Support for the Creation of a Primary mLDP P2MP LSP in the Class-Specific Topology

This section describes the creation of an mLDP P2MP master tree in the class-specific topology.


Both NG MVPN over mLDP P2MP and VPLS over mLDP P2MP provide dual-root 1+1 protection. If an mLDP P2MP master tree fails, traffic rapidly switches to a backup tree, which reduces service traffic loss.

mLDP P2MP searches a unicast routing table created in the base topology for root routes, which may cause a protection failure. If unicast routes from the two roots to a leaf node partially overlap and the overlapping link fails, dual-root 1+1 protection fails. Adjusting the unicast routes to prevent a protection failure stemming from an overlapping link but adversely affects existing unicast services.

An apparent solution is to divide a physical network into different logical topologies for different services. This is called multi-topology. Each class-specific topology in a public network address family contains an independent routing table. The class-specific topology allows protocol routes to be added, deleted, and imported. Based on the multi-topology and class-specific topology, the class-specific topology can be configured to address the dual-root 1+1 protection failure for mLDP P2MP tunnels.

mLDP P2MP is a typical application of the class-specific topology. A primary mLDP P2MP LSP can be configured in the class-specific topology. Routes then only partially depend on the unicast routing table. Route priorities can be adjusted in the class-specific topology to prevent the overlapping link, which does not affect unicast services.

The mLDP P2MP master tree is created in the class-specific topology, whereas an mLDP FRR LSP is created in the base topology because the FRR LSP is established using unicast techniques.

Basic Concepts

  • Base topology: is created by default on a public network and cannot be configured or deleted.
  • Class-specific topology: can be added, deleted, or imported.


On the network shown in Figure 1, in the base topology, the master tree PE1 -> P1 -> PE3 and the backup tree PE2 -> P1 -> PE3 share the P1 -> PE3 link. If this link fails, both the master and backup trees are interrupted, causing traffic loss. To prevent the overlapping link, properly plan the network deployment and prevent the master and backup trees from overlapping. Change the path of the PE2-to-PE3 tunnel from PE2 -> P1 -> PE3 to PE2 -> P2 -> PE3. After the route is changed in the base topology, all service paths are updated to PE2 -> P2 -> PE3. In this situation, existing unicast services are adversely affected. To address this problem, create the master tree in the class-specific topology. Deploy the class-specific topology on each router and adjust the master tree to PE1 -> P1 -> PE3 and the backup tree to PE2 -> P1 -> PE3, which addresses the overlapping link issue and does not affect existing unicast services along the path PE2 -> P1 -> PE3.
Figure 1 Typical topology


  • Deployment can be modified to prevent mLDP P2MP dual-root 1+1 protection failures stemming from the overlapping link issue, without unicast services affected.

Copyright © Huawei Technologies Co., Ltd.
Copyright © Huawei Technologies Co., Ltd.
< Previous topic Next topic >