MPLS Fundamentals Review (Chapter I).

Chapter I : The evolution of MPLS.

Definition of MPLS

Multi Protocol Label Switching

The MPLS labels are advertised between routers so that they can build a label-to-label mapping. These labels are attached to the IP packets, enabling the routers to forward traffic by looking at the label and not the destination IP address. The packets are forwarded by label switching instead of by IP switching.

The fact that the MPLS labels are used to forward the packets and no longer the destination IP address have led to the popularity of MPLS.

Benefits of MPLS.

• The use of one unified network infrastructure
• Better IP over ATM integration
• BGP free core
• The peer-to-peer model for MPLS VPN
• Optimal traffic flow
• traffic engineering (TE).

The use of ONE Unified Network Infrastructure.

With MPLS, the idea is to label ingress packets  based on their destination address or other preconfigured criteria and switch all the traffic over a common infrastructure.

By using MPLS with IP, you can extend the possibilities of what you can transport. Adding labels to the packet enables you to carry other protocols than just IP over an MPLS-enabled Layer 3 IP backbone. MPLS can transport IPv4, IPv6, Ethernet, HDLC,PPP, and other layer 2 Technologies.

The feature whereby any Layer 2 frame is carried across the MPLS backbone is called Any Transport over MPLS (AToM). The routers that are switching the AToM traffic do not need to be aware of the MPLS payload; they just need to be able to switch the labeled traffic by looking at the label on top of it.

In esscense, MPLS label switching is a simple method of switching multiple protocols in one network. You need to have a forwarding table consisting of incoming labels to be swapped by outgoing labels and a next hop.


AToM enables the service provider to provide the same layer 2 service toward the customers as with any specific non-MPLS network. At the same time, the service provider need only one unified network infrastructure to carry all kinds of customer traffic.


BGP-Free Core.

When the IP network of a service provider must forward traffic, each router must look up the destination IP address of the packet. If the packets are sent to destination that are external to the service provider network, those external IP prefixes must be present in the routing table of each router. BGP carries external prefixes, such as the customer prefixes or the Internet Prefixes. This means that all routers in the service provider network must run BGP.

MPLS however, enables the forwarding of packets based on label lookup rather than a lookup of the IP addresses, MPLS enables a label to be associated with an egress router rather than with the destination IP address of the packet.  The core routers no longer need to have the information to forward the packets based on the destination IP address. Thus, the core routers in the service provider network no longer need to run BGP.

The router at the edge of the MPLS network still needs to look at the destination IP address of the packet and hence still needs to run BGP.

Each BGP prefix on the ingress MPLS routers has a BGP next-hop IP address associated with it. This BGP next-hop IP address is an IP address of an egress MPLS router.The label that is associated with an IP packet is the label that is associated with this BGP next-hop IP address. Because every core router forwards a packet based on the attached MPLS label that is associated with the BGP next-hop IP address, each BGP next-hop IP address of an egress MPLS router must be known to all core routers. Any interior gateway routing protocol, such as OSPF or IS-IS, can accomplish this task.

Peer-to-Peer VPN model / Overlay VPN Model.

A VPN is a network that emulates a private network over a common infrastructure. The private network requires all customer sites to be able to interconnect an be completely separate from other VPNs.

Service providers can deploy two major VPN models to provide VPN services to their customers:

• Overlay VPN model
• Peer-to-Peer VPN model.

Overlay VPN model

In the overlay model, the service provider supplies a service of point-to-point links or virtual circuits across his network between the routers of the customer.  The customer routers form routing peering between them directly across the links or virtual circuits from the service provider. The routers or switches from the service provider carry the customer data across the service provider network, but no routing peering occurs between a customer and a service provider router. The result of this is that the service provider routers never see the customer routes.

These point-to-point services could be of Layer 1 (TDM,E1,E3,SONET,SDH). Layer 2 (X.25,ATM,Frame-Relay).

The overlay service can also be provided over the IP layer 3 protocol. Most commonly used tunnels to build the overlay network on IP are generic routing encapsulation (GRE) tunnels. These tunnels encapsulate the traffic with a GRE header and an IP header. The GRE header, among other things, indicates what the transported protocol is. The IP header is used to route the packet through the service provider network.

Peer-to-Peer VPN model.

In the peer-to-peer VPN model, the service provider routers carry the customer data across the network, but they also participate in the customer routing. The service provider routers peer directly with the customer routers at Layer 3. The result is that one routing protocol neighborship or adhacency exists between the customer and the service provider router.

The privateness in MPLS VPN networks is achieved by using the concept of virtual routing/forwarding (VRF) and the fact that the data is forwarded in the backbone as labeled packets. The VRFs ensure that the routing information from the different customer is kept separate, and the MPLS in the backbone ensures that the packets are forwarding based on the label information and not the information in the IP header.

Disadvantages of the peer-to-peer VPN model compared to the overlay VPN model

• The customer share the routing responsibility with the service provider.
• The edge devices of the service provider have an added burden.

Traffic Engineering.

The basic idea behind traffic engineering is to optimally use the network infrastructure, including links that are underutilized , because they do not lie on the preferred path. This means that traffic engineering must provide the possibility to steer traffic thorugh the network on paths different from the preferred path, which is the least-cost path provided by IP routing. With traffic engineering implemented in the MPLS network, you could have the traffic that is destined for a particular prefix or with a particular quality of service flow from point A to point B along a path that is different from the least-cost path. The result is that the traffic can be spread  more evenly over the available links in the network and make more use of underutiilized links in the network.

An extra advantage of running MPLS traffic engineering is the possibility of Fast ReRouting (FRR). FRR allows you to reroute labeled traffic around a link or router that has become unavailable. The rerouting of traffic happens in less than 50 ms.

History of MPLS in Cisco IOS.

Tag Switching to MPLS

Cisco systems started of with putting labels on top of IP packets in what was then called tag switching. The first implementation was released in Cisco IOS 11.1(17)CT in 1998. A tag was the name for what is now known as a label. This implementation could assign tags to networks from the routing table and put those tahgs on top of the packet that was destined for that network. Tag switching built a Tag Forwarding Information Base (TFIB). which is , in essence , a table that stores input-to-output label mappings. Each tag-switching router had to match the tag on the incoming packet, swap it with the outgoing tag , and forward the packet.

IETF standardized taq switching into MPLS (RFC 2547. "BGP/MPLS VPNs" 1999). Terminology changed.

• Tag Switching = MPLS
• Tag  = Label
• TDP  = LDP
• TFIB = LFIB
• TSR  = LSR
• TSC  = LSC
• TSP  = LSP

MPLS applications

The first release of tag switching in Cisco IOS allowed for traffic engineering, but it was first called Routing with Resourse Reservation (RRR or R^3) The first implementation of traffic engineering in Cisco IOS was static. A later implementation made traffic engineering more dynamic by using extensions to the link state routing protocols. The operator no longer had to statically configure the traffic engineering tunnels hop by hop. The link state routing protocol carried extra information , so that the tunnels could be created in a more dynamic way.

To date, the MPLS VPN application is still the most popular of all the MPLS applications.

The next big addition to the family of MPLS appliactions was AToM.