Multiprotocol Label Switching (MPLS) is an advanced networking technology used by many enterprises and service providers. MPLS improves network performance and scalability by directing traffic based on labels instead of network addresses.
As MPLS gains popularity, knowledge of MPLS is becoming essential for network engineers and IT professionals. This article provides answers to the more than 100 MPLS interview questions to help you prepare for technical interviews and certification exams.
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MPLS Interview Questions and Answers
Here are more than 100 MPLS interview questions divided into sections with answers.
Q1. What is MPLS?
MPLS stands for Multi-Protocol Label Switching. It is a data-carrying mechanism which emulates some properties of a circuit-switched network over a packet-switched network. MPLS works by appending packets with an MPLS header containing one or more labels.
Q2. What are the key benefits of MPLS?
- Traffic engineering – MPLS allows explicit routing of packets.
- VPN support – MPLS enables creation of VPNs with overlapping IP addresses.
- Fast reroute – MPLS can reroute packets around network failures.
- Quality of Service – MPLS can provide differentiated service levels for traffic.
Q3. How does MPLS work?
MPLS works by assigning labels to packets at the ingress router in an MPLS domain. These labels are used to forward packets across MPLS network. Labels are removed at egress router when packet leaves MPLS domain.
Q4. What are the components of an MPLS network?
The key components are:
- Label Edge Router (LER) – Classifies and marks IP packets with labels.
- Label Switch Router (LSR) – Forwards labeled packets based on label value.
- Forwarding Equivalence Class (FEC) – Group of packets forwarded in same manner.
- Label – Short fixed length identifier attached to packets in MPLS network.
Q5. What are the differences between IP and MPLS forwarding?
|Forwarding decision based on network layer header
|Forwarding decision based on labels
|Routing decision at each hop
|Label swapping at each hop
|Best effort packet delivery
|Traffic engineering capability
|All packets treated equally
|Different service levels possible
Q6. What is a label in MPLS?
A label is a short, fixed-length identifier attached to packets in an MPLS network that determines the forwarding of packets. The label value is used by Label Switch Routers (LSRs) to make forwarding decisions.
Q7. What is label stacking in MPLS?
Label stacking refers to the ability to attach multiple labels to the same MPLS packet. This allows hierarchical or nested labeling of MPLS packets. The label stack is read from bottom to top. The last label is popped at each LSR.
Q8. How many bits are there in an MPLS label?
MPLS labels are 32 bits long. This includes:
- 20 bit label value
- 3 bit Traffic Class field
- 1 bit bottom of stack
- 8 bit TTL field
Q9. What is the maximum number of labels that can be stacked on an MPLS packet?
The maximum number of labels that can be stacked is 3. This allows for up to 3 levels of label hierarchy.
Q10. What is label swapping in MPLS?
Label swapping refers to the process where a Label Switch Router or P (Provider) replaces the incoming label with an outgoing label before forwarding the packet to the next hop. This allows the packet to be forwarded based on the label value.
MPLS LSP and Tunneling
Q11. What is a label switched path (LSP) in MPLS?
A label switched path (LSP) is a path created by routers in an MPLS network between source and destination, where packets are forwarded based on labels rather than layer 3 destination address.
Q12. What is tunneling in MPLS?
Tunneling refers to the transmission of packets through an MPLS domain. Packets entering MPLS network are assigned to a FEC and given a label. This label determines the LSP the packet takes. The core MPLS network is unaware of original packet headers.
Q13. What is an MPLS tunnel?
An MPLS tunnel is a unidirectional path between two MPLS nodes set up by MPLS signaling protocols. Packets entering tunnel are assigned MPLS label by ingress LER. Transit routers forward packets based on this label. Labels are removed by egress LER.
Q14. What are the different types of MPLS tunnels?
Different types of MPLS tunnels include:
- P2P (Point-to-Point) tunnels
- P2MP (Point-to-Multipoint) tunnels
- TE (Traffic Engineering) tunnels
- Fast reroute tunnels
- Inter-AS tunnels
Q15. What protocols can be used to setup MPLS tunnels?
