350-501 Architecture Practice Test 2 — 15 Questions

Question 1

Refer to the exhibit. A network engineer configures a BGP route-map to set communities on routes advertised to a neighbor. After applying the configuration, the engineer checks the BGP table on the neighbor router and does not see the communities. What is the most likely reason?

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

Accepted Answer

The neighbor is missing the 'send-community' command. C is correct because BGP communities are not sent to a neighbor by default. Even if a route-map sets the community values correctly, the neighbor will not receive them unless the 'send-community' command is configured under the neighbor statement. This command enables the advertisement of the community attribute in BGP updates.

Answer

The prefix-list does not match the exact prefix

Answer

The community values are not in the format 'AA:NN'

Answer

The route-map needs to be applied inbound

Question 2

Refer to the exhibit. An engineer configures MPLS LDP on a router. The router has two interfaces with IP addresses 10.0.0.1/30 and 10.0.0.5/30. The engineer notices that LDP sessions are not established. The OSPF neighbor adjacencies are up. What is the most likely cause?

Accepted Answer

LDP is not enabled on the interfaces. D is correct because MPLS LDP requires explicit interface-level activation under the `mpls ip` command. Even if OSPF adjacencies are up and the global LDP process is configured, LDP will not form sessions on interfaces where `mpls ip` is missing. The engineer likely enabled LDP globally but forgot to enable it on the specific interfaces, which is a common oversight.

Answer

The OSPF network statements do not cover the interfaces correctly

Answer

The MPLS MTU is set to 1500, which is too low

Answer

The label range is too small

Question 3

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 4

You are a network engineer at a service provider that offers L3VPN services. A customer complains that their VPN traffic is intermittently dropping. The network uses OSPF as the IGP, LDP for label distribution, and MP-BGP for VPNv4 route exchange. The PE routers are connected to two core routers (P1 and P2) in a redundant design. Upon investigation, you find that the BGP next-hop for some VPN prefixes is set to the loopback interface of the remote PE, but the route to that loopback is sometimes unreachable due to a flapping link between P1 and P2. The IGP converges, but the BGP prefixes take longer to recover. What is the most likely cause of the intermittent drops?

Accepted Answer

BGP next-hop tracking is not enabled, causing BGP to rely on IGP convergence only. The intermittent drops occur because BGP next-hop tracking (NHT) is not enabled. Without NHT, BGP relies solely on IGP convergence to detect reachability changes for the BGP next-hop (the remote PE loopback). When the link between P1 and P2 flaps, OSPF converges quickly, but BGP does not immediately re-evaluate its next-hop reachability; it waits for the next BGP scan interval (default 60 seconds) or until a BGP update is received. This delay causes the VPN prefixes to remain installed with an unreachable next-hop, leading to traffic drops until BGP eventually withdraws or re-advertises the routes.

Answer

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

Answer

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

Answer

The LDP session between the PEs is down

Question 5

A service provider is redesigning its MPLS core to support both L3VPN and L2VPN services. They want to minimize the number of labels in the forwarding table while maintaining per-VRF label allocation. Which label allocation mode should be used for the L3VPN service?

Accepted Answer

Per-VRF label allocation. Per-VRF label allocation assigns a single MPLS label per VRF, meaning all routes within a VRF share the same label. This minimizes the number of labels in the forwarding table compared to per-prefix allocation, while still maintaining per-VRF isolation for L3VPN services. It is the recommended mode when the goal is to reduce label consumption without sacrificing VPN separation.

Answer

Per-CE label allocation

Answer

Per-prefix label allocation

Answer

Per-instance label allocation

Question 6

A service provider is deploying Segment Routing over IPv6 (SRv6) and needs to ensure that the forwarding plane can support SRv6 encapsulated packets. Which hardware capability is most critical for SRv6 at the line rate?

