Multiprotocol support and MPLS

Multiprotocol support

Multiprotocol support means that a network device or technology can carry and handle traffic of more than one network-layer protocol at the same time, such as IPv4, IPv6, IPX or AppleTalk, over the same links. A multiprotocol router keeps a separate routing table for each protocol; a dual-stack router runs IPv4 and IPv6 together. In MPLS, multiprotocol means the technology works independently of both the network-layer protocol above it (IP, IPv6, Ethernet frames, ATM cells) and the link-layer technology below it.

How MPLS works

Multiprotocol Label Switching (MPLS) forwards packets using short fixed-length labels instead of looking up the IP destination address at every router. It is called a "layer 2.5" technology because the label is inserted between the layer 2 header and the IP header.

MPLS header (32 bits): Label (20 bits), Traffic Class (3 bits, QoS), S bit (1 bit, bottom of label stack), TTL (8 bits).

Terms

• LER (Label Edge Router): at the edge of the MPLS domain; adds (push) or removes (pop) labels.
• LSR (Label Switch Router): core router that forwards by label (swap).
• FEC (Forwarding Equivalence Class): group of packets forwarded the same way (e.g. same destination prefix).
• LSP (Label Switched Path): the one-way path through the network.
• LDP / RSVP-TE: protocols to distribute labels. The routers build a label forwarding table (LFIB).

Operation

1. IGP (OSPF/IS-IS) builds routes and LDP assigns labels for each prefix, building the LSP.
2. Ingress LER classifies the incoming IP packet into a FEC and pushes a label.
3. Each LSR looks at only the label, finds the outgoing label and interface in its table, swaps the label and forwards.
4. Egress LER pops the label and forwards the packet by a normal IP lookup. (With penultimate hop popping, the last LSR before the egress removes the label instead, so the egress LER does only the IP lookup.)

 IP pkt        LSP
  -->[LER1]-->[LSR2]-->[LSR3]-->[LER4]--> IP
     push 17   swap     swap     pop
              17->22   22->31

Advantages: fast forwarding with a simple lookup, traffic engineering (explicit paths), VPNs (layer 2 and layer 3), QoS and fast reroute. It is widely used in ISP and enterprise WAN backbones.