What are Label-Switched Paths?

By June 27, 2017MPLS


Traditional IP-routed networks leave something to be desired when it comes to more effective transport; Multi-Protocol Label Switching, MPLS, can take a routed network and make it akin to a switched network. This method establishes pathways between sources and their respective destinations rather than forwarding packets on a hop-by-hop basis. These pathways are predetermined and are called label-switched paths LSPs, they are what make MPLS work.


As the name suggests, hop-by-hop router configurations take things as they go. They will receive a packet, take a look at its header, and then they will move it forward to the next hop in regards to the packet’s destination address.

In contrast, a label-switched network will take a source-destination pair into account and establish a pathway for the packet, eliminating superfluous steps in the process.

An IP-routed network schematic is quite simple, but inefficient: when router 4 receives traffic from router 1, it will determine which router to forward those packets to, until they ultimately land at router 9.

Label-switch network have that much needed efficiency however: a path directly from router 1 to 9 is established and all packet traffic will move down that path. This eliminates the need for routing notes to perform forwarding lookups as packets enter a router.

This will result in switching tables being smaller than routing tables, as nodes will hold onto only the information that was assigned to it on the pathway determined by the label-switched network.

Using the aforementioned schematic, Router 4 will not need to consult its routing table to know which router needs to send particular packets to router 6 it will move the packets along to the next hop as soon as they arrive.


The paths that these networks establish are called label-switched paths (LSPs) and they are what make the MPLS process run. MPLS routers will trade MPLS information along source-destination paired pathways.

The LSP needs to be viewed uniformly by every router in a network. Switched paths will be more efficient than a traditional IP if each router on the LSP can switch the packet to the next.

Many refer to MPLS as a “layer 2.5 technology.” The reason for this is that it shares characteristics of both routing and switching, which are layers 2 and 3 respectively. The predetermination of paths will make MPLS look like a layer 2 protocol; but its signaling protocol capabilities, which depend on the knowledge of routing for LSP establishment and adaptive traffic engineering, will make it look more like layer 3.


The sending of information from end host A to B, can be indicated by various factors including bandwidth availability, congestion, jitter, and latency on the instructions of a routing table. This is another benefit to an LSP. It will actually consider committed and peak traffic loads, loss probability, and maximum latency, among other things.

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