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An ingress router is a label switch router that is a starting point (source) for a given label-switched path (LSP). An ingress router may be an egress router or an intermediate router for any other LSP(s). Hence the role of ingress and egress routers is LSP specific.
The router which first prefixes the MPLS header to a packet is 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 switch routers (LSRs).
An egress router may be an ingress router or an intermediate router for any other LSP(s). Hence the role of egress and ingress routers is LSP specific. Usually, the MPLS label is attached with an IP packet at the ingress router and removed at the egress router, whereas label swapping is performed on the intermediate routers.
It is a packet-based network technology that provides a framework for recovery through the creation of point to point paths called Label Switched Paths (LSP). These LSPs creation are between a head-end and a tail-end label switch router (LSR). In the former case, the head-end router is the input or ingress router.
Two routers with an established session are called LDP peers and the exchange of information is bi-directional. LDP is used to build and maintain label-switched path (LSP) databases that are used to forward traffic through MPLS networks. LDP can be used to distribute the inner label (VC/VPN/service label) and outer label (path label) in MPLS.
Traffic aggregates of varying granularity are associated with a label-switched path at an ingress node, and packets/cells within each label-switched path are marked with a forwarding label that is used to look up the next-hop node, the per-hop forwarding behavior, and the replacement label at each hop.
These changes and additions impact basic label-switched path (LSP) properties: how labels are requested and communicated, the unidirectional nature of LSPs, how errors are propagated, and information provided for synchronizing the ingress and egress LSRs.
In Fig.1 (right), there is a primary path (label-switched path, or LSP) from A to E via B and D. The traffic of customers connected to A and E will take this path in normal operation. There is also a secondary path (LSP) from A to E via C. This path can be either pre-signaled or not. For the primary LSP, FRR (Fast ReRoute) is enabled.