Juniper JUNOSE, Руководство по настройке

Страница: 245 / 738

■
RFC 4364—BGP/MPLS IP Virtual Private Networks (VPNs) (February 2006)
■
RFC 4379—Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures
(February 2006)
■
RFC 4661—Signaling Requirements for Point-to-Multipoint Traffic-Engineered
MPLS Label Switched Paths (LSPs) (April 2006)
■
RFC 4875—Extensions to Resource Reservation Protocol - Traffic Engineering
(RSVP-TE) for Point-to-Multipoint TE Label Switched Paths (LSPs) (May 2007)
NOTE: IETF drafts are valid for only 6 months from the date of issuance. They must
be considered as works in progress. Please refer to the IETF Web site at
http://www.ietf.org for the latest drafts.
MPLS Label Switching and Packet Forwarding
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.
MPLS Label Switching and Packet Forwarding
■ 209
Chapter 2: MPLS Overview