MPLS tunnels can be setup using signaling protocols like LDP (Label Distribution Protocol) and RSVP-TE (Reservation Protocol with Traffic Engineering).
Q16. How are VPNs implemented using MPLS?
MPLS VPNs are created by assigning a unique VPN identifier to packets entering MPLS network at ingress router. Core MPLS routers forward packets based on label value and are unaware of VPN membership. At egress, VPN identity is used to determine routing to customer network.
Q17. What is the role of Route Distinguishers and Route Targets in MPLS VPN?
Route Distinguishers (RDs) allow customer routes with overlapping IP addresses to be unique within VPN. Route Targets are used to control distribution of VPN routes between PEs attached to that VPN.
Q18. What is MP-BGP and its role in MPLS VPN?
Multi-Protocol BGP is an extension to BGP for distributing VPN routes. MP-BGP carries VPN IPv4 prefixes along with associated RDs, Route Targets and next hop. MP-BGP allows PEs to learn customer routes and associated VPN metadata.
Q19. What are the different MPLS VPN architectures?
The main MPLS VPN architectures are:
- Overlay Model – Customer edge (CE) routers do not participate in MPLS VPN routing.
- Peer Model – CEs run MP-BGP and act as peers to Provider Edge routers.
Q20. What is VRF in MPLS VPN context?
VRF (Virtual Routing and Forwarding) is a routing table instance on PE routers separate from global routing table. Each VRF contains routing details of one VPN customer. This separation of routing information allows overlapping IP addresses between VPNs.
MPLS TE and Fast ReRoute
Q21. How does MPLS Traffic Engineering work?
MPLS TE provides mechanism to route traffic along explicitly defined LSPs according to resource requirements. MPLS TE LSPs are created by RSVP signaling protocol. Constraint-based routing is used to meet TE requirements.
Q22. How does constraint-based routing work in MPLS TE?
In constraint-based routing, when calculating LSP paths, constraints like bandwidth requirements, link attributes and administrative policies are taken into account to meet TE requirements. CSPF (Constrained Shortest Path First) algorithm is used for this.
Q23. What is CSPF algorithm in MPLS TE?
CSPF (Constrained Shortest Path First) is a shortest path first algorithm extended for MPLS TE. It computes shortest paths subject to constraints like bandwidth requirement. CSPF considers both link attributes and administrative constraints when calculating TE tunnels.
Q24. What is Fast ReRoute in MPLS?
Fast ReRoute provides mechanisms for routers to reroute traffic around network element failures, typically in tens of milliseconds. Backup tunnels are pre-established to bypass failed elements. When failure occurs, protected traffic is rerouted onto backup tunnels by the router upstream of failure.
Q25. What are the different types of backup tunnels?
Backup tunnels can be Next Hop (NHOP) backup tunnels or Next Next Hop (NNHOP) backup tunnels depending on the type of protected interface. NHOP tunnels bypass the directly connected next hop. NNHOP tunnels bypass next hop’s downstream node.
MPLS Resiliency and High Availability
Q26. How does Graceful Restart help in achieving high availability in MPLS networks?
Graceful Restart allows routers to recover from failures without impacting data plane operation. When enabled, control plane signaling and forwarding states are preserved during restart, allowing forwarding to continue across outage.
Q27. What is Non-Stop Forwarding in MPLS?
NSF (Non-Stop Forwarding) is a high availability feature that preserves data forwarding plane during control plane restart or failure. NSF works by creating backup replicas of forwarding information while the control plane is active.
Q28. What are the different NSF modes supported in MPLS?
The NSF modes are:
- NSF/SSO (Non-Stop Forwarding with Stateful Switchover) – Forwarding plane preserved when active control plane fails over to standby.
- NSF/ISSU (In Service Software Upgrade) – Forwarding plane preserved during software upgrades.
Q29. What is ECMP load balancing in MPLS?
ECMP (Equal Cost Multi Path) allows traffic across multiple LSPs between same ingress and egress routers. Hashing algorithms are used to divide traffic across equal cost LSPs. ECMP provides load balancing and resiliency in MPLS networks.
Q30. How does Any Transport over MPLS (AToM) provide resiliency?