Accepted Answer

IPv6 extension header processing. SRv6 (Segment Routing over IPv6) encapsulates packets with an IPv6 header containing a Segment Routing Header (SRH), which is a type of IPv6 extension header. For line-rate forwarding, the hardware must natively process IPv6 extension headers in the forwarding plane without punting to the CPU, as software processing would cause performance degradation. Option C is correct because this capability is the most critical for SRv6 at line rate.

Answer

MPLS label swap capability

Answer

VXLAN tunnel termination

Answer

NAT64 translation support

Question 7

A network engineer is troubleshooting an OSPF issue in a service provider network. The network uses OSPFv2 with multiple areas. Routers in area 0 are able to ping each other, but routers in area 1 cannot reach routers in area 2. All routers are OSPF-enabled and have correct network type configurations. What is the most likely cause?

Accepted Answer

Missing or misconfigured ABRs between area 1 and area 0, or area 2 and area 0. In OSPFv2, inter-area communication must traverse area 0 (the backbone). Routers in area 1 and area 2 can only reach each other if Area Border Routers (ABRs) exist between each non-backbone area and area 0, and those ABRs are correctly configured and have full adjacencies. Since area 0 routers can ping each other but area 1 and area 2 cannot, the most likely cause is a missing or misconfigured ABR on either side, preventing the exchange of Type 3 summary LSAs between the areas.

Answer

Mismatched OSPF area IDs on routers in area 1 and area 2

Answer

Lack of BGP peering between ABRs

Answer

OSPF network type mismatch preventing adjacency formation

Question 8

A service provider is implementing MPLS-TE with FRR (Fast Reroute) using one-to-one backup tunnels. After activation, they notice that the number of labels in the forwarding table has increased significantly. What is the most likely reason for this increase?

Accepted Answer

One-to-one backup creates a unique backup tunnel for each protected LSP. One-to-one backup tunnels in MPLS-TE FRR create a dedicated backup LSP for each protected primary LSP. This means every primary LSP gets its own unique set of labels for the backup path, leading to a significant increase in the number of labels in the forwarding table. In contrast, facility backup shares a single bypass tunnel among multiple protected LSPs, which conserves labels.

Answer

RSVP-TE signaling is not properly aggregating labels

Answer

Penultimate Hop Popping (PHP) is disabled

Answer

Facility backup is being used instead of one-to-one

Question 9

A service provider is deploying EVPN-MPLS for L2VPN services. The customer requires that MAC addresses learned from one PE are not advertised to other PEs unless they are active. Which EVPN route type is used for MAC address withdrawal?

Accepted Answer

Route Type 2: MAC/IP Advertisement. EVPN Route Type 2 (MAC/IP Advertisement) carries both MAC addresses and their associated IP addresses, and it supports a 'sticky' or 'withdraw' mechanism via the BGP Withdraw message. When a MAC address becomes inactive on a PE, the PE sends a BGP Withdraw for the specific Route Type 2 route, effectively removing that MAC from the control plane of other PEs. This ensures that only active MAC addresses are advertised, meeting the customer requirement.

Answer

Route Type 5: IP Prefix

Answer

Route Type 1: Ethernet Auto-Discovery (A-D)

Answer

Route Type 3: Inclusive Multicast Ethernet Tag

Question 10

Which TWO statements about MPLS label operations in a service provider core are correct? (Choose two.)

Accepted Answer

The ingress router pushes a label stack onto the IP packet. The ingress router (LER) in an MPLS network performs a push operation, adding a label stack (typically one or more labels) onto the incoming IP packet. This label stack is used to direct the packet along a Label Switched Path (LSP) through the core, enabling MPLS forwarding based on labels rather than IP routing.

Answer

The egress router receives an MPLS packet with two labels by default

Answer

The penultimate hop performs label swap for the top label

Answer

The penultimate hop is disabled by default in MPLS networks

Question 11

Which THREE of the following are required components for implementing MPLS Layer 3 VPN (L3VPN) in a service provider network? (Choose three.)

