35 free Cisco 300-515 SPVI practice questions from the CertStash pack of 147, each with the correct answer and a full explanation. The rule is the same for every exam: ten free on the product page, 35 free here, all 147 in the pack, and each step includes the one before it.
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Question 1
Which utility can you use to validate an LSP in an MPLS environment?
Show answer and explanation
Correct answer: B. MPLS LSP ping
MPLS LSP ping is a diagnostic tool specifically designed to validate Label Switched Paths in MPLS networks by sending echo requests along the LSP to verify connectivity and proper label forwarding.
Why the other options are wrong
- A. uRPF (Unicast Reverse Path Forwarding) is a security mechanism for preventing spoofed packets, not for validating LSPs.
- C. Logging is a general diagnostic feature that records events but does not actively validate LSP functionality.
- D. RSVP is a signaling protocol used to establish LSPs but is not itself a validation utility.
Question 2
What is the primary function of a VRF on a router?
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Correct answer: A. It enables the router to support multiple separate routing tables, which allows the device to handle overlapping IP addresses.
VRF (Virtual Routing and Forwarding) allows a router to maintain multiple isolated routing tables, enabling it to support overlapping IP address spaces across different VPN instances or network segments.
Why the other options are wrong
- B. VRF does not directly enable running BGP with distance vector protocols simultaneously, nor is this its primary function.
- C. VLANs are configured locally on switches for layer 2 segmentation; VRF operates at layer 3 for routing table separation.
- D. Label switching and MPLS forwarding are separate from VRF; VRF provides routing table isolation rather than label-based forwarding optimization.
Question 3
In an Ethernet Virtual Circuit environment, which restriction do bridge domains have when STP is running?
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Correct answer: C. The STP mode must be MSTP
When STP runs on bridge domains in an EVC environment, Cisco supports only MSTP as the spanning tree mode. Bridge domains are then mapped into MST instances through their VLANs.
Why the other options are wrong
- A. RSTP and PVST+ are not supported for bridge domains in an EVC environment.
- B. Bridge domains are not required to map to different VLANs as a condition of running STP.
- D. Different bridge domains can share an MST instance, so they are not required to belong to different instances.
Question 4
An engineer is investigating an EVPN traffic flow issue. Which type of traffic should the engineer allow in an EVPN Tree Service in order to fix this issue?
Show answer and explanation
Correct answer: D. known unicast from a root to another root
In an EVPN E-Tree service, every attachment circuit is marked as either root or leaf, and the only traffic that is blocked is leaf-to-leaf. Root-to-root and root-to-leaf traffic must be permitted for the service to work. Known unicast between two roots is therefore the traffic that should be allowed.
Why the other options are wrong
- A. Leaf-to-leaf traffic of any kind is filtered by design in an E-Tree service, so it should not be allowed.
- B. Unknown unicast between leaves is still leaf-to-leaf traffic and is blocked by the E-Tree leaf filtering.
- C. Multicast between leaves is also leaf-to-leaf traffic and is filtered by the E-Tree leaf label.
Question 5
An engineer is investigating an MPLS LDP issue. Which command should an engineer use on a Cisco IOS XE device to display the contents of the LFIB?
Show answer and explanation
Correct answer: A. show mpls forwarding-table
The 'show mpls forwarding-table' command displays the LFIB (Label Forwarding Information Base), which contains the actual label forwarding entries used for packet switching in an MPLS network.
Why the other options are wrong
- B. show mpls ldp neighbors displays adjacency information with LDP peers, not forwarding table contents.
- C. show mpls ldp labels displays label bindings learned from LDP, not the LFIB forwarding entries.
- D. show mpls ldp bindings shows label-to-prefix bindings advertised by LDP, not the actual forwarding table used for switching.
Question 6
An engineer is troubleshooting an ongoing network outage. Which command should he use that can display the live log files for a process or service running on a network device?
Show answer and explanation
Correct answer: D. debug
The debug command displays live log output for processes and services running on network devices in real-time, allowing engineers to observe ongoing events and troubleshoot active issues.
Why the other options are wrong
- A. traceroute traces the network path to a destination and does not display live process logs.
- B. show run displays the running configuration file, not live process or service logs.
- C. ping tests connectivity to a remote host and does not provide live process logging output.
Question 7
In a typical service provider environment, which two tools are used to help scale PE router connectivity requirements? (Choose two.)