AToM allows non-IP traffic like Ethernet over MPLS. This allows Layer-2 traffic to leverage TE and FRR capabilities of MPLS. AToM provides resiliency by rerouting layer-2 traffic onto backup LSPs using MPLS Fast Reroute.
Q31. What protocols are used in an MPLS network?
Key protocols used in MPLS networks include:
- Label Distribution Protocol (LDP) – Distribution of labels
- Resource Reservation Protocol (RSVP) – Signaling of TE LSPs
- Routing protocols like OSPF, ISIS – Distribution of routes
- MP-BGP – Distribution of VPN routes
- ICMP – Detect MPLS forwarding problems
Q32. What is Label Distribution Protocol?
LDP is the protocol used for label binding and distribution between MPLS routers. LDP discovers LSR neighbors and establishes LDP sessions. It advertises labels for FECs to LSRs which create label mappings.
Q33. What are the different LDP modes?
LDP modes are:
- DU (Downstream unsolicited) – LSR advertises labels to peers without waiting for request
- DoD (Downstream on Demand) – LSR sends label to peer only on request
- Upstream – LSR allocates local label after receiving mapping from upstream
Q34. What information is contained in LDP messages?
Key LDP message types are:
- Hello – Discover LDP neighbors
- Initialization – Establish LDP session parameters
- Keepalive – Maintain LDP sessions
- Label Mapping – Advertise and request label mappings
- Notification – Signal events like errors
Q35. What is RSVP protocol and its role in MPLS?
RSVP (Resource Reservation Protocol) is a signaling protocol used to establish Label Switched Paths in MPLS networks. RSVP carries information like bandwidth requirements allowing creation of TE tunnels with QoS parameters.
Advanced MPLS Concepts
Q36. What is MPLS Segment Routing and how does it work?
Segment Routing greatly simplifies MPLS by removing need for complex signaling protocols. Packets are routed through Explicit Route hops or segments identified by segment IDs. Segment Routing Capable Routers push/pop segment IDs onto routed packets.
Q37. What are the benefits of MPLS Segment Routing?
- simplified network operation
- flexible explicit routing
- fast recovery using TI-LFA
- easy traffic engineering
- better scaling without label distribution protocols
Q38. What is Traffic Engineering with Segment Routing?
Segment Routing for Traffic Engineering steers traffic along paths based on segment stack. Controller calculates optimal paths and enables segments on routers. Routers impose segment IDs representing paths satisfying TE requirements.
Q39. How does Topology Independent Loop Free Alternate work in Segment Routing?
TI-LFA computes backup paths without topology information, avoiding micro-loops after re-convergence. Routers store post-convergence paths as segments. On failure, packets are steered to backup segments avoiding failed elements.
Q40. What are the components of segment routing architecture?
- Segment Routing Capable Routers
- Controller – Computes optimal paths
- Segment Routing Domain – Routers under same admin control
- Segment Lists – Ordered list of segments encoding paths
- Segment Identifiers – IPv6 address or MPLS label identifying path hop
Q41. How do you troubleshoot MPLS LSP issues?
- Verify LSP existence on head-end, transit and tail-end routers
- Check MPLS forwarding plane using ping and traceroute with exp bits
- Examine LSP last hop on egress LER
- Validate LDP session status and labels
- Check for errors in routing protocol neighbors and LSP path
Q42. What are the different MPLS ping and traceroute options?
MPLS ping and traceroute can specify:
- Exp bits to test specific LSP
- TTL to test up to specific hops
- DSCP for QoS verification
- FEC stack to validate LSP binding
Q43. How does MPLS Ping work?
MPLS LSP ping uses ICMP echo request with MPLS echo request extension. MPLS shim header with EXP bits is added to test specific LSP. Replies indicate LSP connectivity. Sequence numbers detect packet loss.
Q44. What are the key identifiers in MPLS traceroute?
MPLS traceroute shows:
- Outgoing label – Label assigned to FEC
- Incoming label – Label received from previous hop
- Outgoing interface – Interface to next hop
This helps validate LSP and label path.
Q45. What steps help troubleshoot VPN connectivity issues?
