350-501 Practice Test 1 — 25 Questions

Question 1

A customer is experiencing intermittent packet loss in an MPLS L3VPN network. The PE routers show no errors on interfaces. Which tool can help isolate the issue by showing the complete path a packet takes through the MPLS network?

Accepted Answer

MPLS traceroute. MPLS traceroute (often implemented via LSP ping with the 'traceroute' option or the 'mpls traceroute' command) is the correct tool because it uses MPLS echo request/reply messages (RFC 8029) to traverse the label-switched path (LSP) hop by hop, revealing the exact label stack and transit LSRs. This allows identification of a failing or misbehaving node in the MPLS core that standard IP traceroute would not see, since IP traceroute relies on TTL expiry in the IP header, which may not occur in an MPLS network where labels are swapped and IP TTL is often copied from the label TTL.

Answer

Traceroute

Answer

Ping

Answer

MPLS ping

Question 2

Which TWO statements about EVPN Route Type 2 (MAC/IP advertisement) are correct?

Accepted Answer

It is used for MAC learning and ARP suppression. EVPN Route Type 2 is used to advertise MAC and IP addresses of hosts, enabling MAC learning and ARP suppression in EVPN-VXLAN fabrics. By distributing host MAC and IP bindings via BGP, it allows VTEPs to learn remote MAC addresses without data-plane flooding and to suppress ARP requests by responding locally when the IP-to-MAC mapping is known.

Answer

It provides load balancing via aliasing

Answer

It includes the Ethernet Segment Identifier (ESI)

Answer

It is not used for host mobility

Question 3

In Segment Routing, what is the role of the 'Prefix-SID'?

Accepted Answer

It identifies a prefix in the IGP. A Prefix-SID is a Segment Routing identifier that is globally unique within the SR domain and is bound to an IGP prefix (e.g., a loopback interface). It instructs routers to forward packets along the shortest path to that prefix, as computed by the IGP (OSPF or IS-IS). Option D correctly states this role, while Option C is identical but marked as incorrect in the answer set, making D the correct choice.

Answer

It identifies a specific adjacency

Answer

It is used for service chaining

Answer

It identifies a prefix in the IGP

Question 4

Refer to the exhibit. Which statement is true about this configuration?

Accepted Answer

LDP is automatically enabled on the interface. Option C is correct because the `mpls ip` command under interface configuration enables LDP on that interface, allowing it to automatically discover LDP neighbors and exchange label bindings. This command implicitly enables LDP without requiring manual neighbor configuration, which is the default behavior for label distribution in many MPLS deployments.

Answer

LDP is manually configured

Answer

MPLS VPN is enabled

Answer

MPLS forwarding is disabled

Question 5

Which THREE components are required to deploy MPLS Layer 3 VPN?

Accepted Answer

VRFs on PE routers. VRFs (Virtual Routing and Forwarding) are required on PE routers to maintain separate, isolated routing tables for each customer VPN. This allows overlapping IP addresses between different customers and ensures that traffic from one VPN does not leak into another. Without VRFs, the PE router cannot distinguish between customer routes, making Layer 3 VPN operation impossible.

Answer

GRE tunneling

Answer

OSPF

Question 6

Refer to the exhibit. The router has an outgoing label of 'Pop' for prefix ***********. What does this imply?

Accepted Answer

The router is the penultimate hop and will remove the label before forwarding. When a router has an outgoing label of 'Pop' for a prefix in MPLS, it indicates that the router is the penultimate hop (PHP) in the LSP. PHP means the router will remove (pop) the top label before forwarding the packet to the next hop, reducing the burden on the egress LSR. This is standard MPLS behavior as per RFC 3031, where the egress router signals an implicit null label (label 3) to its upstream neighbor.

Answer

The router has not learned a label for that prefix

Answer

The router will not forward traffic for that prefix

Answer

The router will push a new label for that prefix

Question 7

In a carrier-supporting carrier (CSC) architecture, the customer carrier runs MPLS in its network. Which technology is used to exchange VPN routes between the provider carrier and the customer carrier?

Accepted Answer

MP-BGP with VPNv4 address family. In a carrier-supporting-carrier (CSC) architecture, the customer carrier runs MPLS in its network and needs to exchange VPN routes (e.g., customer VPN prefixes) with the provider carrier. MP-BGP with the VPNv4 address family is the correct technology because it carries both the IPv4 prefix and the Route Distinguisher (RD), enabling the provider carrier to distinguish overlapping VPN routes from different customer VPNs. This is defined in RFC 4364 and is the standard inter-AS VPN option for CSC scenarios.

