Label Switched Path
Encyclopedia
In MPLS networking, a Label Switched Path (LSP) is a path through an MPLS network, set up by a signaling protocol such as LDP
, RSVP-TE
, BGP or CR-LDP. The path is set up based on criteria in the forwarding equivalence class
(FEC).
The path begins at a Label Edge Router
(LER), which makes a decision on which label to prefix to a packet based on the appropriate FEC. It then forwards the packet along to the next router in the path, which swaps the packet's outer label for another label, and forwards it to the next router. The last router in the path removes the label from the packet and forwards the packet based on the header of its next layer, for example IPv4
. Due to the forwarding of packets through an LSP being opaque to higher network layers, an LSP is also sometimes referred to as an MPLS tunnel.
The router which first prefixes the MPLS header to a packet is called an ingress router
. The last router in an LSP, which pops the label from the packet, is called an egress router
. Routers in between, which need only swap labels, are called transit routers or Label Switching Router (LSR)s.
Note that LSPs are unidirectional; they enable a packet to be label switched through the MPLS network from one endpoint to another. Since bidirectional communication is typically desired, the aforementioned dynamic signaling protocols can set up an LSP in the other direction to compensate for this.
When protection is considered, LSPs could be categorized as primary(working), secondary(backup) and tertiary (LSP of last resort). As described above, LSPs are normally P2P (Point to Point). A new concept of LSPs, which are known as P2MP (Point to Multi Point), was introduced recently. These are mainly used for multicasting purposes.
Label Distribution Protocol
Label Distribution Protocol is a protocol in which two Label Edge Routers exchange label mapping information. The two LERs are called LDP peers and the exchange of information is bi-directional....
, RSVP-TE
RSVP-TE
Resource Reservation Protocol - Traffic Engineering is an extension of the resource reservation protocol for traffic engineering. It supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature of the packet...
, BGP or CR-LDP. The path is set up based on criteria in the forwarding equivalence class
Forwarding equivalence class
A Forwarding Equivalence Class is a term used in Multiprotocol Label Switching to describe a set of packets with similar and / or identical characteristics which may be forwarded the same way; that is, they may be bound to the same MPLS label....
(FEC).
The path begins at a Label Edge Router
Label Edge Router
A label edge router is a router that operates at the edge of an Multiprotocol Label Switching network....
(LER), which makes a decision on which label to prefix to a packet based on the appropriate FEC. It then forwards the packet along to the next router in the path, which swaps the packet's outer label for another label, and forwards it to the next router. The last router in the path removes the label from the packet and forwards the packet based on the header of its next layer, for example IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
. Due to the forwarding of packets through an LSP being opaque to higher network layers, an LSP is also sometimes referred to as an MPLS tunnel.
The router which first prefixes the MPLS header to a packet is called an ingress router
Ingress router
An "ingress router" is a Label Switch Router that is a starting point for a given Label Switched Path . An ingress router may be an egress router or an intermediate router for any other LSP. Hence the role of ingress and egress routers is LSP specific...
. The last router in an LSP, which pops the label from the packet, is called an egress router
Egress router
An egress router is a Label Switch Router that is an end point for a given Label Switched Path . An egress router may be an ingress router or an intermediate router for any other LSP. Hence the role of egress and ingress routers is LSP specific...
. Routers in between, which need only swap labels, are called transit routers or Label Switching Router (LSR)s.
Note that LSPs are unidirectional; they enable a packet to be label switched through the MPLS network from one endpoint to another. Since bidirectional communication is typically desired, the aforementioned dynamic signaling protocols can set up an LSP in the other direction to compensate for this.
When protection is considered, LSPs could be categorized as primary(working), secondary(backup) and tertiary (LSP of last resort). As described above, LSPs are normally P2P (Point to Point). A new concept of LSPs, which are known as P2MP (Point to Multi Point), was introduced recently. These are mainly used for multicasting purposes.
External links
- MPLS Architecture, RFC 3031
- LSP Hierarchy with Generalized MPLS Traffic Engineering, RFC 4206
- Detecting MPLS Data Plane Failures, RFC 4379 (LSP Ping)