- Verify VRF instances on PE routers
- Check CE-PE routing protocol adjacencies
- Validate VPN prefixes and labels in VRF using show commands
- Use ping and traceroute with VRF option to test VPN connectivity
- Check MP-BGP sessions and VPN routes exchange
Q46. How does MPLS Traffic Engineering debugging help troubleshoot issues?
Debug commands for MPLS TE:
- debug mpls traffic-eng link-management
- debug ip rsvp signal
- debug mpls traffic-eng tunnels events
- debug mpls traffic-eng tunnels signaling
Can help validate RSVP signaling, tunnel events and TE link attributes.
Q47. What information does debugging MPLS VPNs provide?
Useful debug commands are:
- debug mpls ldp bindings – local label bindings
- debug mpls ldp transport – LDP message errors
- debug mpls packets – MPLS packet labels
- debug ip bgp vpnv4 – MP-BGP VPN prefixes
Helps troubleshoot VPN label and route exchange.
Q48. What are some common MPLS issues?
Common issues include:
- LDP session failures
- Improper label allocation
- Routing protocol convergence problems
- Access-list blocking MPLS packets
- Traffic blackholing due to MPLS loops
- TE tunnel setup failures
- VPN route target filtering errors
Q49. What tools can be used to monitor MPLS networks?
Monitoring tools include:
- SNMP – Interface stats, LDP information
- IP SLA – Network performance metrics
- Netflow – Traffic analysis
- Logging – Trend analysis
- Packet capture – Detailed protocol inspection
MPLS Interview Questions – Layer 2 VPNs and VPLS
Q50. What is Virtual Private LAN Service (VPLS)?
VPLS provides Layer 2 MPLS VPN service allowing sites to share an Ethernet broadcast domain over MPLS network. VPLS enables multipoint Ethernet connectivity between sites using MPLS pseudowires between provider edge devices.
Q51. How does VPLS emulate LAN over WAN?
In VPLS, PEs connect Ethernet segments from multiple sites into single bridged domain. MAC learning, flooding and forwarding across pseudo-wires provide multipoint L2 connectivity. This creates single logical Ethernet switch across geographically dispersed sites.
Q52. What protocols are used in VPLS?
Key protocols are:
- LDP for signaling pseudo-wires between PEs
- BGP for auto-discovery and signaling of VPLS sites
- Spanning tree protocol for loop prevention
Q53. What information do BGP NLRI and extended communities carry for VPLS?
NLRI contains Route Distinguisher and VPLS endpoint identifier. Extended community includes site ID and optional multipoint capability. This provides discovery and grouping of VPLS sites.
Q54. What is MAC flushing in VPLS and why is it required?
When a PE learns a MAC over pseudo-wire, it must flush same MAC over other pseudo-wires to avoid duplication. This ensures frames to destination are sent only over pseudo-wire associated with source MAC.
Q55. What are the advantages of VPLS?
- Single broadcast domain between dispersed sites
- Layer 2 connectivity over MPLS core
- Redundant multi-homing capabilities
- Scaling without spanning tree issues
- Separation from customer addressing
MPLS Interview Questions – Carrier Supporting Carrier and H-VPLS
Q56. What is Carrier Supporting Carrier in MPLS networks?
CSC allows one MPLS service provider to transparently support customers of another MPLS provider. Customer MPLS core can be transparently carried over server provider core using multiple levels of labels.
Q57. What are the benefits of Carrier Supporting Carrier?
- Customer can retain MPLS core with VPN services
- Server provider can offer enhanced services
- Scaling as core load is distributed
- Customer independence and security
Q58. What is Hierarchical VPLS (H-VPLS)?
H-VPLS provides ability to interconnect multiple VPLS domains and form a hierarchical VPLS service. This allows large scale L2VPNs to be built
Here are more MPLS interview questions on Carrier Supporting Carrier and H-VPLS:
Q59. How does H-VPLS achieve inter-domain connection?
In H-VPLS, VPLS domains are interconnected using MPLS pseudowires between Route Reflectors. The Route Reflectors control pseudowires and Autonomous System Border Routers (ASBRs) forward traffic between VPLS domains.
