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[Junos] Understanding kernel routes

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Article ID: KB36167 KB Last Updated: 14 Sep 2020Version: 2.0
Summary:

This article throws some light on routes that are marked as "kernel" routes on devices running Junos OS.

 

Solution:

In devices that run Junos OS, the following type of routes can be seen marked with “Kernel” in the routing table:

inet.0: 17 destinations, 17 routes (17 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

30.30.30.0/24      *[Kernel/4294967294] 00:00:11
                    > to 1.1.1.2 via ge-0/0/0.0

When RPD restarts, KRT reads nexthops and routes from the kernel that were added by a previous instance of RPD. These are used for graceful restart. These routes are called Remnant routes.

During graceful restart, the kernel and the PFE continue to use the route/nexthops that were added for the previous instance of RPD.

As the new RPD instance performs graceful restart recovery, it will add new routes/nexthops. Upon successful restart, the new route/nexthops information will be the same as the old information. If the RPD generates a new route, it overrides the remnant route (for the same prefix).

Eventually, KRT will delete the remnant routes. Upon unsuccessful restart, the remnant routes will continue to remain in the table as per the stale-time (that is, long lived graceful restart knob set for the BGP restarter).

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