Show answer and explanation
Correct answer: A, E. route reflectors; confederations
Route reflectors and confederations are both BGP scaling techniques used in service provider environments to reduce the mesh of BGP sessions between PE routers, allowing for better scalability of connectivity requirements. Route reflectors allow selective route propagation while confederations divide AS numbers to organize router hierarchies.
Why the other options are wrong
- B. VPNv4 address family is used for carrying VPN routes but is not a scaling tool for PE connectivity requirements.
- C. Originator ID is an attribute used in route reflection to prevent loops but is not itself a connectivity scaling tool.
- D. Cluster ID is used within route reflection to identify clusters but is not a primary scaling tool for PE connectivity.
Question 8
An ISP provides a major client MPLS VPN for managed services. The MPLS engineering team needs to use the advanced VPN feature of selective VRF import so that only specific prefixes are present in the required VPNs. Which aspect of this feature must the team consider?
Show answer and explanation
Correct answer: D. A route is imported into the VRF only when at least one RT that is attached to the route matches one RT that is configured in the VRF and the route is permitted by the import route map.
Selective VRF import requires that a route must satisfy two conditions: it must match at least one route target configured in the VRF AND it must be permitted by the import route map filter, ensuring granular control over which prefixes enter the VPN.
Why the other options are wrong
- A. The import route map is applied first, then the route target filter, not the reverse order.
- B. While BGP and IGP routes can be imported, this does not specifically describe the selective import feature requirement.
- C. The import-map command is configured under the VRF itself, not under the PE interface connecting to the CE.
Question 9
While configuring the VRF Selection feature, you get an error message after typing the below statement: Router(config)#no vrf selection source 172.16.0.0 255.255.0.0 vrf VRF1 Which action caused this message?
Show answer and explanation
Correct answer: C. an attempt to remove a VRF Selection entry that does not exist
The 'no' command is used to remove configuration entries. The error occurs because the engineer is attempting to remove a VRF Selection entry using 'no vrf selection source' for a specific network that was never configured in the first place. VRF Selection must be added before it can be removed, so attempting to delete a non-existent entry produces an error.
Why the other options are wrong
- A. Inconsistent IP address and mask combinations are typically accepted; the error is not about validation of the network parameters themselves.
- B. VRF Selection and VRF instances are different features; this error is not related to interface-level VRF assignment conflicts.
- D. The VRF Selection table exists as a feature; the error is specific to this particular entry, not the table as a whole.
Question 10
An engineer needs to improve MPLS network management by implementing a set of tools to support the NOC engineers in troubleshooting network failures. Which feature should the engineer implement to check the connectivity of the MPLS LSP between the ingress and egress PE routers?
Show answer and explanation
Correct answer: A. MPLS OAM
MPLS OAM (Operations, Administration, and Maintenance) provides tools specifically designed to verify and troubleshoot MPLS LSP connectivity between ingress and egress PE routers. It includes ping and traceroute functionality tailored for MPLS networks, enabling NOC engineers to validate LSP path integrity and detect failures.
Why the other options are wrong
- B. MPLS-TP is a profile of MPLS for transport networks with additional features, but it is not the primary tool for checking LSP connectivity.
- C. LDP auto-discovery is a protocol mechanism for discovering peers and establishing LDP sessions, not a troubleshooting tool for connectivity verification.
- D. Extended ping is a general IP diagnostic tool and lacks MPLS-specific capabilities needed to trace LSP paths in MPLS networks.
Question 11
How does a virtual machine move seamlessly within a network in an EVPN environment?
Show answer and explanation
Correct answer: A. The IP-to-MAC binding for the default gateway is maintained across the network independent of the location of the virtual machine.
In EVPN environments, virtual machine mobility is achieved by maintaining the IP-to-MAC binding for the default gateway across the entire network independent of the VM's physical location. This ensures that regardless of where the VM moves, its Layer 3 gateway remains reachable using the same MAC address binding, enabling seamless communication without requiring address reassignment or extensive network updates.
Why the other options are wrong
- B. Broadcasting after movement is not how EVPN handles VM mobility; EVPN uses BGP control plane updates rather than broadcast mechanisms.
- C. Automatic IP address assignment via dial-out is not part of EVPN mobility solutions and would disrupt ongoing connections.
- D. Manual CAM table updates by administrators contradict the seamless and automated nature of EVPN mobility.
Question 12
In Layer 3 MPLS VPN implementations, if some of the VPNv4 routes on one PE router do not appear on another PE router, what could be the problem?