Q60. What is MTU splitting in H-VPLS?
MTU splitting in H-VPLS allows configuring different MTUs for core pseudowires and access pseudowires. This prevents fragmentation when customer MTU is larger than core MTU.
Q61. How are routing loops prevented in Carrier Supporting Carrier?
In CSC,customer network uses its own IGP and MPLS tunnels. To prevent loops, customer IGP metrics are increased for CSC links to prefer backdoor links without CSC tunnel.
Q62. What is the advantage of EoMPLS in Carrier Supporting Carrier?
EoMPLS (Ethernet over MPLS) uses traffic engineering capabilities of MPLS for customer Ethernet traffic. This provides benefits like fast reroute for protected sub-50ms convergence.
Q63. What information do NLRI and extended communities carry in H-VPLS?
For H-VPLS, NLRI contains RD and VPLS ID. Extended community includes site ID, VPLS domain ID and tunnel type indicating transport tunnel endpoint.
Q64. How does MAC addressing work in H-VPLS?
In H-VPLS, MAC addresses are learned by individual VPLS domains. Route Reflectors maintain global MAC table and propagate MACs between domains for flooding/forwarding.
Q65. What are the scaling benefits of H-VPLS?
H-VPLS provides distributed architecture allowing large scale deployment. Domain MAC tables scale better than single VPLS. Route Reflectors abstract MP-BGP peering complexity.
MPLS Interview Questions – Services and Integration
Q66. What services can be delivered using an MPLS network?
MPLS enables delivery of services like:
- IP VPN for site-to-site connectivity
- Metro Ethernet for cloud access
- Mobile backhaul from cell sites
- Managed services like IPTV, VoIP
- Carrier interconnect offerings
Q67. How does MPLS integrate with network access technologies?
Access technologies like:
- Cellular networks through backhaul links
- Broadband networks via wholesale and enterprise services
- Optical transport using packet transport network
- Carrier Ethernet services
Can be seamlessly integrated over an MPLS core.
Q68. How can enterprises connect to MPLS provider networks?
Enterprise connectivity options include:
- IPsec VPN over Internet
- Point-to-point links like T1/E1, Ethernet
- Metro Ethernet services
- Multiprotocol BGP over DMVPN tunnels
- Layer 2 or Layer 3 MPLS VPN
Q69. How does MPLS interact with multicast technologies?
MPLS can leverage multicast capabilities like PIM, IGMP snooping to deliver multi-destination services like IPTV video and content delivery network replication.
Q70. How does MPLS integrate with Software Defined Networking?
MPLS integrates with SDN via controllers that program network elements. SDN controllers can orchestrate MPLS tunnels, segment routing policies, and service provisioning.
New and Emerging MPLS Technologies
Q71. What benefits does Segment Routing for Traffic Engineering provide?
SR TE enables simplified, agile traffic engineering by combining SDN programmability of MPLS with segment routing. Optimal explicit paths can be computed and realized dynamically using segment stack.
Q72. How does MPLS support 5G mobile networks?
- Backhaul with fast sub-50ms protection enhances reliability
- Mobile transport network evolution through segment routing and SDN
- Network slicing enabled using MPLS tunnels
Q73. What is the role of MPLS in cloud and data center networks?
In data centers, EVPN with MPLS provides multi-tenant connectivity, segmentation using VXLAN encapsulation and integration with orchestration platforms.
Q74. How does MPLS enhance network automation capabilities?
MPLS integrates with controllers for centralized management. Automation handles tunnel bring up, policy based routing using segments, zero touch provisioning and closed loop remediation.
Q75. What is the role of MPLS in Edge Computing?
Edge compute requires security, reliability and low latency connectivity to distributed sites. MPLS provides encryption, rapid restoration and explicit steering to satisfy edge application demands.
