Juniper JUNOSE 11.2.X BGP AND MPLS, Руководство по настройке

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MPLS Label Switching and Packet Forwarding Overview
MPLS is not a routing protocol; it works with layer 3 routing protocols (BGP, IS-IS, OSPF)
to integrate network layer routing with label switching. An MPLS FEC consists of a set of
packets that are all forwarded in the same manner by a given label-switching router
(LSR). For example, all packets received on a particular interface might be assigned to
a FEC. MPLS assigns each packet to a FEC only at the LSR that serves as the ingress
node to the MPLS domain. A label distribution protocol binds a label to the FEC. Each
LSR uses the label distribution protocol to signal its forwarding peers and distribute its
labels to establish an LSP. The label distribution protocol enables negotiation with the
downstream LSRs to determine what labels are used on the LSP and how they are
employed.
Labels represent the FEC along the LSP from the ingress node to the egress node. The
label is prepended to the packet when the packet is forwarded to the next hop. Each
label is valid only between a pair of LSRs. A downstream LSR reached by a packet uses
the label as an index into a table that contains both the next hop and a different label to
prepend to the packet before forwarding. This table is usually referred to as a label
information base (LIB).
The LSR that serves as the egress MPLS node uses the label as an index into a table that
has the information necessary to forward the packet from the MPLS domain. The
forwarding actions at the egress LSR can be any of the following:
•
Forward the packet based on the inner header exposed after popping the label. This
can be accomplished either by doing a routing table lookup or forwarding based on
the exposed inner MPLS label.
•
Forward the packet to a particular neighbor as directed by the table entry, for example
in a Martini layer 2 transport case.
NOTE: Forwarding of traffic for labeled IPv6 unicast routes with native IPv6 next-hop
addresses does not work.
Figure 47 on page 219 shows a simple MPLS domain, consisting of multiple LSRs. The
LSRs serving as ingress and egress nodes are also referred to as
label edge routers (LERs).
The ingress router is sometimes referred to as the tunnel head end , or the head-end router.
The egress router is sometimes referred to as the tunnel tail end , or the tail-end router.
LSPs are unidirectional , carrying traffic only in the downstream direction from the ingress
node to the egress node.
Copyright © 2010, Juniper Networks, Inc. 218
JunosE 11.2.x BGP and MPLS Configuration Guide