JNCIS-SP MPLS Concepts
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 actions 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. MBB (Make-before-Break) - Default Junos behavior where the new LSP is fully signaled and verified before traffic is switched over from the old path. 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. ...