Automation and Orchestration
76. How can zero-touch provisioning be enabled on MPLS devices?
Zero-touch MPLS provisioning can leverage:
- gNMI and gRPC for configuration automation
- NETCONF and YANG models for programmability
- PCEP for automated tunnel provisioning
- BGP Link State and TE policies for traffic steering
- Ansible, Puppet or Chef for infrastructure orchestration
77. What programmable aspects of MPLS networks can be automated?
Automation can be applied to:
- Device onboarding and configuration
- Label distribution protocol provisioning
- RSVP-TE tunnel and policy creation
- Path computation and optimization
- Service provisioning and modification
78. What benefits does model-driven management provide for MPLS?
Benefits of model-driven MPLS management include:
- Abstract network semantics from protocols
- Use reusable and modular data models
- Enable declarative configuration
- Facilitate network validation and error checking
- Unify operations across vendors and domains
79. How can closed-loop automation improve MPLS traffic engineering?
Closed-loop automation can:
- Dynamically optimize TE based on network state
- Use streaming telemetry as input to traffic models
- Compute optimal paths with segment routing or PCE
- Update paths and policies with BGP Link State or PCEP
- Validate and monitor using telemetry
80. What role do controllers play in SDN-based MPLS automation?
SDN controllers for MPLS automation:
- Provide centralized network visibility
- Implement common abstractions and data models
- Perform path computation and optimization
- Use protocols like PCEP and BGP Link State to program network
- Integrate with orchestration systems and applications
MPLS Scenarios and Use Cases
Q81. What factors drive adoption of MPLS VPNs by enterprises?
Drivers for MPLS VPN:
- Connecting remote sites and multi-cloud networks
- Leveraging QoS and reliability of MPLS networks
- Integrating security services like firewall in VPN
- Offloading WAN complexity to service provider
- Usage based flexible billing models
Q82. How can VPLS expand business reach and flexibility?
- Rapid integration of acquired sites or M&A
- Expanding networks without redesigning LAN
- Multi-location connectivity as Point-of-Sale terminals
- Disaster recovery sites with active-active datacenters
- Flexible work models with secure remote access
Q83. Where does SD-WAN fit in alongside MPLS services?
SD-WAN complements MPLS by:
- Enabling hybrid networks with MPLS and Internet
- Extending reach to smaller branch sites
- Providing single pane of management
- Routing based on business policies and metrics
Q84. Why adopt 5G backhaul based on segment routing?
Benefits of SR for 5G:
- Exceeding reliability objectives using TI-LFA
- Drastically simplified transport operations
- Flexible explicit path steering
- Automation through centralized control
- Scale to meet capacity demands
Q85. How can MPLS support IoT scale and requirements?
- MPLS reliability, QoS overcomes intermittent connectivity
- Large scale MPLS deployments connect massive endpoints
- Traffic engineering steers data to analytic platforms
- End-to-end security including encryption
General Networking Questions important to understand MPLS
Q86. What is convergence in relation to routing protocols?
Convergence refers to the time taken for routing protocols to react to topology changes by updating routing tables and reconverging on a stable topology view. Fast convergence minimizes traffic loss and network outages.
Q87. What is IP precedence and DSCP?
IP precedence uses 3 precedence bits in TOS field for classifying packets into priority levels. DSCP uses 6 bits in DiffServ field to provide 64 service levels. Both used for applying QoS policies.
Q88. What are the advantages of a hierarchical network design?
- Improved scalability and performance
- Ease of troubleshooting due to modular design
- Enabling advanced routing capabilities
- Consistent configurations using templates
Q89. What are the main components in a WAN architecture?
Typical components are:
- Customer edge (CE) routers
- Provider edge (PE) routers
- WAN distribution layer
- WAN core layer
- Internet edge for DMVPN, IPsec connectivity
Q90. What is traffic shaping and policing?
Shaping buffers excess traffic and schedules transmission to control rate. Policing discards traffic exceeding configured rate parameters. Used to limit and control utilization.
Scenario Based MPLS Questions
Q91. Your customer wants to connect a new office to their existing MPLS VPN network using Metro Ethernet. How will you design this?
- Extend MPLS VPN to new site using VPLS
- Connect new site using DMVPN over Internet
- Setup Ethernet over MPLS pseudowire between sites
- Implement SD-WAN appliance to connect sites over Internet
Q92. A customer has a dual router site connected via fiber ring to MPLS network. How will you enhance resiliency here?