Answer

OSPF

Answer

EIGRP

Answer

IPv4 BGP

Question 8

Which protocol is used to exchange label binding information in a classic MPLS network without Segment Routing?

Accepted Answer

LDP. In a classic MPLS network without Segment Routing, LDP (Label Distribution Protocol) is the standard protocol used to exchange label binding information. LDP assigns and distributes labels for each FEC (Forwarding Equivalence Class) based on the underlying IGP routing table, enabling label-switched paths (LSPs) without requiring RSVP or Segment Routing extensions.

Answer

OSPF

Answer

BGP

Answer

IS-IS

Question 9

A service provider is designing a new MPLS L3VPN service. The customer requires that their VPN traffic be isolated from other customers and that the provider edge routers maintain separate routing tables for each VPN. Which architectural component is essential for this separation?

Accepted Answer

VRF (Virtual Routing and Forwarding). VRF (Virtual Routing and Forwarding) is the essential architectural component that enables a provider edge router to maintain separate, isolated routing tables for each VPN customer. Each VRF instance contains its own routing table, CEF (Cisco Express Forwarding) table, and associated interfaces, ensuring that traffic from one customer is never forwarded using another customer's routing information. This per-VPN isolation is fundamental to MPLS L3VPN services as defined in RFC 4364.

Answer

MPLS label stacking

Answer

VLAN tagging on the customer-facing interfaces

Answer

BGP route reflectors

Question 10

A service provider's network is experiencing suboptimal routing for BGP prefixes received from a customer. The customer is multihomed to two different PE routers in the same AS. Which BGP feature can ensure that traffic ingresses via the correct PE?

Accepted Answer

MED. MED (Multi-Exit Discriminator) is the correct BGP attribute for influencing inbound traffic from a multihomed customer. When the customer advertises the same prefix to two PE routers in the same AS, setting a lower MED value on one PE will cause the customer's upstream AS to prefer that path for returning traffic. This allows the service provider to control which PE receives the ingress traffic without altering any outbound routing policies.

Answer

Local Preference

Answer

AS-path prepend

Answer

Weight

Question 11

A network engineer is troubleshooting OSPFv3 on a service provider's IPv6 network. The router shows that OSPFv3 adjacency never reaches FULL, says 'Init'. The neighbor is directly connected over a point-to-point link. What is the most likely cause?

Accepted Answer

Missing 'ipv6 ospf' interface command on one side. The 'Init' state in OSPFv3 indicates that the router has received a Hello packet from the neighbor but the neighbor has not received a Hello packet back. On a point-to-point link, the most common cause is that the 'ipv6 ospf area ' interface command is missing on one side, which prevents OSPFv3 from sending Hellos on that interface. Without this command, the interface is not enabled for OSPFv3, so the neighbor never sees a Hello and the adjacency cannot progress to FULL.

Answer

LSA throttling prevents exchanges

Answer

Mismatched OSPF router-id

Answer

Mismatched area IDs

Question 12

Which TWO of the following are characteristics of MPLS LDP?

Accepted Answer

Label distribution based on IP routing table. B is correct because MPLS LDP distributes labels for routes found in the IP routing table, not for BGP routes. LDP peers exchange label bindings for each prefix in the IGP routing table, enabling label-switched paths (LSPs) for those destinations. This is fundamental to LDP's operation as defined in RFC 5036.

Answer

Distributes labels for BGP routes

Answer

Requires an IGP like OSPF for session reachability

Answer

TCP for both discovery and session

Question 13

A customer reports intermittent packet loss on a MPLS L3VPN connection. The PE router shows 'show mpls forwarding' entries for the CE prefix, but ping from the PE to the CE fails intermittently. Which action should be taken to isolate the issue?

Accepted Answer

Check 'show ip route vrf CUSTOMER' and 'show bgp vpnv4 unicast vrf CUSTOMER' to confirm the VRF routes.. Option A is correct because the intermittent packet loss suggests a control-plane issue rather than a data-plane problem. By checking 'show ip route vrf CUSTOMER' and 'show bgp vpnv4 unicast vrf CUSTOMER', you can verify that the VRF route for the CE prefix is present in the routing table and that BGP is advertising the correct VPNv4 route with the proper next-hop and label. This isolates whether the failure is due to missing or incorrect route propagation, which is a common cause of intermittent reachability in MPLS L3VPN.

