MPLS Overivew

Key Concepts

• MLS (Multiprotocol Label Switching): Packet forwarding technology using label switching instead of network addresses.
• Label Switching: Forwarding traffic based on labels attached to packets.
• Layer 3 VPNs: A primary application of MLS, enabling virtual private networks over a shared infrastructure.
• PE (Provider Edge) Router: Router at the edge of the provider network that interfaces with customer equipment.
• P (Provider) Router: Router within the MLS backbone responsible for transport.
• CE (Customer Edge) Router: Router on the customer's network that connects to the PE router.
• VRF (Virtual Routing and Forwarding Instance): A virtual router within a physical router, isolating customer routing tables.
• Route Distinguisher (RD): A unique identifier within a VRF that allows overlapping IP address spaces.
• Route Target: An extended BGP community attribute used to control the import and export of routes between VRFs.
• Label Distribution Protocol (LDP): Protocol used to distribute labels for destination network mappings.
• Label Switched Path (LSP): The path a labeled packet takes through the MLS network.
• BGP (Border Gateway Protocol): Routing protocol used for exchanging routing information between autonomous systems.
• IGP (Interior Gateway Protocol): Routing protocol used within an autonomous system (e.g., OSPF, ISIS).
• Hub and Spoke Topology: A traditional network design where all traffic passes through a central hub.

Traditional Networking Challenges

• Physically Separate Networks: Each customer required dedicated circuits and hardware, leading to high costs and long turn-up times.
• Inefficient Bandwidth Utilization: Provisioned bandwidth was reserved for a customer even if it wasn't fully utilized.
• Hub and Spoke Limitations: Single point of failure at the hub, increased latency for spoke-to-spoke communication, and scalability issues.
• High Cost: Redundancy required doubling circuits and head-end routers.
• Low Bandwidth: Limited by the capacity of individual circuits (e.g., T1s, DS3s).

MLS Topology and Operation

• Layer 3 Provider Network: The provider handles routing, allowing customers to share infrastructure.
• Shared Hardware: Multiple customers can coexist on the same provider hardware (PE routers) while maintaining isolation.
• Virtual Routing: VRFs create logical separation between customer networks.
• Route Exchange: Customers exchange routes with the provider's PE routers.
• Label Assignment: The ingress PE router assigns a label to incoming IP packets based on their destination and VRF.
• Label Forwarding: Core P routers forward packets based solely on the label, without inspecting the IP header.
• Label Removal: The egress PE router removes the label before forwarding the packet to the destination CE router.

MLS Benefits

• Elimination of Redundancy: Reduces the need for multiple dedicated networks and long turn-up times.
• Improved Efficiency: Pools resources for efficient use of bandwidth and network hardware.
• Rapid Service Activation: Enables quick provisioning, modification, and decommissioning of networks through configuration changes.
• Scalability: Allows the network to grow, change, or shrink without requiring new hardware or circuits.
• Improved Reliability: Offers features like rapid fail detection and traffic engineering for alternate path routing.
• Cost Reduction: Reduces the need for dedicated routers and WAN circuits.
• Overlapping IP Space: Customers can use the same private IP address ranges without conflict.
• QoS Support: Enables prioritization of traffic based on application requirements.

Router Types in MLS

• P Router (Provider Router):
  • Located within the MLS backbone.
  • Performs transport functions.
  • Runs an IGP (OSPF or ISIS).
  • No BGP peering with customers.
  • Does not have knowledge of customer routes.
• PE Router (Provider Edge Router):
  • Located at the edge of the provider network.
  • Connects to customer CE routers.
  • Performs label switching.
  • Has knowledge of customer routes.
  • Peers directly with customer equipment.
• CE Router (Customer Edge Router):
  • Located on the customer's network.
  • Connects to the provider's PE router.
  • Typically has an eBGP peering with the PE router.
  • Runs standard IP routing software.
  • No MLS functionality required.
• Route Reflector:
  • Overcomes the limitations of full-mesh iBGP peering.
  • PE routers establish iBGP sessions only with route reflectors.
  • Route reflectors perform routing lookups for PE routers.
  • Scalable because it only exchanges route information, not customer traffic.

