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

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3.
The source address of ICMP packet is set to the router ID of the router on which the
TTL expired.
4.
The first 128 bytes of the MPLS packet including the IP payload encapsulated in the
MPLS packet are copied into the payload of the ICMP packet, followed by the entire
label stack of the original packet.
The ICMP packet is label-switched to the end of the LSP. From that location, the packet
is forwarded back to the source of the IP packet. This behavior enables IP trace-route to
work even when the LSR in the middle of the LSP does not have an IP route to the source
address of the IP packet.
Related Topics MPLS Label Distribution Methodology on page 227
•
•
IP Data Packet Mapping onto MPLS LSPs Overview on page 229
•
MPLS Label Switching and Packet Forwarding Overview on page 218
•
MPLS Forwarding and Next-Hop Tables Overview on page 233
•
MPLS Interfaces and Interface Stacking Overview on page 236
•
Topology-Driven LSPs Overview on page 255
MPLS Label Distribution Methodology
The JunosE implementation of MPLS supports the following methods of label distribution:
• Downstream-on-demand, ordered control with RSVP-TE
• Downstream-unsolicited, independent control or ordered control with LDP; ordered
control is the default. BGP accepts only downstream-unsolicited, ordered control
Downstream-on-demand means that MPLS devices do not signal a FEC-to-label binding
until requested to do so by an upstream device. Upstream is the direction toward a
packet’s source; the ingress node in an MPLS domain is the farthest possible upstream
node. Downstream is the direction toward a packet’s destination; the egress node in an
MPLS domain is the farthest possible downstream node. The egress node is sometime
referred to as the
tunnel endpoint .
Downstream-on-demand conserves labels in that they are not bound until they are
needed and the LSR receives label mappings (also known as label bindings) from a
neighbor that is the next hop to a destination; it is used when RSVP is the signaling
protocol.
Ordered control means that an LSR does not advertise a label for a FEC unless it is the
egress LSR for the FEC or until it has received a label for the FEC from its downstream
peer. In this manner the entire LSP is established before MPLS begins to map data onto
the LSP, preventing inappropriate (early) data mapping from occurring on the first LSR
in the path.
An LSR is an egress LSR for a FEC when the FEC is its directly attached interface or when
MPLS is not configured on the next-hop interface.
227 Copyright © 2010, Juniper Networks, Inc.
Chapter 3: MPLS Overview