate the S1AP control packets.

This is a classic mobile backhaul issue. Standard pings use small packets and will succeed even with a low MTU. However, S1 Application Protocol (S1AP) messages, especially during initial setup, can be larger. The MPLS transport path adds label overhead (typically 4 bytes per label). If the path MTU is not configured to be larger than the standard 1500 bytes to account for these labels, larger S1AP packets will be fragmented or dropped, preventing the S1 interface from establishing, even though basic IP connectivity works.