How to Reserve Bandwidth on an LSP Using Junos

By Walter J. Goralski, Cathy Gadecki, Michael Bushong

One of the best parts about RSVP is that you can specify a minimum bandwidth that must be supported on each transit router in the LSP. This specification helps ensure that you have enough allocated bandwidth from ingress to egress, especially for traffic that is particularly sensitive to latency or drop.

For example, imagine that you’re streaming video across your network that you’re using to conduct simulations in multiple branch offices. For the simulation to work well, every site must receive the same feed with the same timing.

You may want to ensure that the LSPs you’re using to stream the video across your MPLS core all have enough bandwidth to support the rather high requirements for your media. In this case, you can use RSVP to guarantee the bandwidth along the path from source to egress router.

To configure this bandwidth constraint, you can build off your basic MPLS configuration. In the previous MPLS example, you configure an RSVP LSP and verify that it is operating. Taking that LSP, add to it such that the bandwidth requirement is 5MB:

[edit protocols]
mpls {
   label-switched-path router1-to-router5 {
      to 10.0.0.5;
      bandwidth 50m;
   }
}

The simple inclusion of the bandwidth statement adds that value to the reservation messages that each transit router must respond to. If a router in the network doesn’t have the required bandwidth (either because its interfaces don’t support that much throughput or because bandwidth has already been allocated to other LSPs), RSVP won’t use that router in the LSP.

Instead, it sends messages to other routers until it finds a path that meets the bandwidth requirements. Although this path may not be the shortest in terms of hops or overall latency, you’ll know that each path segment can support the required bandwidth.