Accepted Answer

MP-BGP between PE routers to exchange VPNv4 routes. MP-BGP (Multiprotocol BGP) is essential for carrying VPNv4 routes between PE routers. It uses address family VPNv4 to transport customer IPv4 prefixes along with their Route Distinguisher (RD) and Route Target (RT) attributes, enabling the separation and distribution of multiple customer VPNs across the MPLS core.

Answer

IBGP between PE and P routers for label distribution

Answer

VRF configuration on P routers

Question 12

A customer reports that CE routers attached to PE1 and PE2 in the same VRF cannot ping each other. Based on the exhibit, what is the most likely cause?

Accepted Answer

LDP is not enabled on the core interfaces between PE1 and PE2. B is correct because LDP must be enabled on the core interfaces between PE1 and PE2 to establish LDP sessions, which are required to exchange MPLS labels for the transport LSP. Without LDP, the MPLS forwarding path between the PEs is broken, preventing CE-to-CE ping even if BGP VPNv4 routes are correctly advertised.

Answer

Missing address-family ipv4 for BGP neighbor

Answer

Mismatched route distinguisher (RD) values on PE1 and PE2

Answer

Route target (RT) import/export mismatch

Question 13

Based on the exhibit, which label operation will occur when a packet destined to 10.2.2.1 enters PE1 with label 17?

Accepted Answer

Swap label 17 to label 18. The exhibit shows an MPLS VPN scenario where PE1 receives a labeled packet destined for 10.2.2.1. The incoming label 17 corresponds to the VPNv4 route for 10.2.2.1/32, and the LFIB on PE1 indicates a swap operation to label 18, which is the transport label used to forward the packet across the MPLS core toward the next-hop PE.

Answer

Forward with no label (Untagged)

Answer

Pop the label and forward as IP

Answer

Push label 17 onto the packet

Question 14

Refer to the exhibit. The router is configured as a PE router in an MPLS VPN network. Which option correctly identifies a configuration error that would prevent proper operation?

Accepted Answer

The OSPF network statement uses an incorrect wildcard mask. The OSPF network statement uses a wildcard mask of 0.0.0.0, which matches only the exact IP address 10.1.1.1, but the interface has a /30 prefix (255.255.255.252), so the correct wildcard mask should be 0.0.0.3 to include the entire subnet. This misconfiguration prevents OSPF from advertising the correct network, breaking MPLS VPN CE-to-PE routing.

Answer

The neighbor remote-as should be the same AS number for iBGP

Answer

The interface lacks the 'mpls ip' command

Answer

The BGP network statement does not match the interface prefix

Question 15

A service provider is deploying an MPLS L3VPN solution for a customer with two sites. The PE routers are running OSPF as the IGP and LDP for label distribution. The provider wants to ensure that customer traffic is load-balanced across two equal-cost paths between the PEs. The network team notices that all traffic is taking only one path despite the IGP showing equal-cost routes. The team has verified that the MPLS forwarding table on the P router shows only one label entry for the BGP next-hop. The team suspects a load-balancing issue. Which action best resolves the problem?

Accepted Answer

Enable per-flow load balancing on all routers with 'load-balance per-flow'. The issue is that MPLS L3VPN traffic is not being load-balanced despite equal-cost IGP paths. This typically occurs when the MPLS forwarding table on the P router has only one label entry for the BGP next-hop, indicating that per-flow load balancing is not enabled. Enabling 'load-balance per-flow' on all routers ensures that the CEF (Cisco Express Forwarding) uses per-flow hashing to distribute traffic across multiple equal-cost LSPs (Label Switched Paths), rather than per-packet or default behavior that may favor a single path.

Answer

Increase the OSPF maximum-paths value to 4

Answer

Enable BGP additional-paths on the PE routers

Answer

Configure LDP to use the 'mpls ldp igp sync' command