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    Point-to-Multipoint LSPs Overview

    A point-to-multipoint MPLS LSP is an RSVP-TE LSP with a single ingress LSR and one or more egress LSRs. You can use point-to-multipoint LSPs to avoid unnecessary duplication of packets at the ingress router by allowing non-ingress LSRs to replicate the incoming data on one or more outgoing interfaces. Point-to-multipoint LSPs for multicast VPNs are supported for intra-autonomous system (AS) environments (within an AS), but are not supported for inter-AS environments (between ASs).

    Although you can use point-to-point LSPs to provide point-to-multipoint services, this type of configuration can cause data replication at the ingress LSR or duplicate traffic within the network. You can use the traffic engineering (TE) capability of LSPs to achieve consistent QoS control and efficient use of network resources, and create point-to-multipoint LSPs to deliver data from one ingress LSR to multiple egress LSRs. The flow of traffic in a point-to-multipoint LSP is not restricted to the paths that are followed for multicast or shortest path routing; instead, you can explicitly configure the values to determine the path. Packet replication takes place only when packets are forwarded to two or more different destinations requiring different network paths.

    A point-to-multipoint TE tunnel is composed of multiple point-to-multipoint LSPs. To scale to a large number of nodes or branches in a point-to-multipoint LSP, each LSP is uniquely identified by a point-to-multipoint ID, which is unique for the entire LSP, regardless of the number of branches or leaves it contains. A point-to-multipoint LSP is composed of multiple source-to-leaf sub-LSPs. These sub-LSPs are formed between the ingress and egress LSRs to form the point-to-multipoint LSP.

    Point-to-multipoint LSPs can be signaled using one or more path messages. If a path message signals only one sub-LSP, it targets only one leaf in the point-to-multipoint tunnel. Because a single path message might not be large enough to contain all the sub-LSPs in the tunnel and also because you can create path messages specific to a sub-LSP in the tunnel, you can use multiple path messages. However, if you want to minimize the number of control messages required to configure a point-to-multipoint tunnel, you need to use a single path message to signal multiple sub-LSPs.

    The following are some of the benefits of using point-to-multipoint LSPs:

    • A point-to-multipoint LSP allows you to use MPLS TE for point-to-multipoint data distribution. This functionality provides better control over the path chosen to transmit traffic than that provided by IP multicast.
    • You can 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.
    • You can enable 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 quickly switched to the bypass path.
    • You can configure branch LSPs statically, dynamically, or as a combination of static and dynamic LSPs.
    • You can enable graceful restart on point-to-multipoint LSPs.

    Using E Series Routers as Egress LSRs

    You can use E Series routers as egress LSRs in a point-to-multipoint LSP. To create a point-to-multipoint LSP and to use E Series routers as egress LSRs, no special configuration is required. The configuration that you made for point-to-point LSPs, which enables MPLS RSVP-TE on the interface that must signal an LSP in that virtual router context, is sufficient.

    Figure 1 shows a point-to-multipoint LSP with multiple egress LSRs. The multicast source sends a packet to the ingress router, LSR 1, which in turn sends the packet on the point-to-multipoint LSP to the branch router, LSR 2. The branch router, LSR 2, is connected to another branch router, LSR 3. Here, LSR 3 is not directly connected to the ingress router, LSR 1, but only to the branch router, LSR 2. These branch routers, in turn, replicate the packet and forward it to E Series routers, LSRs 4 through 7, configured as egress LSRs.

    The configuration shown in Figure 1 is an example of an LSP that contains segments that run from ingress LSR to one or more branch and egress LSRs. For example, sub-LSPs exist between LSR 1 and LSR 2, and between LSR 2 and LSR 4. The sub-LSP between LSR 2 and LSR 4 is an egress sub-LSP that transmits the replicated packet from branch router, LSR 2, to egress E Series router, LSR 4. Egress LSRs can also be directly connected to the ingress LSR. In this figure, the connection between LSR 8 and LSR 1 is an example of this type.

    Note: You cannot use E Series routers as core or ingress LSRs. You need to use Juniper Networks routers running Junos OS to function as core or ingress LSRs in the point-to-multipoint LSP.

    Figure 1: Simple MPLS Domain

    Simple MPLS Domain

    Use the show mpls rsvp tunnels p2mp role tail command to view the status and configuration information for point-to-multipoint egress tunnels.

    Published: 2014-08-18