MPLS key terms

A group of packets that share the same forwarding treatment because they share a destination prefix, a tunnel, or a service. A label is bound to a FEC so downstream routers can forward without re-examining the IP header.

A router that forwards packets by swapping MPLS labels instead of performing an IP longest-prefix lookup. Edge LSRs push or pop labels; core LSRs usually swap them and are sometimes called transit or P routers.

An LSR at the boundary of an MPLS domain that classifies incoming IP packets, assigns them to a FEC, and pushes the first label. On egress it removes the label stack and forwards the packet natively.

The ordered sequence of LSRs and labels a packet traverses from ingress to egress for a given FEC. An LSP may be hop-by-hop, signalled explicitly, or layered beneath another LSP as a tunnel.

An ordered set of 32-bit MPLS shim headers prepended to a packet. The outermost label is processed first; deeper labels are only visible to the routers that pop the outer one, enabling tunnelling and service separation.

An optimisation where the second-to-last router pops the outer label so the egress router receives an unlabelled packet and avoids a double lookup. It is requested through an implicit-null label advertised by the egress.

The core MPLS forwarding operation: an LSR looks up the incoming label in its LFIB, replaces it with the outgoing label, and sends the packet out the associated interface and next hop.

The data-plane table an LSR consults to map an incoming label to an outgoing label, interface, and next hop. It is populated from label bindings learned via LDP, RSVP-TE, or BGP.

The association of a label with a FEC that a router advertises to its neighbours. Bindings can be local (locally assigned) or remote (learned from a peer) and drive the contents of the LFIB.

A hop-by-hop protocol in which LSRs exchange label bindings for IGP-learned prefixes over a TCP session. It builds LSPs that follow the IGP shortest path and is the default label distribution method in many MPLS networks.

An LDP advertisement mode where a router distributes label bindings to all LDP peers without waiting for a request. It is the default and most common mode, as opposed to downstream-on-demand.

An LDP behaviour where a router keeps all received label bindings, even those not currently used for forwarding, so it can switch to an alternative next hop quickly after a topology change.

A signalling protocol that establishes explicitly routed LSPs with bandwidth reservations. It carries LABEL_REQUEST, LABEL, and ROUTE objects so each hop installs the label and reserves resources along the path.

A field in RSVP-TE Path messages that lists the hops, or loose hops, an LSP must traverse. It lets a headend pin a tunnel away from the IGP shortest path to satisfy bandwidth or policy constraints.

A path computation algorithm that prunes links failing constraints such as bandwidth, affinity, or admin group before running shortest path. It supplies the ERO for RSVP-TE tunnels and is used by SR-TE controllers.

A unidirectional LSP signalled by RSVP-TE from a headend to a tailend, with reserved bandwidth and explicit path constraints. Traffic is steered into it via static routes, autoroute, or forwarding adjacencies.

An RSVP-TE mechanism that signals a new LSP before tearing down the old one, so traffic moves to the new path with minimal loss during reoptimisation or bandwidth changes.

A local protection scheme that pre-computes a backup path and redirects traffic within tens of milliseconds when a link or node fails, before the headend can resignal the tunnel.

The router adjacent to a protected link or node that detects the failure and redirects traffic onto a pre-established backup LSP. It is the headend of the bypass or detour tunnel.

The router where a backup LSP rejoins the primary LSP after bypassing a failed link or node. It receives traffic from the PLR and continues forwarding along the original path.

An FRR technique where one bypass tunnel protects a whole link or node and is shared by many primary LSPs, as opposed to a one-to-one detour tunnel per protected LSP.

A protection style that pre-installs an alternative next hop for a prefix or label so forwarding can switch immediately on failure. It is common in segment routing TI-LFA and IP fast reroute.

A per-customer routing and forwarding instance on a PE router with its own RIB and FIB. It isolates overlapping address space and binds interfaces to a customer VPN.

An 8-byte value prepended to a customer IPv4 prefix to make it unique in MP-BGP. It identifies the VRF that originated the route but does not control which VRFs import it.

An extended BGP community attached to VPN routes that controls import and export between VRFs. A PE exports routes with one or more RTs and imports routes whose RTs match its configured import list.

Multiprotocol BGP extensions that carry address families beyond IPv4 unicast, including VPNv4 and VPNv6. It distributes customer routes between PE routers with RD, RT, and label information attached.

The MP-BGP address family used for MPLS L3VPN. Each route carries an RD, an IPv4 prefix, extended communities for RTs, and an MPLS label used by the egress PE for forwarding.

An emulated point-to-point circuit that carries Layer 2 frames across an MPLS network. The two provider edge devices encapsulate the attachment circuit and exchange labels to form the tunnel.

The customer-facing interface or VLAN on a PE that connects to the Layer 2 VPN. Frames arriving on the AC are encapsulated into the pseudowire for transport across the MPLS core.

A method for signalling pseudowires and VPLS membership using MP-BGP auto-discovery routes. It removes the need for manual peer configuration and supports multihoming and dynamic discovery of PEs in a VPLS domain.

An MPLS Layer 2 service that emulates a multipoint Ethernet LAN across the provider network. Each PE learns MAC addresses and forwards frames over pseudowires in a full mesh or hub-and-spoke topology.

An OAM mechanism that verifies pseudowire connectivity and path liveness by sending control-channel packets with special first nibbles that identify the PW being tested.

OAM tools that send labelled echo requests along an LSP to test reachability and map the path. They reveal the forwarding labels and next hops at each hop, helping isolate black holes and label mismatches.

An MPLS evolution where the source encodes the path as an ordered list of segments, each identified by a SID. Routers forward on the SID list without per-path signalling state in the core.

A label or index that identifies a segment such as a node, adjacency, or prefix in segment routing. Node SIDs use the global SRGB, while adjacency SIDs are locally significant.

The label range reserved on each SR router for global segments. Consistent SRGB configuration across the domain lets a node SID map to the same label on every router.

An FRR method that uses segment routing to build a backup path guaranteed loop-free for any topology, by combining a post-convergence path with repair segments. It provides sub-50 ms protection.