350-501 Practice Exam 60 — 20 Questions

Question 1

A network engineer is troubleshooting a BGP route selection issue on a router that receives the same prefix from two different peers. Which THREE BGP attributes are considered in the route selection process before the 'prefer the route with the lowest metric' step?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Highest weight. The BGP best-path selection algorithm considers attributes in a specific order before using the metric (MED). The sequence is: highest Weight, highest Local Preference, prefer locally originated routes, shortest AS_PATH, lowest Origin type, and then lowest metric. Therefore, all five options A, B, C, D, and E are evaluated before the metric step.

Question 2

Refer to the exhibit. What is the role of this router for prefix 10.0.1.0/24?

Accepted Answer

Penultimate hop.. The router is the penultimate hop for prefix 10.0.1.0/24 because it performs a 'Pop tag' operation (implicit null label). In MPLS, the penultimate hop pops the label before forwarding the unlabeled IP packet to the egress LSR. Therefore, this router is not the egress; it is the penultimate Label Switching Router (LSR).

Answer

Transit LSR.

Answer

Egress LSR.

Answer

Ingress LSR.

Question 3

Which QoS mechanism marks packets at the edge of the network to classify traffic into different service classes?

Accepted Answer

Marking. Marking is the QoS mechanism that sets the Differentiated Services Code Point (DSCP) or IP Precedence bits in the packet header at the ingress edge of the network. This classification allows downstream devices to apply per-hop behaviors (PHBs) based on the marked value, as defined in RFC 2474 and RFC 2597. Unlike queuing, shaping, or policing, marking does not delay or drop packets; it only modifies the packet header to indicate the desired service class.

Answer

Queuing

Answer

Shaping

Answer

Policing

Question 4

In Segment Routing over IPv6 (SRv6), what is the function of the SRH (Segment Routing Header)?

Accepted Answer

It contains the segment list (ordered list of SIDs). The Segment Routing Header (SRH) is a new IPv6 extension header defined in RFC 8754 that carries an ordered list of Segment Identifiers (SIDs). When a packet is forwarded along a segment-routed path, the SRH encodes the explicit path by listing each segment (node or adjacency SID) in sequence, and the active segment is updated as the packet traverses each hop. This allows the source node to steer packets through a specific path without intermediate nodes needing to maintain per-flow state.

Answer

It replaces the IPv6 header to reduce overhead

Answer

It provides OAM capabilities for path monitoring

Answer

It uses the IPv6 flow label to encode the path

Question 5

Which Cisco platform is designed for the core/backbone of large service provider networks and offers high scalability with IOS XR?

Accepted Answer

Cisco CRS. The Cisco CRS (Carrier Routing System) is a high-end core router designed for the backbone of large SP networks.

Answer

Cisco NCS 5500

Answer

Cisco 8000 series

Answer

Cisco ASR 9000

Question 6

Drag and drop the steps to configure OSPF on a Cisco IOS router into the correct order.

Accepted Answer

Enter global configuration mode, then enable the OSPF process with 'router ospf' command, then advertise networks under the OSPF process using 'network' command.. OSPF configuration requires entering global config mode, enabling the OSPF process, and then advertising networks under the OSPF process.

Answer

Advertise networks using 'network' command, then enter global config mode, then enable OSPF process.

Answer

Enable OSPF process, then advertise networks, then enter global config mode.

Answer

Enter global configuration mode, then advertise networks, then enable OSPF process.

Question 7

An engineer notices that MPLS VPN traffic is taking a suboptimal path because the MPLS label binding for the BGP next-hop loopback is missing. What is the most likely cause?

Accepted Answer

LDP is not enabled on the core-facing interfaces.. MPLS VPN traffic relies on LDP to distribute labels for the BGP next-hop loopback addresses used in the VPNv4 route exchange. If LDP is not enabled on the core-facing interfaces, the local PE cannot obtain a label binding for the remote PE's loopback, causing the traffic to be forwarded via IP (without a label) or taking a suboptimal path. Enabling LDP on all core-facing interfaces resolves the missing label binding and restores optimal label-switched forwarding.

Answer

The remote PE is configured with a different VPN ID.

Answer

The local PE does not have a route to its own loopback.

Answer

LDP is not enabled on the PE-CE interface.

Question 8

An SP is managing a large MPLS network and needs to verify the MPLS forwarding entries for a specific VRF. Which command provides the most complete information for the VRF?

Accepted Answer

show mpls forwarding-table vrf . The 'show mpls forwarding-table vrf ' command displays the complete MPLS forwarding entries for a specific VRF, including the incoming label, outgoing label, next-hop, and outgoing interface. This command is specifically designed to verify MPLS label-switched paths (LSPs) for a VRF, which is essential for troubleshooting MPLS VPN forwarding. It provides the most complete information for MPLS forwarding, as it shows both the label operations (swap, pop, push) and the VRF-specific forwarding details.

