MPLS Introduction

Drawback of Traditional IP Routing

Traditional IP Forwarding

                          

• In traditional IP networks, routing protocols are used to distribute Layer 3 routing information. 
• Regardless of the routing protocol, packet forwarding is based on the destination address alone. 
• Therefore, when a packet is received by the router, it determines the next-hop address using the packet's destination IP address along with the information from its own forwarding/routing table. 
• This process of determining the next hop is repeated at each hop (router) from the source to the destination.
• Every router in the path performs a destination-based routing lookup in a large forwarding table.
• Each router performs an IP lookup (“routing”), determines a next-hop based on its routing table, and forwards the packet to that next-hop.
• Because of which, every router may need full Internet routing information (i.e., more than 100,000 routes)

IP over ATM

• The figure illustrates a worst-case scenario where Layer 2 (L2) and Layer 3 (L3) topologies do not overlap.
• The result is that a single packet could be propagated with three L2 hops but instead requires 7 hops.
• The reason is that L2 devices have static information about how to interconnect L3 devices.
• Routers use a routing protocol to propagate L3 routing information through the intermediary router.

Traffic Engineering with Traditional IP Forwarding

• This figure illustrates a topology with unequal links. Traffic patterns illustrate that most of the traffic goes between sites A and B.
• Most traffic goes between large sites A and B and only uses the primary link.
• Destination-based routing does not provide any mechanism for load balancing across unequal paths.
• Policy-based routing can be used to forward packets based on other parameters, but this is not scalable solution in high volume traffic due to performance limitations.

Definition of MPLS

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 the 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 label switching technique is not new. Frame Relay and ATM use it to move frames or cells
• throughout a network.
• In Frame Relay, the frame can be any length, whereas in ATM, a fixed length cell consists of a header of 5 bytes and a payload of 48 bytes.
• The header of the ATM cell and the Frame Relay frame refer to the virtual circuit that the cell or frame resides on.
• The similarity between Frame Relay and ATM is that at each hop throughout the network, the “label” value in the header is changed.
• This is different from the forwarding of IP packets. When a router forwards an IP packet; it does not change a value that pertains to the destination of the packet: that is, it does not change the destination IP address of the packet.
• 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

• Bogus Benefit: One of the early reasons for a label-swapping protocol was the need for speed. Switching IP packets on a CPU were slower than switching labelled packets by looking up just the label on top of a packet.
• 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.
• Better IP over ATM integration
• Border (BGP)- Gateway Protocol free core
• The peer-to-peer model for MPLS VPN
• Optimal traffic flow
• Traffic engineering

History of MPLS in Cisco IOS

Tag Switching to MPLS

• Cisco Systems started off 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 tags 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.
• Later, the IETF standardized tag switching into MPLS. The IETF released the first RFC on MPLS—RFC 2547, “BGP/MPLS VPNs”—in 1999. The result of this was that much of the terminology changed.

Old and New Terminology for Tag Switching/MPLS

Old Terminology	New Terminology
Tag Switching	MPLS
Tag	Label
TDP = Tag Distribution Protocol	LDP = Label Distribution Protocol
TFIB = tag forwarding information base	LFIB = label forwarding information base
TSR = tag switching router	LSR = label switching router
TSC = tag switch controller	LSC = label switch controller
TSP = tag switched path	LSP = label switched path

 MPLS Application

• The first release of tag switching in Cisco IOS allowed for traffic engineering, but it was first called Routing with Resource Reservation (RRR or R3). The first implementation of traffic engineering in Cisco IOS was static. This meant that you as the operator of the router had to configure all the hops that a certain flow of traffic had to follow through the network. 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. This greatly reduced the amount of work the operator had to do, which made MPLS traffic engineering more popular.
• Until the coming of MPLS VPN, tag switching or MPLS was not widespread. When Cisco came out with Cisco IOS Software Release 12.0(5)T, the first Cisco IOS release containing support for MPLS VPN in 1999, it became an instant success because many service providers immediately started to implement MPLS VPN. 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 applications was AToM. Cisco implemented AToM in Cisco IOS Release 12.0(10)ST, released in 2000, to carry ATM AAL 5 over an MPLS backbone.

Why MPLS?

• What we want that forwarding decision should not only based on Destination IP.
• Needed a single infrastructure that support multitude of applications in a secure manner.
• Provide a highly scalable mechanism that was a topology driven rather than a flow driven.
• Load balance traffic to utilize network bandwidth efficiently.
• Allow core routers/networking devices to switch packet based on some simplified header.

What is MPLS used for?

MPLS is used to create a transport network. It provides an underlay medium for overlay services. 
The main services that we run with the MPLS are:

• Layer 2 MPLS VPN with Pseudowires (VPWS, VPLS)
• EVPN
• Layer 3 MPLS VPN
• Inter-AS MPLS VPN
• Carrier Supporting Carrier
• MPLS Traffic Engineering with RSVP and Segment Routing
• RSVP-FRR, TI-LFA
• Seamless MPLS/Unified MPLS

Important Point Regarding MPLS

• RFC 3031 specifies Tag Switching, which later called as MPLS.
• Defined by IETF in 1998
• Multi-Protocol Label Switching is technology for delivery of IP services.
• MPLS technology switches packet (IP packets, AAL5 frames) instead of routing packets to transport the data.
• MPLS is a new forwarding mechanism in which packets are forwarded based on labels.
• MPLS provide high performance forwarding mechanism.
• MPLS uses best of both: “Switch, if possible, Route if necessary “
• MPLS is a scalable and protocol-independent solution, that can carry Layer 3 IP and Non-IP and Layer 2 traffic, PPP, HDLC, Frame-Relay, Ethernet, all are possible.
• MPLS provides transport and can be considered one of the tunneling mechanisms.
• MPLS transport protocols as of 2022, are LDP, RSVP, Segment Routing and BGP LU.
• MPLS is globally enabled on cisco routers.
• By default, MPLS traffic will follow the same path as regular IP traffic
• By default, all IGP routes (IGP, Connected, Static) will have a label assigned.
• MPLS is easy and accessible. It means MPLS is simple but also complex. The complexity of MPLS is due to the services/function of the MPLS which add the complexity.