Show answer and explanation
Correct answer: B. RT export and import configuration errors
In Layer 3 MPLS VPN, if VPNv4 routes from one PE do not appear on another PE, the problem is typically Route Target (RT) export and import configuration errors. Route Targets control which VPNv4 routes are imported into specific VRFs; if the exporting PE's RT does not match the importing PE's RT import policy, routes will not be redistributed between the sites even if connectivity is otherwise correct.
Why the other options are wrong
- A. RD mismatch does not prevent route propagation between PEs; RD is used only to make prefixes unique and does not affect which routes are imported or exported.
- C. VRF name mismatch between PE routers does not affect route propagation; the BGP process exchanges VPNv4 routes regardless of local VRF naming.
- D. RD does not have export and import configurations; RD is assigned per VRF and does not control route filtering between PEs.
Question 13
A service provider that implements large L2VPNs received a request to implement EVPN Native to improve a customer’s Layer 2 communication across the core. MPLS is configured in the core to support VPN transport. Which action must the engineer take as part of the implementation?
Show answer and explanation
Correct answer: D. Configure the customer EVPN to advertise MAC addresses over the core.
When implementing EVPN Native for Layer 2 communication, the engineer must configure the EVPN instances to advertise MAC addresses over the core network. This allows MAC learning and forwarding to occur dynamically across the MPLS infrastructure through EVPN BGP advertisements, enabling efficient Layer 2 service delivery without requiring manual provisioning of individual MAC entries.
Why the other options are wrong
- A. MPLS TE configuration is not needed for EVPN Native implementation, which uses BGP for MAC address distribution.
- B. Pseudowire configuration between CEs would bypass EVPN and contradict the purpose of implementing EVPN Native service.
- C. EVPN supports both Layer 2 and Layer 3 traffic; disabling MPLS would break the transport mechanism required for the service.
Question 14
What is the role of an EVC?
Show answer and explanation
Correct answer: A. It serves as a Layer 2 circuit.
An Ethernet Virtual Connection (EVC) is a Layer 2 service that creates a logical point-to-point or multipoint connection across a service provider network. It operates at Layer 2 and provides connectivity between customer sites, functioning similarly to a traditional circuit but delivered over Ethernet infrastructure.
Why the other options are wrong
- B. Telemetry data provision is a monitoring function, not the primary role of an EVC which focuses on connectivity.
- C. Port security and access control are security functions unrelated to the core purpose of an EVC.
- D. EVC provides native Ethernet connectivity, not an overlay for VPN traffic which is a different service model.
Question 15
Service provider A is a Tier-2 carrier with an MPLS-enabled network that uses the IS-IS and BGP routing protocols. The service provider is planning to offer a new E-Line service to business customers, and the lead architect has decided to use MP-BGP for the E-Line implementation. Which action must an engineer take to start the implementation process?
Show answer and explanation
Correct answer: A. Configure the PE nodes to support AFI=25 and SAFI=70.
To implement E-Line services using MP-BGP, the PE nodes must be configured to support AFI (Address Family Identifier) 25 and SAFI (Subsequent Address Family Identifier) 70, which correspond to L2VPN Signaling for VPWS (Virtual Private Wire Service). This is the standard MP-BGP configuration required to enable Ethernet point-to-point services.
Why the other options are wrong
- B. Upgrading to 100Gbps links is an infrastructure enhancement not required to begin MP-BGP E-Line implementation.
- C. BGP authentication is a security best practice but not a mandatory first step for starting the implementation process.
- D. IS-IS multitopology configuration is unnecessary for MP-BGP E-Line services which operate independently of Interior Gateway Protocol topology extensions.
Question 16
The CTO of a company requires the support of a network consultant to deliver an MPLS solution without resigning to a certain degree of redundancy and scalability. Which solution effectively scales to hundreds or thousands of sites?
Show answer and explanation
Correct answer: D. L3VPN using a hierarchical topology of N-PEs and U-PEs.
L3VPN using a hierarchical topology with Network Provider Edge (N-PE) routers and User Provider Edge (U-PE) routers effectively scales to hundreds or thousands of sites. This hierarchical design reduces the number of direct PE-to-PE connections required, preventing the mesh explosion that would occur with direct LSP connectivity between all PEs, while maintaining full redundancy and scalability across large networks.
