Interface goes down FRR holds back on the static kernel routes

[maisheri]

When an interface goes down / brought down with 4.x Linux based kernel, the static default routes associated with the respective interfaces are taken down from the kernel, while FRR / Zebra (8.3.1) holds back the static default routes.

Following are the steps followed:

1. Static default route configured
2. Interface administratively down
3. Check the kernel routes, kernel cleans up the kernel routes.
4. Check routes in FRR, FRR/Zebra holds the default routes.

Curious to know the primary reason to holding back on to the default route entry in FRR/Zebra, when kernel route was deleted?

Also, if there would be any knob to make sure the kernel routes and FRR routes are synced at regular intervals without restarting FRR ?

Step 1 :

root@localhost:/# sudo ip route add default src 10.223.104.111 metric 5 nexthop via 172.31.94.40 dev p0 weight 1 nexthop via 172.31.94.168 dev p1 weight 1
root@localhost:/# sudo ip route add 10.223.104.154 src 10.223.104.111 metric 5 nexthop via 172.31.94.40 dev p0 weight 1 nexthop via 172.31.94.168 dev p1 weight 1

root@localhost:/# ip r | grep default
default src 10.223.104.111 metric 5
root@localhost:/# vtysh -c "show ip route" | grep -A 3 0.0.0.0
K>* 0.0.0.0/0 [0/5] via 172.31.94.40, p0, src 10.223.104.111, weight 1, 00:00:38
 *         via 172.31.94.168, p1, src 10.223.104.111, weight 1, 00:00:38
B  0.0.0.0/0 [20/10] via 172.31.94.40, p0, weight 1, 00:09:58
           via 172.31.94.168, p1, weight 1, 00:09:58
root@localhost:/# ip r | grep 10.223.104.154
10.223.104.154 src 10.223.104.111 metric 5
root@localhost:/# vtysh -c "show ip route" | grep -A 3 10.223.104.154
K>* 10.223.104.154/32 [0/5] via 172.31.94.40, p0, src 10.223.104.111, weight 1, 00:00:50
 *             via 172.31.94.168, p1, src 10.223.104.111, weight 1, 00:00:50
B  10.223.104.154/32 [20/0] via 172.31.94.40, p0, weight 1, 00:10:18
               via 172.31.94.168, p1, weight 1, 00:10:18

Step 2:

root@localhost:/# ip link set p0 down
root@localhost:/# ip link set p1 down

Step 3:

Kernel cleans up the associated route.
root@localhost:/# ip r | grep default
root@localhost:/#

Step 4:

root@localhost:/# vtysh -c "show ip route" | grep -A 3 0.0.0.0
K * 0.0.0.0/0 [0/5] via 172.31.94.40, p0 inactive, src 10.223.104.111, weight 1, 00:01:57
 *         via 172.31.94.168, p1 inactive, src 10.223.104.111, weight 1, 00:01:57
root@localhost:/# ip r | grep 10.223.104.154
root@localhost:/# vtysh -c "show ip route" | grep -A 3 10.223.104.154
K * 10.223.104.154/32 [0/5] via 172.31.94.40, p0 inactive, src 10.223.104.111, weight 1, 00:02:18
 *             via 172.31.94.168, p1 inactive, src 10.223.104.111, weight 1, 00:02:18

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