Answer

show ip cef vrf <name>

Answer

show mpls vrf <name>

Answer

show bgp vpnv4 unicast all labels

Question 9

Which component of the IOS XR architecture is responsible for managing the system configuration and providing a separation of control plane for multiple tenants?

Accepted Answer

Default SDR. The Default Secure Domain Router (SDR) in IOS XR manages the system configuration and provides logical separation of the control plane for multiple tenants. Other options: XR Config Manager is a tool for configuration management but not for tenant separation; Admin Plane handles administrative functions like system monitoring; LRO (Label Rewrite Engine) is used for label switching operations, unrelated to configuration or tenant separation.

Answer

XR Config Manager

Answer

Admin Plane

Answer

LRO (Label Rewrite Engine)

Question 10

Based on the exhibit, which prefix is NOT reachable via MPLS forwarding?

Accepted Answer

10.5.5.5/32. (10.5.5.5/32) is correct because the exhibit shows that the LDP label bindings are only present for prefixes 10.1.1.1/32, 10.2.2.2/32, and 10.3.3.3/32. The prefix 10.5.5.5/32 is not in the LDP label information base (LIB), meaning no MPLS label has been assigned to it, so it cannot be forwarded via MPLS and must be forwarded using standard IP routing instead.

Answer

10.1.1.1/32

Answer

10.3.3.3/32

Answer

10.2.2.2/32

Question 11

A network architect is designing a model-driven telemetry solution for a large SP network. Which three factors are critical to consider when configuring telemetry subscriptions? (Choose three.)

Accepted Answer

The collection protocol (gRPC vs gNMI vs native TCP).. The collection protocol (gRPC, gNMI, or native TCP) is critical because it determines the transport mechanism, security model, and compatibility with the telemetry collector. gRPC offers high-performance streaming with TLS, gNMI provides a standardized interface for both telemetry and configuration, and native TCP is simpler but less flexible. Choosing the wrong protocol can lead to interoperability issues or performance bottlenecks in a large SP network.

Answer

The size of the YANG data model.

Answer

The encoding format (GPB, JSON, XML).

Question 12

You are a network architect for a large service provider. The network consists of multiple core routers (P routers) and edge routers (PE routers) supporting both L3VPN and L2VPN services. The network currently uses LDP for label distribution and has a flat IGP (OSPF) design. Recently, there have been scalability issues: the IGP convergence time has increased significantly, and the OSPF link-state database (LSDB) has grown large, causing high CPU utilization on core routers. Additionally, the LDP sessions are taking longer to establish after a router reboot. You are tasked with redesigning the network to improve scalability and convergence. The budget allows for significant changes but cannot replace all hardware. Which approach best addresses the scalability issues while minimizing disruption?

Accepted Answer

Implement Segment Routing (SR-MPLS) to replace LDP and reduce IGP overhead. Segment Routing (SR-MPLS) eliminates the need for LDP by encoding MPLS labels directly in the IGP (OSPF or IS-IS) using extensions, which reduces control-plane overhead and speeds up convergence. This directly addresses the large OSPF LSDB and slow LDP session establishment because SR-MPLS does not require a separate label distribution protocol and can leverage a single IGP for both routing and label information, improving scalability without hardware replacement.

Answer

Upgrade OSPF to OSPFv3 to support IPv6 and improve performance

Answer

Introduce IS-IS as a second IGP to offload traffic from OSPF, and use route redistribution between the two protocols

Answer

Replace OSPF with BGP as the core IGP to reduce link-state overhead

Question 13

A service provider's network core runs IS-IS as the IGP. After adding a new router, some routers have incomplete LSP databases. The new router's interfaces are up, and IS-IS adjacency is up with neighbors. What is the cause? The network has a mix of interface MTUs, with some links having MTU 1500 and others 4470.

Accepted Answer

MTU mismatch on the interface. An MTU mismatch on the interface causes incomplete LSP databases. IS-IS uses a maximum LSP size derived from the interface MTU minus the IS-IS header overhead (typically 3 bytes for the LSP header). When a router with a smaller MTU (e.g., 1500) receives an LSP that was generated on a larger MTU link (e.g., 4470), the LSP may be too large to be stored or processed, leading to fragmentation or rejection. This results in an incomplete LSP database on some routers, even though adjacencies are up.

Answer

IS-IS overload bit set on the new router

Answer

IS-IS LSP flooding is inhibited on the new router

Answer

The new router's system-id is a duplicate

Question 14

During an MPLS network migration from LDP to Segment Routing, an engineer notices that some routers are not advertising Prefix-SIDs for certain loopbacks. The IGP is OSPF. What configuration is required on these routers to advertise Prefix-SIDs?