Why the other options are wrong
- A. L2VPN with broadcast traffic processed at the ingress PE creates significant scaling issues because broadcast handling at a single ingress point does not distribute load efficiently across hundreds or thousands of sites.
- B. L3VPN with direct LSP connectivity between all PEs requires a full mesh, which scales poorly and becomes unmanageable with hundreds or thousands of PEs due to exponential growth in connections.
- C. L2VPN with multiple frame format encapsulation and interworking addresses frame format compatibility but does not inherently provide the hierarchical scaling mechanism needed for massive networks.
Question 17
A network engineer is implementing Layer 3 MPLS VPNs on Cisco IOS/IOS XE PE routers. Which PE-to-CE routing protocol requires a separate routing process to be created for each VRF?
Show answer and explanation
Correct answer: B. OSPF
OSPF requires a separate routing process instance to be created for each VRF in MPLS VPN implementations. Unlike BGP and EIGRP which can manage multiple VRFs within a single process, OSPF architecture on Cisco IOS/IOS XE necessitates distinct process numbers for each VRF to maintain routing isolation and prevent cross-VRF route leakage.
Why the other options are wrong
- A. RIPv2 does not require separate processes per VRF; it can operate within VRF contexts using a single process model.
- C. BGP uses a single process with address-family contexts to handle multiple VRFs efficiently without requiring separate processes.
- D. EIGRP can manage multiple VRFs within a single autonomous system number and process without requiring separate process creation per VRF.
Question 18
The network engineering group of a large ISP needs to harden the management plane of its Cisco 9000 Series ASRs. While addressing IPv6 ICMP issues, they realized they have to limit the rate at which IPv6 ICMP error messages are sent out on the network. Which command do they need to apply?
Show answer and explanation
Correct answer: D. ipv6 icmp error-interval 50 20
The ipv6 icmp error-interval command limits the rate at which IPv6 ICMP error messages are sent. The syntax 'ipv6 icmp error-interval 50 20' sets the interval to 50 milliseconds with a burst of 20 packets, effectively rate-limiting error messages on the management plane to harden the router against ICMP-based attacks.
Why the other options are wrong
- A. This syntax is incorrect; ICMP rate-limiting for IPv6 does not use this command format.
- B. This command does not exist; IPv6 rate-limiting is configured specifically for ICMP messages.
- C. This command is for IPv4 ICMP rate-limiting, not IPv6 as required in the question.
Question 19
An engineer is troubleshooting an EoMPLS circuit on a Cisco IOS XR router interface that removes a VLAN from the distribution layer. Which configuration should the engineer apply in order to accomplish the task?
Show answer and explanation
Correct answer: A. interface GigabitEthernet 0/10.l2transport encapsulation dot1q 10 rewrite ingress tag pop 1 symmetric l2vpn xconnect group 103588 p2p 103588 interface GigabitEthernet 0/10.10 neighbor ipv4 10.10.10.2 pw-id 103588
For EoMPLS circuits that remove a VLAN from the distribution layer on Cisco IOS XR, the configuration must use l2transport encapsulation with dot1q, include the rewrite ingress tag pop 1 symmetric command to strip the VLAN tag, and define the L2VPN xconnect group with the peer neighbor. Option A correctly specifies the subinterface with l2transport mode, dot1q 10 encapsulation, the symmetric pop 1 rewrite to remove the VLAN tag, and the xconnect configuration pointing to the remote PE.
Why the other options are wrong
- B. Missing l2transport keyword on the interface subinterface configuration, which is required for Layer 2 transport mode on IOS XR.
- C. Omits the rewrite ingress tag pop 1 command needed to actually remove the VLAN tag from frames, making VLAN removal not functional.
- D. Uses tag translate 1-to-1 dot1ad 10 which translates the tag rather than removing it, not accomplishing the task of VLAN removal.
Question 20
Which condition must be met before an environment can support CSC?
Show answer and explanation
Correct answer: C. The CSC-PE and CSC-CE devices must be able to send labels to one another using BGP.
Carrier Supporting Carrier requires the CSC-PE and CSC-CE to exchange MPLS labels for the customer carrier's internal routes, and when BGP is used for this, both routers must support and run BGP label distribution. Without label exchange across that link, the customer carrier's labelled traffic cannot be carried through the backbone VRF.
Why the other options are wrong
- A. Pinging an interface in the global table only proves basic reachability and is not the defining requirement for CSC.
- B. CSC works with IPv4 and does not require IPv6 support on either router.
