Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks that directs data from one node to the next based on labels rather than network addresses.
It is a multilayered protocol for computer networks.
MPLS Characteristics
Labels are used to determine specific routes while addresses are used to identify endpoints. On each hop, the label changes. This approach existed already on ATM and Frame Relay.
MPLS was designed to improve ATM, and it has substituted it in much of the market.
It would stand between layers 2 (data link) and 3 (transport) of the OSI model. Sometimes is referred to be in layer 2.5.
MPLS is multiprotocol because it is compatible with different network communication technologies, such as ATM, Frame Relay, Ethernet, and SONET.
It enables traffic engineering, service levels and VPN.
MPLS Nodes
A label switch router (LSR) or transit router is a device located in the core of the MPLS network that perform high-performance routing based on level-2 label commuting. It cannot communicate with non-MPLS routers.
A label edge router (LER) or edge LSR acts as an entry or exit point within an MPLS network and it is located at the edges. It can communicate with both MPLS and non-MPLS routers.
A provider router or provider edge router is a subcase of LERs in the context of MPLS-based VPN.
MPLS Message
MPLS is defined in standard RFC 3032.
The header is composed of the following fields:
- TTL
Time to live (TTL) is composed of 8 bits.
MPLS Working
The Forwarding Equivalence Class (FEC) is a set of packets with similar or identical characteristics which may be forwarded the same way, i.e. the label may be the same.
When an unlabeled packet is received in a router the FEC is calculated and then is labeled accordingly.
The characteristic to calculate the FEC may vary on the router, but it is usually the destination IP.
Label distribution protocol (LDP) is a protocol in which routers capable of MPLS exchange label mapping information.
Label information base (LIB) is the software table maintained by IP/MPLS capable routers to store the details of port and the corresponding MPLS router label to be popped/pushed on incoming/outgoing MPLS packets.
A label switch path (LSP) is a virtual connection that is established in the context of MPLS.
A FEC tends to correspond to a single label-switched path (LSP), but a LSP may correspond to different FECs.
MPLS requires other protocols like OSPF and BGP.
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External References
- Wikipedia community; “Multiprotocol Label Switching“; en.wikipedia.org