Approaches to enhance resiliency:
- Deploy node and link protection using MPLS Fast Reroute
- Configure Carrier Ethernet link aggregation or MC-LAG
- Enable non-stop forwarding for stateful switchover
- Implement ECMP and load balancing across paths
Q93. Your customer is migrating to cloud and needs multi-cloud interconnect. How can MPLS help?
MPLS can enable multi-cloud through:
- Interconnecting virtual private clouds via MPLS VPN
- Establishing direct links to Equinix, Megaport exchanges
- Integrating software defined WAN solutions
- Secure encrypted connectivity between branches, clouds
Q94. A service provider wants to use segment routing but first needs to validate it. What is your deployment recommendation?
For segment routing trial:
- Enable segment routing on edge/core routers in test topology
- Configure policies, traffic engineering using controller
- Test explicit paths for fast restoration using TI-LFA
- Define network slices using segment stacks
- Monitor performance metrics, telemetry data
Q95. Your customer runs MPLS-TP in metro network and MPLS in core. How can these interact?
Integration options include:
- Establish IP/MPLS core with MPLS-TP metro access
- Use MPLS-TP for cell site transport and MPLS for backhaul
- Connect MPLS-TP network over RSVP-TE tunnels in core
- Extend end-to-end visibility using Y.1711 OAM mechanisms
Q96. A services provider wants to improve 5G RAN transport. How can MPLS meet these needs?
MPLS benefits for 5G RAN transport:
- Meet stringent 50ms protection timing needs
- Simplified operations with segment routing
- Network slicing using MPLS tunnel separation
- Edge computing integration via low latency VPNs
- Handling massive bandwidth needs via 100G+ links
Q97. A large enterprise needs to connect 500 branches to data center. What is your recommendation?
For large scale branch connectivity:
- Implement MPLS Layer 3 VPN service
- Use VPLS for legacy multipoint connectivity
- Enable route reflectors for MP-BGP scaling
- Traffic shape and QoS to meet application SLAs
- Plan optimal hub & spoke topology for transport
Q98. Your customer sees frequent fiber cuts in metro. How can resilience be improved?
To enhance metro resilience:
- Deploy dense fiber rings for path diversity
- Enable metro link and node protection
- Implement Fast Reroute on provider edge
- Use Ethernet Link Aggregation and MC-LAG
- Consider wireless microwave backup links
Q99. A service provider wants to migrate from legacy to MPLS. What transition strategies can help?
- Support dual stack cores during transition
- Move services incrementally based on geography
- Use MPLS tunnels to integrate islands
- Provide customer premise gateways for hybrid access
- Leverage MPLS overlays for security and connectivity
Q100. How can VXLAN integrate with an existing MPLS environment?
Options for VXLAN transition:
- Map VXLAN to MPLS tunnels for transport
- Use EVPN control plane with MPLS forwarding
- Integrate SDN controllers for network automation
- Enable NSX firewall services over MPLS VPNs
- Support incremental adoption on newer hardware
Bonus MPLS Questions –
Monitoring and Visibility
101. What telemetry and monitoring capabilities support MPLS networks?
Monitoring capabilities include:
- SNMP for polling interface and MPLS data
- IP SLA for active monitoring and probing
- Netflow for traffic monitoring
- BGP-LS and Link State Tracking for topology visibility
102. What alternatives provide visibility in MPLS networks without SNMP?
Options for non-SNMP visibility include:
- gRPC/gNMI for streaming telemetry
- sFlow and IPFIX for flow monitoring
- BGP-LS for topology and traffic engineering data
103. How can segment routing monitoring be enhanced?
Segment routing can be enhanced with:
- SRv6 embedded monitoring with active probes
- Use of Hybrid Intelligent Loss Monitoring
- Export of segment routing data into BGP-LS
104. What mechanisms help detect MPLS SLA violations?
Detecting SLA violations involves:
- Proactive threshold-based monitoring
- IP SLA probes to measure delay, jitter, loss
- MPLS LSP ping and traceroute for faults
- Analytics to correlate KPIs and infrastructure data
105. What are considerations when monitoring MPLS VPNs?
MPLS VPN monitoring considerations:
- Per-VPN metrics and visibility
- Integration with route targets and VRFs
- Tracking VPN-specific SLAs and KPIs
- Correlating infrastructure and VPN service data
MPLS Design and Architecture
106. What considerations are important when designing an MPLS core?
Important design considerations include:
- Required services – VPN, TE, multicast, etc
- Redundancy and availability requirements
- Scalability needs – growth, capacity, label space
- Convergence speed and fast reroute requirements
- Integration with SDN controllers and orchestration tools
- Multi-vendor interoperability and migration strategy
107. What factors should be considered when designing an MPLS VPN service?
Key factors for MPLS VPN design:
- Number of VPN sites and expected growth
- Bandwidth requirements for VPN traffic
- Routing protocols used in VPN sites
- VPN security requirements and policies
- Integration with existing networks
- High availability and resiliency needs
- Management and monitoring requirements
108. What are considerations when integrating SDN and MPLS?
Considerations when integrating SDN and MPLS:
- SDN controller selection – OpenDaylight, Cisco ACI etc.
- Mechanism for topology visibility – BGP-LS, IGP-TE
- Uni-directional or bi-directional integration
- MPLS label stack depth for hierarchy
- Programming model – PCEP, BGP, APIs, NETCONF/YANG
- Migration strategy from legacy to SDN/MPLS
109. What are some methods to improve convergence times in MPLS networks?
Methods to improve MPLS convergence include:
- Enable Bidirectional Forwarding Detection (BFD)
- Tune IGP timers for faster detection
- Use IP Event Dampening to suppress flaps
- Implement MPLS Fast Reroute to bypass failures
- Deploy MPLS TE Fast Reroute with backup tunnels
- Use segment routing for pre-computed backup paths
110. How would you integrate enterprise SD-WAN with existing MPLS VPNs?
To integrate enterprise SD-WAN with MPLS:
- Deploy SD-WAN gateways at branches with VPN overlay
- Connect SD-WAN hubs to MPLS PE routers
- Use SD-WAN policies to load balance MPLS and broadband
- Provide end-to-end network visibility via SD-WAN controller
- Implement SD-WAN overlays like Viptela or SilverPeak
- Enable hybrid networking using MPLS
MPLS – Troubleshooting Scenarios
111. Two MPLS PE routers are not establishing an LDP session. What are some troubleshooting steps?
Troubleshooting steps include:
- Check interface configuration with
show run int
- Verify LDP enabled on interfaces with
show mpls ldp interface
- Check discovery hellos with
show mpls ldp discovery
- Confirm TCP session status using
show mpls ldp neighbor
- Verify no ACLs blocking TCP port 646
112. CE router has MPLS connectivity to PE1 but not PE2. How would you troubleshoot?
- Check LDP status between PEs with
show mpls ldp neighbor
- Verify end-to-end LSP using
- Check PE2 label bindings with
show mpls ldp binding
- Confirm PE2 MPLS forwarding table has prefix using
show mpls forwarding
- Verify CE route propagated to PE2 using
show ip route
113. New MPLS TE tunnel is down. How would you troubleshoot?
- Verify RSVP-TE headend and tailend configuration
- Check RSVP-TE neighbors and session status
- Confirm tunnel active on headend using
show mpls traffic-eng tunnels
- Check Tunnel ID on tailend router
- Verify Tunnel interfaces are up on headend and tailend
debug mpls traffic-eng tunnels eventsand
debug ip rsvp signaling
114. Some traffic is black holing across MPLS VPN. What could be the issue?
Potential causes include:
- Missing or incorrect BGP VPNv4 routes
- Issue with route targets preventing VPN route exchange
- Wrong MPLS forwarding information on transit routers
- Improper VRF configuration on PE routers
- Firewall or ACL incorrectly applied to VPN traffic
115. MPLS traffic is being dropped due to insufficient labels. How would you resolve this?
Options to increase MPLS label space:
- Extend label range using
mpls label rangecommand
- Reduce label space allocated per prefix
- Summarize routes to reduce label space requirements
- Enable label merging to reuse labels
- Deploy LDP label filtering to limit unneeded label routes
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