- D. OSPFv3 is not required, since CSC can use BGP or an IGP with LDP for the CSC-PE to CSC-CE exchange.
Question 21
What is the purpose of an Ethernet flow point?
Show answer and explanation
Correct answer: C. Identify similar QoS flow frames.
An Ethernet flow point identifies and classifies similar frames based on Quality of Service (QoS) characteristics. It is used to group frames with similar QoS requirements so they can be processed and prioritized consistently across the network, enabling proper traffic management and service level differentiation.
Why the other options are wrong
- A. While flow points may be involved in processing, their primary purpose is identification and classification rather than header modification.
- B. Reducing routing table entries is a Layer 3 function and not related to Ethernet flow point operations.
- D. MAC address table reduction is a forwarding optimization unrelated to the QoS classification purpose of flow points.
Question 22
Which statement describes the no bgp default route-target filter command?
Show answer and explanation
Correct answer: D. Prefixes that are received with route targets that are not imported at the PE are accepted.
The no bgp default route-target filter command disables the default filtering behavior on a PE router. By default, PEs filter incoming VPN routes and only accept those with route targets that match an import policy. When this command is applied, prefixes with route targets that are NOT imported at the PE are accepted anyway, allowing the router to receive all advertised VPN routes regardless of local import policies.
Why the other options are wrong
- A. This describes behavior when route target filtering is enabled, not disabled.
- B. Disabling the filter results in acceptance, not rejection, of prefixes.
- C. This incorrectly describes the default filtering behavior; the command actually overrides default filtering to accept all routes.
Question 23
A network architect is troubleshooting the L2TPv3 tunneling security due to the untrusted nature of the underlaying network. Which two L2TPv3 features does the architect deploy to address the ongoing issues? (Choose two.)
Show answer and explanation
Correct answer: B, D. control message hashing; control message rate limiting
L2TPv3 offers two control plane protections for untrusted networks: control message hashing, which authenticates and protects the integrity of control messages with a shared secret, and control message rate limiting, which protects the router CPU from floods of control messages. Together they defend against spoofed and denial of service traffic aimed at the tunnel endpoints.
Why the other options are wrong
- A. TCP MD5 authentication protects TCP sessions such as BGP, and L2TPv3 does not run over TCP.
- C. CHAP authentication belongs to PPP and legacy L2TPv2 tunnel authentication, not to the L2TPv3 security feature set.
- E. Asymmetric mutual authentication with a PSK is not an L2TPv3 feature; L2TPv3 uses a shared digest secret for message hashing.
Question 24
What is a benefit of Inter-AS L3VPN Option AB?
Show answer and explanation
Correct answer: B. It allows one MP-BGP session between ASBRs reducing CPU utilization.
Inter-AS L3VPN Option AB allows for a single MP-BGP session between ASBRs (Autonomous System Border Routers) rather than multiple connections per customer. This reduces the number of BGP sessions and associated CPU utilization on border routers, making it more efficient than Option A which requires per-customer EBGP sessions.
Why the other options are wrong
- A. Option AB does not use multiple physical interfaces per customer; it consolidates routing into fewer BGP sessions.
- C. While SLAs may be supported, this is not a specific benefit unique to Option AB compared to other Inter-AS options.
- D. Option AB does not reduce CPU by moving VRF traffic to global data plane; it reduces CPU by minimizing BGP sessions.
Question 25
Which action must an engineer take when configuring SR-TE using OSPF?
Show answer and explanation
Correct answer: A. Enable segment routing under the OSPF routing process.
When configuring Segment Routing Traffic Engineering (SR-TE) using OSPF, the engineer must enable segment routing under the OSPF routing process. This configuration step activates SR capabilities within OSPF, allowing the router to advertise segment identifiers and participate in SR-TE path computation.
Why the other options are wrong
- B. PIM-SM is used for multicast and is not required for SR-TE deployment with OSPF.
- C. SR-TE does not require direct neighbor relationships between all OSPF devices; standard OSPF neighbor relationships suffice.
- D. OSPF links do not need to be configured as point-to-multipoint; standard link types work with SR-TE.
Question 26
With Layer 3 MPLS VPN implementations on Cisco IOS XR PE routers, an interface is assigned to a VRF using the vrf command in which configuration mode?
