PLANET XGS3-24042 - Manual (Page 650)

79-4
With the LSR mapping multiple incoming labels to the same FEC, all these incoming labels will correspond
with the same outgoing label and egress port. As a result, when packets with different labels reach the LSR,
all outgoing packets will carry the same label. This process is called Label Merging. Label Merging can
decrease the label number in the MPSL domain, but maybe at the cost of losing ingress port information of the
packets.
If the LSR doesn’t support label merging, when there are multiple label requests, it will initiate a new label
request to the downstream LSR once for each of them, no matter they have the same FEC or not. Otherwise,
only one label request will be implemented.
Label Distribution Protocol
LDP (Label Distribution Protocol) is the MPLS control protocol, like singling protocols in traditional networks,
whose function includes classifying FEC, distributing labels, creating and maintaining LSP and etc.
MPSL supports multiple label distribution protocols, including protocols specially designed for distributing
labels, like LDP, CR-LDP (Constraint-Based Routing using LDP), and existing ones capable of it after
extension, like BGP (Border Gateway Protocol), RSVP (Resource Reservation Protocol). Besides, manually
configured static LSP is allowed.
LSP Tunnel Technology
MPLS supports LSP tunnel technology. Even if the path between an upstream LSR and a downstream LSR in
a LSP is not provided by the routing protocol, MPLS allows creating a new LSP connecting the two, making
them the start and end of it separately. This new LSP is a LSP tunnel, which avoids encapsulating the tunnel
via traditional network layer.
If the routes passed by a tunnel are the same as those from the routing protocol, this tunnel is Hop-by-Hop
Routed Tunnel; or, it is an Explicitly Routed Tunnel.
Multi-layer Label Stack
If a packet is transmitted in more than one layer of LSP tunnel, it will carry multiple layers of labels – Label
Stack. At the ingress and egress of each tunnel, MPLS will PUSH or POP a label accordingly.
The label stack follows the “Last-In-First-Out” principle, so MPLS will process labels from the stack top.
MPLS sets no limit to the label stack depth. If the label stack depth of a packet is m, the label at the stack
bottom is level 1, and the one at the stack top will be level m. A packet without pushing any label will be
treated as having an empty label stack (the label stack depth is 0).
79.1.2 MPLS Network Introduction
MPSL Network Structure
As demonstrated in the next figure, the basic unit composing the MPLS network is LSR; and a network
consists of LSR is called a MPLS domain.
The LSR at the edge of a MPLS domain, connecting other customer networks is called LER (LER，Label Edge
Router) , and the internal LSR is a core LSR. Core LSRs can either be routers supporting MPLS or ATM-LSR
upgraded from ATM routers. LSRs in the domain communicate with each other via MPLS, while the MPLS
domain edge is adapted via LER and traditional IP technologies.
Packets will be transmitted along a LSP composed of a series of LSRs after the ingress LER pushes a label to
it. The ingress LER is called Ingress, egress LER called Egress, and routers in the middle called Trasit.