Accepted Answer

Configure 'segment-routing mpls' globally and assign a SID index under the OSPF router process for the loopback.. In OSPF, Prefix-SIDs for loopbacks are advertised by configuring 'segment-routing mpls' globally and then assigning a SID index under the OSPF router process using the 'prefix-sid index' command for the specific loopback network. This ties the SID to the OSPF prefix advertisement, enabling SR-MPLS forwarding without LDP.

Answer

Enable 'mpls ldp autoconfig' on the loopback interface.

Answer

Enable 'segment-routing mpls' globally and configure 'prefix-sid index' under the loopback interface.

Answer

Configure 'segment-routing mpls set-adjacency-sid' on the loopback.

Question 15

A service provider is configuring VRF-lite between two CE routers connected to the same PE. The CE routers are in different VRFs. Which command allows the PE router to forward traffic between the VRFs?

Accepted Answer

ip route vrf RED 0.0.0.0 0.0.0.0 vrf BLUE. The command `ip route vrf RED 0.0.0.0 0.0.0.0 vrf BLUE` creates a static inter-VRF route on the PE router, allowing traffic from VRF RED to be forwarded to VRF BLUE without requiring MPLS or BGP. This is the standard method for VRF-lite inter-VRF communication on the same PE, using a static route that points to the next-hop VRF instead of an IP address.

Answer

vrf forward RED

Answer

route-target export RED:100 import BLUE:100

Answer

ip route vrf RED 0.0.0.0 0.0.0.0 10.1.1.1 global

Question 16

A team uses Ansible to automate configuration of Cisco devices. They want to ensure that configurations are applied only if the device is reachable and the current configuration differs from the intended. Which Ansible module or feature is best suited for this?

Accepted Answer

ios_config with check_mode. The `ios_config` module with `check_mode` (enabled via the `--check` flag or `check_mode: yes`) allows Ansible to first verify device reachability and then compare the intended configuration against the running configuration without making changes. Only when a difference is detected and `check_mode` is disabled will the module apply the configuration changes. This directly meets the requirement of applying configurations only when the device is reachable and the current configuration differs from the intended.

Answer

ios_system

Answer

net_get

Answer

ios_command

Answer

ios_facts

Question 17

Which three are benefits of using MPLS Layer 3 VPNs over traditional VPNs? (Choose three.)

Accepted Answer

Scalability. MPLS Layer 3 VPNs use a full-mesh of MP-BGP sessions between Provider Edge (PE) routers to exchange VPNv4 routes, which allows the network to scale to thousands of VPNs and customers without requiring a full mesh of tunnels. This is far more scalable than traditional VPNs that rely on point-to-point tunnels (e.g., IPsec or GRE) which create an O(n²) problem for connectivity.

Answer

Lower cost

Answer

Reduced security

Question 18

A service provider is deploying MPLS Layer 3 VPN and needs to ensure that BGP next-hop resolution works correctly for VPNv4 prefixes learned from a route reflector. The PE routers are directly connected to the RR via iBGP, and there is an IGP running within the MPLS core. Which condition must be met for the PE to install the VPNv4 prefix into its routing table?

Accepted Answer

The next-hop must be reachable via the IGP with an MPLS label.. For a PE router to install a VPNv4 prefix learned from a route reflector into its routing table, the BGP next-hop (typically the remote PE) must be reachable via the IGP with an associated MPLS label. This ensures that the transport LSP exists to forward traffic toward the next-hop, which is required for MPLS L3VPN operation. Without an MPLS label in the IGP for the next-hop, the PE cannot build the necessary label stack and will not install the VPNv4 route.

Answer

The next-hop must be a directly connected interface.

Answer

The PE must have a VPN label for the next-hop.

Answer

The IGP must be IS-IS, not OSPF.

Question 19

An SP engineer is configuring BGP Graceful Shutdown (GSHUT) for maintenance on a router. What does the GSHUT community do to the BGP best path selection process?

Accepted Answer

It sets the local preference to 0. BGP GSHUT uses a well-known community (0xFFFF0000) that lowers the local preference by 100 (to 0 by default) to make the routes less preferred, thus draining traffic gracefully.

Answer

It prepends the AS path

Answer

It removes the route from the BGP table

Answer

It increases the MED value by 100

Question 20

A service provider is deploying BGP PIC (Prefix Independent Convergence) for edge routers. What is the primary benefit of BGP PIC?

Accepted Answer

It provides fast convergence by pre-installing backup paths in the FIB. BGP PIC provides fast failover by precomputing backup paths in the forwarding table, enabling sub-second convergence upon failure, independent of the number of prefixes.

Answer

It reduces the number of BGP updates during a flap

Answer

It reduces the memory usage on the router

Answer

It eliminates the need for BFD