Show answer and explanation
Correct answer: B. RP/0/RP0/CPU0:PE(config-if)#
On Cisco IOS XR PE routers, an interface is assigned to a VRF using the vrf command in interface configuration mode (config-if). This is the standard IOS XR method where you enter the specific interface or subinterface context and apply the vrf command to bind that interface to the desired VRF.
Why the other options are wrong
- A. BGP configuration mode (config-bgp) is used for BGP protocol settings, not for assigning interfaces to VRFs.
- C. BGP address-family mode (config-bgp-af) is used for configuring address families within BGP, not for interface VRF assignment.
- D. VRF configuration mode (config-vrf) is used to define VRF properties and policies, not to assign interfaces to VRFs.
Question 27
Which mechanism reduces the network flooding caused by host ARP learning behavior?
Show answer and explanation
Correct answer: A. ARP suppression
ARP suppression is a mechanism that reduces network flooding by preventing unnecessary ARP broadcasts. It works by intercepting ARP requests and responding locally with cached information when available, rather than allowing the request to flood across the network, thereby reducing the amount of broadcast traffic caused by host ARP learning behavior.
Why the other options are wrong
- B. Storm control limits overall broadcast, multicast, and unicast traffic but is not specifically designed to suppress ARP learning behavior.
- C. Root guard is a spanning tree protection mechanism unrelated to ARP learning or flooding.
- D. BPDU guard is a spanning tree security feature that disables ports receiving BPDUs and has no relationship to ARP learning or suppression.
Question 28
Which optional information can be included with an IPv6 ping to support the troubleshooting process?
Show answer and explanation
Correct answer: D. IPv6 hostname
When performing an IPv6 ping, optional information that can be included to support troubleshooting includes the IPv6 hostname. This allows network engineers to verify DNS resolution and hostname-to-address mapping as part of the diagnostic process, which is valuable for comprehensive troubleshooting.
Why the other options are wrong
- A. An IPv4 address is not relevant to IPv6 ping operations and cannot be meaningfully included.
- B. Source MAC addresses cannot be specified in ping commands; MAC addressing is handled by the data link layer independently.
- C. Destination MAC addresses are determined dynamically by ARP/NDP and cannot be manually specified as ping parameters.
Question 29
You are writing the requirements for an MPLS L3VPN environment that uses MP-BGP between PE routers. In this environment, route targets and route distinguishers need to be advertised between the PE routers. Which three operations meet these requirements? (Choose three.)
Show answer and explanation
Correct answer: C, D, F. creation of PE-to-PE BGP sessions between loopback IP addresses; full mesh of IBGP sessions; advertisement of extended communities, enabled on the PE-to-PE BGP neighbors
MPLS L3VPN with MP-BGP requires three key elements: PE-to-PE BGP sessions must use loopback IP addresses (not interface IPs) to ensure stable, scalable peering; a full mesh of IBGP sessions between PE routers is necessary to distribute VPN routes; and extended communities (which carry route targets and route distinguishers) must be enabled on PE-to-PE BGP neighbors to properly tag and filter VPN routes across the provider network.
Why the other options are wrong
- A. BGP sessions should use loopback addresses for stability and scalability, not outgoing interface IP addresses.
- B. Standard communities cannot carry route targets and distinguishers; extended communities are required for L3VPN operations.
- E. EBGP sessions are not used between PEs in a single AS; a full mesh of IBGP is the correct topology for PE-to-PE communication.
Question 30
Which BGP feature causes to replace the AS number of originating router with the AS number of the sending router?
Show answer and explanation
Correct answer: D. AS override
AS override is a BGP feature that replaces the AS number of the originating router with the AS number of the sending router. This is typically used in hub-and-spoke MPLS VPN topologies where the hub router needs to strip its own AS number from the AS path before sending routes to spoke routers, allowing spokes to accept routes that would otherwise be rejected due to AS path loop detection.
Why the other options are wrong
- A. Route reflectors allow one router to reflect routes to other clients without modifying the AS path.
- B. Route dampening suppresses unstable routes based on flap history, not related to AS number replacement.
- C. Confederations divide an AS into multiple sub-ASes but do not replace the originating AS number with the sending router's AS.
Question 31
While troubleshooting EoMPLS configuration problems, which three parameters should an engineer match between the two ends of the pseudowire configurations? (Choose three.)
