Technical Documentation

Example: Load-Balanced MPLS Network

When you configure several RSVP LSPs to the same egress router, the LSP with the lowest metric is selected and carries all traffic. If all of the LSPs have the same metric, one of the LSPs is selected at random and all traffic is forwarded over it. To distribute traffic equally across all LSPs, you can configure load balancing on the ingress or transit routers, depending on the type of load balancing configured.

Figure 1 illustrates an MPLS network with four LSPs configured to the same egress router (R0). Load balancing is configured on ingress router R1. The example network uses Open Shortest Path First (OSPF) as the interior gateway protocol (IGP) with OSPF area An IGP is required for the Constrained Shortest Path First (CSPF) LSP, which is the default for the JUNOS software. In addition, the example network uses a policy to create BGP traffic.

Figure 1: Load-Balancing Network Topology

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The network shown in Figure 1 consists of the following components:

  • A full-mesh interior BGP (IBGP) topology, using AS 65432
  • MPLS and RSVP enabled on all routers
  • A send-statics policy on routers R1 and R0 that allows a new route to be advertised into the network
  • Four unidirectional LSPs between R1 and R0, and one reverse direction LSP between R0 and R1, which allows for bidirectional traffic
  • Load balancing configured on ingress router R1

The network shown in Figure 1 is a BGP full-mesh network. Since route reflectors and confederations are not used to propagate BGP learned routes, each router must have a BGP session with every other router running BGP.

For complete configurations for all routers in the example MPLS network, see Router Configurations for the Load-Balanced MPLS Network.

For a description of the situation before and after load balancing is configured in the network to use all four LSPs to forward traffic, see Traffic Flows Before Load Balancing.

Published: 2010-01-30