Juniper JUNOSE, Configuration Manual

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Figure 47 on page 210 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.
Figure 47: Simple MPLS Domain
MPLS LSRs
Each LSR, also known as an MPLS node, must have the following:
■
At least one layer 3 routing protocol
■
A label distribution protocol
■
The ability to forward packets based on their labels
The router can use BGP, IS-IS, or OSPF as its layer 3 routing protocol, and BGP, LDP,
or RSVP-TE as its label distribution protocol.
MPLS Label Switching: Push, Look Up, and Pop
MPLS can label packets by using the existing layer 2 header or an encapsulation
header that carries the MPLS label. During LSP negotiation, the LSRs in an MPLS
domain agree on a labeling method. Labels have only local meaning; that is, meaning
for two LSR peers. Each pair of LSRs—consisting of a label originator and a label
acceptor—must use a label distribution protocol to agree on the label-to-FEC binding.
Because of the local label assignment, packet labels typically change at each segment
in the LSP path, as shown in Figure 48 on page 211. The ingress node, LSR 1, receives
an unlabeled data packet and prepends label
d to the packet. LSR 2 receives the
packet, removes label d and uses it as an index in its forwarding table to find the
next label. LSR 2 prepends label e to the packet. LSR 3 does the same thing, removing
label e and prepending label u . Finally, the egress node, LSR 4, removes label u and
determines where to forward the packet outside the MPLS domain.
210 ■
MPLS Label Switching and Packet Forwarding
JUNOSe 11.0.x BGP and MPLS Configuration Guide