100 MPLS Interview Questions and Answers

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.

MPLS Basics

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?

IP Forwarding	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

MPLS Labels

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).

MPLS VPN

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.

MPLS Protocols

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?

Benefits include:

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?

Components are:

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

MPLS Troubleshooting

Q41. How do you troubleshoot MPLS LSP issues?

Steps include:

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?

Advantages include:

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?

Benefits include:

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?

For 5G:

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?

VPLS enables:

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?

For IoT:

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?

Advantages include:

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?

Options:

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?

Migration approaches:

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?

Troubleshooting steps:

Check LDP status between PEs with show mpls ldp neighbor
Verify end-to-end LSP using traceroute
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?

Troubleshooting steps:

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
Use debug mpls traffic-eng tunnels events and 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 range command
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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Afroz Ahmad

I'm Afroz, a Network Engineer and CCIE with 17 years of experience in Computer Networking, Data Centers, Telecom, Internet Services and ISPs. Currently, I work as a Network Designer for a leading ISP equipment vendor. I share my expertise through my blog and courses, aiming to share my knowledge and simplify complex networking concepts for both beginners and experienced professionals.