By default the Junos software installs LSPs into the inet.3 routing table for next hop resolution of MPLS VPN routes. MP-BGP will always look into inet.3 by default (if it doesn’t see a route it will look into inet.0) and uses LSPs as the next hop for the purpose of PE to PE transport.
In addition to the default behaviour Junos can also manually add static routes into inet.3 that use LSPs as a next hop. The install <prefix> feature can be configured within the MPLS label-switched-path hierarchy to install a static IP route into the inet.3 table and resolve that route using the LSP.
The purpose of this feature is to allow certain traffic to be steered into an LSP for subnets or hosts that are not reachable via an MPLS VPN or need to be reachable via the global routing tables.
In addition to the above there’s also the option to install the static route into inet.0, which is configured using the active knob. The purpose of this feature is to allow IGPs (that operate in inet.0) to access the route with the potential to scale to larger amounts of IP routes that can be transported over a next hop LSP.
The following configuration depicts the addition of the active knob which moves the LSP resolved route into inet.0.
Removing the active knob maintains the route in the inet.3 table only.
install 172.26.0.20 active;