This article explains about the basic JUNOS route selection process and its hierarchy, when multiple routes are available for the same destination.
When a destination has multiple active routes (e.g., direct, static, and dynamic routes), Junos OS follows a specific route selection process based on route preference (administrative distance) and other attributes.
Following attributes will be taken into account to determine the best route:
1) Compare Route Preference (Administrative Distance) >> Lowest preference value is preferred.2) Compare Route Metric (Cost) >> If two routes have the same preference, the route with the lower metric (cost) is selected.3) Compare AS Path (For BGP Routes) >> Shorter AS Path is preferred.4) Compare Next-Hop Reachability >> The next-hop must be reachable for the route to be installed in the forwarding table.5) Load Balancing (ECMP) >> If multiple equal-cost routes exist (same preference, metric, and next-hop reachability), Junos may install multiple next-hops for load balancing.
Route selection hierarchy:
1) Directly Connected Routes (Interface Routes): These are the most preferred routes because they are directly connected to the router's interfaces. Preference value of zero (0).
2) Static Routes:
Manually configured routes by the administrator, having preference value of 5.
3) Dynamic routes or routing protocol derived routes:
Preference values varies based on the type of protocol that is configured on the network: BGP: 170 (External) / 200 (Internal) OSPF: 10 (Internal) / 150 (External) IS-IS: 15 (L1) / 18 (L2)
Example output: (* indicates active route in the routing table)
root@switch> show route 192.168.1.0/24
192.168.1.0/24 *[Direct/0] 00:00:05 > via ge-0/0/0.0
192.168.1.0/24 *[Direct/0] 00:00:15 > via ge-0/0/0.0 [Static/5] 00:00:10 > via 10.1.1.1 >> Though static was configured, direct route is preferred.
192.168.1.0/24 *[Direct/0] 00:00:30 > via ge-0/0/0.0 [Static/5] 00:00:20 > via 10.1.1.1 [OSPF/10] 00:00:15 > via 10.1.1.2
If the interface is down and direct route is removed, static route is installed as active.
192.168.1.0/24 *[Static/5] 00:00:05 > via 10.1.1.1 [OSPF/10] 00:00:15 > via 10.1.1.2
2025-02-11: Categories added
2025-02-08 : Article Created
https://www.juniper.net/documentation/us/en/software/junos/static-routing/topics/concept/protocol-independent-routing-overview.html