Show answer and explanation
Correct answer: D, E, F. pseudowire ID; MTU size; control word usage
An EoMPLS pseudowire only comes up when both PEs signal the same pseudowire (VC) ID, and LDP also checks that the interface MTU matches at both ends. Control word usage must be negotiated consistently, otherwise the pseudowire fails or frames are misinterpreted. These three values are carried in the pseudowire signaling and must agree.
Why the other options are wrong
- A. VLAN names are locally significant labels and are never exchanged in pseudowire signaling.
- B. The xconnect group name is a local configuration container on IOS XR and does not need to match the remote PE.
- C. EFP subinterface numbers are local identifiers and can differ on each end of the pseudowire.
Question 32
Which two differences must an engineer consider when choosing between Layer 2 and Layer 3 VPNs? (Choose two.)
Show answer and explanation
Correct answer: B, D. VPLS is a Layer 2 VPN and MPLS/BGP VPN is a Layer 3 VPN; E-VPN is a Layer 2 VPN and IPsec is a Layer 3 VPN.
VPLS is a Layer 2 VPN technology that emulates an Ethernet switch, while MPLS/BGP L3VPN is a Layer 3 VPN that operates at the IP routing layer. E-VPN is also a Layer 2 VPN providing Ethernet services, while IPsec operates at Layer 3 for IP-level encryption and security. These are correct distinctions between Layer 2 and Layer 3 VPN types.
Why the other options are wrong
- A. VPWS is Layer 2, but VLL in SVC mode is also Layer 2, not Layer 3.
- C. MPLS is not a Layer 2 VPN; it operates at the MPLS layer (between Layer 2 and 3). Frame Relay is not a Layer 3 VPN.
- E. DMVPN is a Layer 3 VPN technology using IPsec, not Layer 2. VPLS is Layer 2, not Layer 3.
Question 33
What must match in the EVPN and L2VPN configuration mode when configuring EVPN native in a router?
Show answer and explanation
Correct answer: D. EVI
When configuring EVPN native in a router, the EVI (EVPN Instance) identifier must match between the EVPN configuration mode and the L2VPN configuration mode. The EVI serves as the unique identifier for the EVPN service instance and ensures that both the EVPN control plane and the L2VPN forwarding plane are associated with the same service.
Why the other options are wrong
- A. Interface is a local parameter and does not need to match between configuration modes; different interfaces can be part of the same EVI.
- B. Address family is configured separately and does not need to match as a paired parameter between EVPN and L2VPN modes.
- C. Bridge domain is a local forwarding construct that does not require matching identifiers between EVPN and L2VPN configuration contexts.
Question 34
You are troubleshooting ARP connectivity issues for an Ethernet interface on an IOS XR network that runs IS-IS. You verify that the IGP protocol is running, but an ARP entry has not yet been created. Which action should you take?
Show answer and explanation
Correct answer: C. ping the connected neighbor
When IGP is operational but ARP entries have not been created, the issue is likely that communication has not actually occurred between neighbors. Pinging the connected neighbor will trigger ARP learning by initiating actual traffic exchange, which causes the neighbor to respond and creates the necessary ARP entry on the local interface.
Why the other options are wrong
- A. Debugging ping packets does not cause ARP entries to be created; it only displays existing ping traffic.
- B. While ARP debugging shows ARP activity, it does not resolve the underlying cause of missing ARP entries and does not trigger learning.
- D. The RIB table routes verify IGP reachability but do not address the local Ethernet link's ARP learning, which occurs at Layer 2.
Question 35
While implementing Layer 3 MPLS VPN, which feature should an engineer use at the PEs to transform the customer IPv4 prefixes into a unique 96bit prefix?
Show answer and explanation
Correct answer: C. RD
The Route Distinguisher (RD) is used in Layer 3 MPLS VPN to transform customer IPv4 prefixes into unique 96-bit VPNv4 addresses by prepending a 64-bit RD to the 32-bit IPv4 prefix. This allows the same IPv4 prefix to be used across different customer VPNs without conflict, as the RD ensures global uniqueness within the MPLS VPN domain.
Why the other options are wrong
- A. Route Target (RT) controls route import and export policies between VRFs but does not transform IPv4 prefixes into unique addresses.
- B. VC ID is used in Layer 2 MPLS implementations (pseudowires) to identify virtual circuits, not for transforming IPv4 prefixes in Layer 3 VPNs.
- D. PW ID (Pseudowire ID) is used in MPLS pseudowire implementations for Layer 2 services, not for creating unique IPv4 prefix identifiers in Layer 3 VPNs.
That was 35 of 147.
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