MPLS Overview

MPLS Overview

Multiprotocol Label Switching (MPLS) is a protocol that uses labels to route packets instead of using IP addresses. In a traditional network, each switch performs an IP routing lookup, determines a next-hop based on its routing table, and then forwards a packet to that next-hop. With MPLS, only the first device does a routing lookup, and, instead of finding the next-hop, finds the ultimate destination along with a path to that destination. The path of an MPLS packet is called a label-switched path (LSP).

MPLS applies one or more labels to a packet so it can follow the LSP to the destination. Each switch pops off its label and sends the packet to the next switch label in the sequence.

MPLS has the following advantages over conventional packet forwarding:

• Packets arriving on different ports can be assigned different labels.
• A packet arriving at a particular provider edge (PE) switch can be assigned a label that is different from that of the same packet entering the network at a different PE switch. As a result, forwarding decisions that depend on the ingress PE switch can be easily made.
• Sometimes it is desirable to force a packet to follow a particular route that is explicitly chosen at or before the time the packet enters the network, rather than letting it follow the route chosen by the normal dynamic routing algorithm as the packet travels through the network. In MPLS, a label can be used to represent the route so that the packet need not carry the identity of the explicit route.

Why Use MPLS ?

MPLS reduces the use of the forwarding table by using labels instead of the forwarding table. The size of forwarding tables on a switch are limited by silicon and using exact matching for forwarding to destination devices is cheaper than buying more sophisticated hardware. In addition, MPLS allows you to control where and how traffic is routed on your network – this is called traffic engineering.

Some reasons to use MPLS instead of another switching solution are:

• MPLS can connect different technologies that would not otherwise be compatible—service providers have this compatibility issue when connecting clients with different autonomous systems in their networks. In addition, MPLS has a feature called Fast Reroute that provides alternate backups for paths – this prevents network degradation in case of a switch failure.
• Other IP-based encapsulations such as Generic Route Encapsulation (GRE) or Virtual Extensible Local Area Networks (VXLAN) support only two levels of hierarchy, one for the transport tunnel and one piece of metadata. Using virtual servers means that you need multiple hierarchy levels. For example, one label is needed for top-of-rack (ToR), one label for the egress port that identifies the server, and one for the virtual server.

How Do I Configure MPLS ?

There are three types of switches you must set up for MPLS:

• Label Edge Router/Switch (LER) or ingress node to the MPLS network. This switch encapsulates the packets.
• Label Switching Routers/Switches (LSR). One or more switches that transfer MPLS packets in the MPLS network.
• Egress router/switch is the final MPLS device that removes the last label before packets leave the MPLS network.

Service providers (SP) use the term provider router (P) for a backbone router/switch doing label switching only. The customer-facing router at the SP is called a provider edge router (PE). Each customer needs a customer edge router (CE) to communicate with the PE. Customer facing routers typically can terminate IP addresses, L3VPNs, L2VPNs/ pseudowires, and VPLS before packets are transferred to the CE.

What Does the MPLS Protocol Do?

Multiprotocol Label Switching (MPLS) is an Internet Engineering Task Force (IETF)-specified framework that provides for the designation, routing, forwarding and switching of traffic flows through the network. In addition, MPLS:

• Specifies mechanisms to manage traffic flows of various granularities, such as flows between different hardware, machines, or even flows between different applications.
• Remains independent of the layer-2 and layer-3 protocols.
• Provides a means to map IP addresses to simple, fixed-length labels used by different packet-forwarding and packet-switching technologies.
• Interfaces to existing routing protocols, such as Resource Reservation Protocol (RSVP) and Open Shortest Path First (OSPF).
• Supports IP, ATM, and Frame Relay layer-2 protocols.
• Uses these additional technologies:
  • FRR: MPLS Fast Reroute improves convergence during a failure by mapping out alternate LSPs in advance.
  • Link Protection/ Next-hop backup: A bypass LSP is created for every possible link failure.
  • Node Protection/ Next-hop backup: A bypass LSP is created for every possible switch (node) failure.
  • VPLS: Creates Ethernet multipoint switching service over MPLS and emulates functions of an L2 switch.
  • L3VPN: IP-based VPN customers get individual virtual routing domains.

How Does MPLS Interface Other Protocols?

Some of the protocols that work with MPLS are:

• RSVP-TE: Resource Reservation Protocol – Traffic Engineering reserves bandwidth for LSPs.
• LDP: Label Distribution Protocol is the defacto protocol used for distribution of MPLS packets and is usually configured to tunnel inside RSVP-TE.
• IGP: Interior Gateway Protocol is a routing protocol. Edge routers (PE-routers) run BGP between themselves to exchange external (customer) prefixes. Edge and core (P) routers run IGP (usually OSPF or IS-IS) to find optimum path toward BGP next hops. P- and PE-routers use LDP to exchange labels for known IP prefixes (including BGP next hops). LDP indirectly builds end-to-end LSPs across the network core.
• BGP: Border Gateway Protocol (BGP) allows policy-based routing to take place, using TCP as its transport protocol on port 179 to establish connections. You do not configure BGP—configuring interfaces with MPLS and LDP/RSVP establishes the labels and the ability to transmit packets. BGP automatically determines the routes packets take.
• OSPF and ISIS: These protocols are used for routing between the MPLS PE and CE. Open Shortest Path First (OSPF) is perhaps the most widely used interior gateway protocol (IGP) in large enterprise networks. IS-IS, another link-state dynamic routing protocol, is more common in large service provider networks. Assuming you’re running L3VPN to your customers, on the SP edge between the PE and the CE you can run any protocol that your platform supports as a VRF aware instance.