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The MPLS architecture and protocols were developed in the late 1990s in response to concerns about the limitations of connectionless IP forwarding Specific issues identified in the MPLS architecture and documented in RFC 3031 include
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It is easier to implement label lookup and replacement than the IP forwarding algorithms This was one of the initial drivers for MPLS (though its importance has often been overstated) since label switching was implemented in hardware (eg, in ATM switches) at a time when IP forwarding was only implemented in software and hence label switches could support higher packet forwarding rates than IP routers Carrier-class IP routers today are able to forward packets at line rate using hardware switching, so this issue is no longer considered important (though it is still true that label switching is simpler than IP forwarding) The ingress router may use information not included in the packet header (and which would thus not be available to a subsequent router) when assigning a packet to an LSP Because subsequent routers (Label Switching Routers or LSRs) along the LSP need to examine only the label, and not the IP header, this can be generalized to enable non-IP traffic to be forwarded on MPLS LSPs (as with Ethernet over MPLS) This is probably the most important driver for MPLS, and is central to the layer 3 VPN technology that has driven much of the deployment of MPLS up to now Packets may be assigned to LSPs that follow explicit paths rather than the path chosen by the IP routing protocols This enables traffic engineering (a full description of traffic engineering is beyond the scope of this chapter) and could only be accomplished in a connectionless network by including a source route with each packet
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MPLS was designed to carry IP packets (though the multiprotocol in its name refers to its ability to work with other network layer protocols) MPLS also uses as much as possible of the existing IP infrastructure in its operation Thus, MPLS uses IP addressing and IP routing protocols, and encapsulates its signalling traffic in IP Indeed, MPLS is often known as IP/MPLS MPLS forwarding operates at Layer 2; however, MPLS is often referred to as Layer 25 because it has some Layer 3 characteristics, and because
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the MPLS header (known as the MPLS label stack) generally sits between the Layer 2 header and the IP (Layer 3) header (see Figure 141) MPLS supports both connection-orientated and connectionless modes of operation, depending on which signalling protocol is used to establish LSPs LSPs established using RSVP-TE, which creates a point-to- point path across the network, or using static provisioning behave in a connection-orientated fashion LSPs established using LDP, which creates a multipoint-to-point path congruent with the shortest path calculated by the Interior Gateway Protocol, or IGP (the IP routing protocol used within the service provider s network) behave in a connectionless fashion although forwarding along the LSP is still based on label swapping RFC 3031 defines the concept of the Forwarding Equivalence Class (FEC) The FEC (generally pronounced as feck rather than as Eff Eee Cee ) is the set of all packets that will follow the same path (or set of equivalent paths) through the network Packets are classified into FECs at ingress into the network, and the FECs are encoded into labels enabling packets to be forwarded along LSPs FECs may be arbitrarily complex for example, packets from different source interfaces, with different QoS requirements, or destined for different IP addresses may be mapped to the same FEC or to different FECs Note that although the most commonly used FEC is the Address Prefix, which maps packets to routes in the IP routing table based on their Layer 3 forwarding information, packets may be mapped to FECs based on Layer 1, Layer 2, or Layer 4 forwarding information The use of Layer 1 or Layer 2 information to map packets to FECs is a key building block in the pseudowire architecture that will be presented in Pseudowires, later in this chapter The MPLS label is a 20-bit value that is included within a 32-bit label stack entry, as defined in RFC3032 and as shown in Figure 142 The fields in the MPLS label stack entry are used as follows:
Each LSR along the LSP decrements the TTL (Time to Live) by one when forwarding a packet If the TTL reaches zero, the packet is discarded The TTL is used to prevent packets from recirculating when loops are created as a result of inconsistent router forwarding tables, such as during a routing transient the period between a change in network topology and the point where all routers have learned the new topology The TTL field is also used for traceroute where a sequence of packets is sent with increasing TTL When sending IP traffic over MPLS, the IP TTL may be copied into the MPLS TTL at ingress to the LSP and then overwritten
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