MPLS Concepts for JNCIS-SP

Wed, 15 Apr 2026 00:00:00 +0000

MPLS (Multiprotocol Label Switching)

MPLS is a forwarding mechanism that uses short, fixed-length labels to make packet-forwarding decisions instead of performing a full IP lookup at every hop. Labels are applied at the ingress of an MPLS domain and stripped at the egress, with each transit router performing only a label swap — making forwarding fast and enabling traffic engineering, VPNs, and QoS capabilities.

Terms

LSR (Label Switching Router) - Any router participating in MPLS forwarding. Performs label push, swap, or pop.
LSP (Label Switched Path) - The unidirectional path a labeled packet takes from ingress to egress LSR.
FEC (Forwarding Equivalence Class) - A group of packets that receive identical forwarding treatment and are assigned the same label at ingress. The ingress router decides the FEC assignment; downstream routers just label-switch.
Ingress LSR - The first router in an LSP. Classifies traffic into FECs and pushes labels.
Egress LSR - The last router in an LSP. Removes the label and forwards the original packet.
Transit LSR (P router) - An interior provider router. Swaps labels and forwards without examining the inner IP header.
PE (Provider Edge) - ISP router at the edge of the MPLS domain that interfaces with customer equipment. Performs label push/pop for customer traffic.
CE (Customer Edge) - Customer device that connects to the PE. Not aware of MPLS.
LIB (Label Information Base) - The full table of all label bindings a router has received. Not all entries are actively used for forwarding.
LFIB (Label Forwarding Information Base) - The active subset of the LIB used for actual forwarding decisions. This is what the data plane uses.
TED (Traffic Engineering Database) - Populated by IGP TE extensions; stores link-state info (bandwidth, admin groups) used by CSPF to calculate constrained paths.

Label Operations

Operation	Description
Push	Add a new label to the top of the label stack. Done by the ingress LSR.
Swap	Replace the top label with a new one. Done by transit LSRs.
Pop	Remove the top label from the stack. Done by the egress LSR or the penultimate hop.

MPLS Label Structure

Each MPLS label is a 32-bit field inserted between the Layer 2 and Layer 3 headers (sometimes called a “shim header”). Multiple labels can be stacked.

Mpls on TTL Expired

MPLS Concepts for JNCIS-SP

MPLS (Multiprotocol Label Switching)

Terms

Label Operations

MPLS Label Structure