350-501 Practice Test 48 — 15 Questions

Question 1

In a BGP/MPLS IP VPN, which technique ensures that customer traffic from different VPNs remains isolated on the same PE router?

Accepted Answer

Virtual Routing and Forwarding (VRF). VRF (Virtual Routing and Forwarding) provides separate routing tables per VPN on a PE router. MP-BGP carries VPN routes with labels, but isolation is achieved by VRF.

Answer

Route Target (RT)

Answer

MPLS labels

Answer

Route Distinguisher (RD)

Question 2

Which TWO queuing mechanisms best meet the requirements for voice traffic (low latency and jitter) in a service provider network?

Accepted Answer

LLQ (Low Latency Queuing). LLQ is correct because it combines a strict priority queue with CBWFQ, ensuring voice traffic (which is delay-sensitive) is always serviced first, thereby minimizing latency and jitter. The priority queue is policed to prevent starvation of other traffic, making it ideal for real-time applications like VoIP in service provider networks.

Answer

WRED (Weighted Random Early Detection)

Answer

CBWFQ without LLQ

Answer

FIFO (First In First Out)

Question 3

A service provider wants to implement Segment Routing in its MPLS core to simplify traffic engineering. What is the primary advantage of Segment Routing over traditional RSVP-TE?

Accepted Answer

No per-flow state on transit routers. Segment Routing (SR) eliminates the need for per-flow state on transit routers by encoding the path as a list of segment identifiers (SIDs) directly in the packet header. In contrast, RSVP-TE requires each transit router to maintain soft-state for every LSP, consuming memory and CPU. This state reduction simplifies network operation and scales better in large MPLS cores.

Answer

Improved ECMP load balancing

Answer

Better support for Fast Reroute

Answer

Requires a central controller for path calculation

Question 4

Which THREE are valid methods for label distribution in an MPLS network?

Accepted Answer

Segment Routing via IGP. Segment Routing via IGP (Option A) is a valid label distribution method because it uses the IGP (OSPF or IS-IS) to advertise prefix-SIDs, which are MPLS labels. The IGP distributes these labels as part of its link-state database, eliminating the need for a separate label distribution protocol like LDP. This is defined in RFC 8660 and RFC 8665 for OSPF and IS-IS respectively.

Answer

BGP

Answer

TDP

Question 5

Match each network automation tool to its primary use.

Accepted Answer

Ansible: An agentless automation tool that uses YAML playbooks for configuration management and application deployment.. These are key automation and programmability tools for service provider networks.

Answer

Ansible: A configuration management tool that uses a declarative language and a client-server model.

Answer

Puppet: A tool that uses YAML playbooks for automation.

Question 6

A service provider is troubleshooting an L2VPN where a CE is unable to ping the remote CE. The PE-CE interfaces are up, and the pseudowire status shows 'up'. Which two actions should be taken to further isolate the issue? (Choose two.)

Accepted Answer

Verify the VC ID match on both PEs. The VC ID (Virtual Circuit ID) must match on both PEs for the pseudowire to be established correctly. If the VC IDs are mismatched, the pseudowire status may show as 'up' due to local signaling, but traffic will not be forwarded between the CEs because the pseudowire endpoints are not logically connected. Verifying the VC ID ensures that the L2VPN circuit is properly configured end-to-end.

Answer

Check the MAC address table on the CE

Answer

Check the LDP session between PEs

Answer

Verify the VFI configuration on the PE

Question 7

A network engineer is troubleshooting an MPLS TE tunnel that is not coming up. The tunnel is configured with a strict explicit path, and the path includes an interface that is currently down. Which action should the engineer take to allow the tunnel to use an alternative path?

Accepted Answer

Change the explicit path to 'loose' for the down interface.. Changing the explicit path from 'strict' to 'loose' for the down interface allows the MPLS TE tunnel to use an alternative next-hop that is reachable, even if the specified interface is down. A strict explicit path requires every hop to be directly connected, so a down interface prevents the tunnel from coming up. By making the hop loose, the router can route around the failed link using the IGP's best path to the next specified node.

Answer

Increase the path-option preference value.

Answer

Disable path protection on the tunnel.

Answer

Configure an affinity constraint to exclude the down interface.

Question 8

Which TWO BGP path attributes are considered before AS-path length in the default best-path selection process?

Accepted Answer

Weight. Weight (D) is a Cisco-proprietary attribute that is checked first in the BGP best-path selection process, before AS-path length. Local Preference (E) is the second attribute evaluated, also preceding AS-path length. Both are considered before the AS-path length comparison in the default BGP decision process.

Answer

Origin code

Answer

Community

Answer

MED

Question 9

In SR-TE, a headend router is configured with an SR Policy using an explicit candidate path that includes a segment list [100, 200, 300]. If all routers have the same SRGB starting at 10000, what is the actual outgoing label stack for the first segment?

Accepted Answer

10100, 10200, 10300. In SR-MPLS, the SID values in the segment list are relative to the SRGB. If the SRGB starts at 10000, a SID value of 100 corresponds to label 10100 (10000 + 100). So the first segment label is 10100.

Answer

100, 200, 300

Answer

10000, 20000, 30000

Answer

Labels depend on LDP allocation

Question 10

In SRv6, which Segment Identifier (SID) format is used to encode a function that performs IPv6 decapsulation and forwarding?

Accepted Answer

End.DT4. SRv6 SIDs are 128-bit and structured as Locator:Function:Arguments. End.DT4 is a function that decapsulates the outer IPv6 header and forwards the inner IPv4 packet based on destination.

Answer

End.X

Answer

End.B6

Answer

End

Question 11

In MPLS, which label operation is performed by a Penultimate Hop Popping (PHP) router?

Accepted Answer

Pop the top label from the stack. In PHP, the penultimate router pops the label before forwarding to the egress, reducing the egress router's work.

Answer

Swap the top label with a new label

Answer

Push a new label onto the stack

Answer

Swap and push an additional label

Question 12

What is the purpose of Constrained Shortest Path First (CSPF) in MPLS-TE?

Accepted Answer

To find a path that satisfies traffic engineering constraints and is as short as possible. CSPF computes paths that meet TE constraints such as bandwidth, affinity, and explicit hops while optimizing the path cost.

Answer

To distribute labels for LDP

Answer

To calculate the best path for L3VPN

Answer

To compute the shortest path without any constraints

Question 13

Which SRv6 SID behavior is used to support EVPN-IRB (Integrated Routing and Bridging) for L2VPN with bridging?

Accepted Answer

End.DT2U. End.DT2U is used for L2VPN bridging (EVPN VPWS) and EVPN-IRB uses End.DT2U for bridge domain.

Answer

End.X

Answer

End.DT4

Answer

End.DT6

Question 14

Which of the following best describes the function of CSPF in MPLS-TE?

Accepted Answer

It prunes links that do not satisfy constraints and then runs SPF on the remaining topology.. CSPF computes a path that meets constraints (bandwidth, affinity, etc.) and avoids links/ nodes not meeting them.

Answer

It distributes labels to LDP neighbors.

Answer

It calculates the shortest path based solely on IGP metric.

Answer

It signals RSVP-TE PATH messages.

Question 15

Which MPLS label operation is performed by the penultimate hop router in PHP (Penultimate Hop Popping)?

Accepted Answer

Pop. In PHP, the penultimate hop router pops the label before forwarding the packet to the egress router, reducing the egress's workload.

Answer

No operation

Answer

Swap

Answer

Push