Label Switching Details

• IP Routing Protocol: Used within the MLS backbone (e.g., OSPF, ISIS) to establish reachability to destination networks.
• Label Distribution: LDP or TDP distributes labels for destination network mappings.
• Ingress PE Router: Receives the IP packet, performs packet classification in a VRF, assigns a label, and forwards the labeled packet into the MLS network.
• Core P Router: Switches packets based on the label value, without knowledge of customer routes.
• Egress PE Router: Removes the label before forwarding the IP packet to the destination CE router.

Label Switched Paths (LSPs)

• Derivation: LSPs derive from the IGP routing protocol.
• Shortest Path: Typically follow the shortest path, but can diverge with MLS Traffic Engineering (TE).
• Label Stack: Used for packet forwarding, with two labels:
  • Top label: Indicates the BGP next hop (IGP label).
  • Second label: Indicates the outgoing interface or VRF (VPN label).

Layer 3 VPNs

• Purpose: Transport customer routes across the MLS backbone while keeping them isolated from other customer networks.
• Definition: An IP network infrastructure delivering private network services over a service provider's infrastructure.
• Key Features:
  • Scalable and easy to provision.
  • Customer IP space can overlap.
  • QoS available for customer applications.
  • Easy to control access.
  • Easy configuration for the customer.

VRF (Virtual Routing and Forwarding Instance) Details

• Location: Typically used on the PE router.
• Function: Creates a virtual routing instance to support customer data.
• Isolation: Maintains separation from the global routing table and other customer VRFs.
• Route Distinguisher (RD): Locally significant identifier that gives routes a unique identifier, allowing overlapping IP address spaces.
• Route Targets: Used to import and export customer routes across PE devices on the MLS backbone, enabling communication between customer sites.
• Configuration:
  • vrf definition <vrf_name>: Defines a VRF.
  • rd <route_distinguisher>: Configures the route distinguisher.
  • route-target import <extended_community>: Configures the import route target.
  • route-target export <extended_community>: Configures the export route target.

MLS VPN Model with Routing

• Customer Peering: Customers typically peer eBGP with the PE router.
• Provider Peering: PE routers peer iBGP with route reflectors within the provider network.
• IGP: An IGP (e.g., ISIS or OSPF) is used within the provider network.
• MLS Core: PE routers use MLS within the core network.
• Plain IP: Plain IP routing is used between the CE and PE routers.

Routing Tables and Route Targets

• PE Routing Tables:
  • Global routing table: Used for communication within the MLS backbone.
  • VRF forwarding tables: Used for communication with customer CE routers.
• Route Targets:
  • Each VRF has import and export route policies configured.
  • The eBGP extended community is used for route target configuration.
  • If the route target in a received update matches any of the import values, the routing update is accepted; otherwise, it is discarded.

MLS VPN Forwarding

• BGP Next Hop Reachability: PE and P routers have BGP next hop reachability through the backbone IGP.
• Label Distribution: Labels are distributed through LDP hop-by-hop.
• Label Stack: Used for packet forwarding:
  • Top label: Indicates the BGP next hop (IGP label).
  • Second label: Indicates the VRF (VPN label).
• P Router Knowledge: P routers do not have any BGP or VPN knowledge.

Configuration Example

• VRF Definition:

  vrf definition red
   rd 100:1
   route-target both 100:1
  !
  interface GigabitEthernet0/0
   vrf forwarding red
   ip address 10.1.1.1 255.255.255.0
  

• BGP Setup:

  router bgp 100
   neighbor 10.1.1.2 remote-as 200
   !
   address-family ipv4 vrf red
    neighbor 10.1.1.2 activate
    neighbor 10.1.1.2 send-community extended
   exit-address-family
  

Conclusion

MLS provides a scalable, efficient, and reliable solution for building VPNs over a shared infrastructure. By using label switching and VRFs, MLS enables service providers to offer private network services to multiple customers while maintaining isolation and security. The key components of an MLS network include PE routers, P routers, CE routers, route reflectors, and the underlying routing protocols (IGP and BGP). Understanding these concepts is crucial for designing, implementing, and troubleshooting MLS-based VPNs. The next steps involve watching the MLS lab videos and working through the lab workbook to gain hands-on experience with configuring and managing an MLS network.