Answer

Use 'show mpls lsp' to verify the LSP to the CE's PE.

Answer

Examine the 'show mpls forwarding vrf CUSTOMER' output to see label operations.

Answer

Review the 'show bgp vpnv4 unicast all' output to verify route advertisement.

Question 14

Refer to the exhibit. An engineer is troubleshooting an MPLS LDP session between two routers. The output shows that the LDP session is operational. However, MPLS labels are not being exchanged. What is the most likely cause?

Accepted Answer

The LDP label advertisement mode is 'downstream on demand'.. The exhibit shows that the LDP session is operational (state = Operational), but no labels are being exchanged. In 'downstream on demand' mode, a router does not advertise labels unless explicitly requested by an upstream neighbor via a Label Request message. Since the session is up but no labels are exchanged, this mismatch in label advertisement mode is the most likely cause.

Answer

The peer's addresses are not bound to the LDP Ident.

Answer

The TCP connection is not using port 646.

Answer

The LDP session is not established.

Question 15

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 16

Which TWO statements about MPLS label switching are correct? (Choose two.)

Accepted Answer

The transit LSR performs label swapping.. Option A is correct because a transit Label Switch Router (LSR) in an MPLS network performs label swapping: it receives a labeled packet, replaces the incoming label with an outgoing label from its LFIB (Label Forwarding Information Base), and forwards the packet toward the egress LSR. This is the fundamental operation of an LSR in the core of an MPLS domain, as defined in RFC 3031.

Answer

The CE receives a frame with an MPLS label.

Answer

PHP (Penultimate Hop Popping) causes the egress router to pop the label.

Answer

The egress LSR performs label swapping before forwarding.

Question 17

Refer to the exhibit. A PE router is showing a VRF route. What does the [200/0] indicate?

Accepted Answer

Administrative distance 200 and metric 0. In the VRF route output, the [200/0] notation represents the administrative distance (200) and the metric (0) for the route. The administrative distance is a trustworthiness value used by the router to select the best path when multiple routing protocols provide routes to the same destination; a lower value is preferred. The metric is a protocol-specific cost, and here it is 0, indicating a directly connected or very low-cost path within the VRF.

Answer

Local preference 200 and MED 0

Answer

Label value 200 and metric 0

Answer

AS path length 200 and weight 0

Question 18

Which THREE of the following are benefits of using Segment Routing over LDP in an MPLS network? (Choose three.)

Accepted Answer

Enables traffic engineering without RSVP-TE.. Option A is correct because Segment Routing (SR) can perform traffic engineering using SR-TE policies (via a controller or PCEP) or SR Flexible Algorithm, without requiring RSVP-TE. This eliminates the complexity of RSVP-TE's soft-state signaling and per-LSP state maintenance, relying instead on source-routed segment lists encoded in the packet header.

Answer

Provides inherent security against label spoofing.

Answer

Requires BGP-LU for label distribution.

Question 19

An ISP is deploying EVPN-VXLAN for its data center interconnect. Which statement about control-plane learning is correct?

Accepted Answer

MAC addresses are learned via BGP MP-BGP EVPN address family. In EVPN-VXLAN deployments for data center interconnect, MAC address learning occurs via the control plane using BGP with the MP-BGP EVPN address family (AFI 25 / SAFI 70). This eliminates the need for traditional data-plane flooding and learning, providing better scalability and control. The correct statement is that MAC addresses are learned via BGP MP-BGP EVPN address family.

Answer

MAC addresses are learned via IRB

Answer

MAC addresses are learned via OSPF

Answer

MAC addresses are learned via VXLAN data plane

Question 20

A service provider is troubleshooting BGP route selection for prefixes received from two different peers. The first peer prepends its AS path twice, making it longer than the second peer's path. However, the router still prefers the route with the longer AS path. Which additional attribute could cause this behavior?

Accepted Answer

The route has a higher local preference. BGP selects the best path based on a sequence of comparison steps. The first step after considering the next-hop reachability is to prefer the route with the highest weight (Cisco proprietary), then the highest local preference. Since the router prefers the longer AS path, a higher local preference (option D) on the first peer's route overrides the shorter AS path of the second peer, as local preference is evaluated before AS path length in the BGP best-path selection algorithm.

Answer

The route has a lower MED

Answer

The route has a higher weight

Answer

The route has a lower origin type

Answer

The route has a lower neighbor router ID