Using MPLS and Label-Switched Paths

Instead of using PIM SM to create multicast trees, you can use MPLS to control the paths that traffic takes to various destinations.

In the traditional Layer 3 forwarding paradigm, as a packet travels from one router to the next, an independent forwarding decision is made at each hop. Each device analyzes the IP network layer header and then chooses the next hop based on the analysis and the information in the routing table.

In an MPLS environment, however, the packet header is analyzed only once, when the packet enters the MPLS network. After analyzing the packet header, the router assigns the packet to a stream that is identified by a label (a short, fixed-length value at the front of the packet). Downstream routers use these labels as lookup indexes for the label-forwarding table. The label-forwarding table stores forwarding information for each label.

A point-to-multipoint MPLS label-switched path (LSP) is an RSVP-signaled LSP with a single source and multiple destinations. By taking advantage of the MPLS packet replication capability of the network, point-to-multipoint LSPs avoid unnecessary packet replication at the ingress router. Packet replication takes place only when packets are forwarded to two or more different destinations requiring different network paths.

Point-to-multipoint LSPs enable you to do the following:

• Use MPLS for point-to-multipoint data distribution similar to that provided by IP multicast.
• Add and remove branch LSPs from a main point-to-multipoint LSP without disrupting traffic. The unaffected parts of the point-to-multipoint LSP continue to function normally.
• Configure a node to be both a transit and an egress router for different branch LSPs of the same point-to-multipoint LSP.
• Use link protection on a point-to-multipoint LSP. Link protection can provide a bypass LSP for each of the branch LSPs that make up the point-to-multipoint LSP. If any of the primary paths fail, traffic can be switched quickly to the bypass.
• Configure subpaths either statically or dynamically.
• Specify graceful restart on point-to-multipoint LSPs.

For additional information about how to configure MPLS point-to-multipoint LSPs, see the JUNOS MPLS Applications Configuration Guide.

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