1.  no interface address, and  add a default route

         ip route add 0.0.0.0/0 via 1.1.1.1

vpp# show ip fib 0.0.0.0/0
ipv4-VRF:0, fib_index:0, flow hash:[src dst sport dport proto ] locks:6
0.0.0.0/0 fib:0 index:0 locks:3
src:CLI refs:1
index:18 locks:2 flags:shared,looped, uPRF-list:14 len:0 itfs:[]
index:18 pl-index:18 ipv4 weight=1 recursive: oper-flags:recursive-loop,
via 1.1.1.1 in fib:0 via-fib:14 via-dpo:[dpo-drop:0]

……

       2. add  interface address

set interface ip addr host-eth1 1.1.1.10/24

       3. but default route still can not work

vpp# show ip fib 0.0.0.0/0
ipv4-VRF:0, fib_index:0, flow hash:[src dst sport dport proto ] locks:6
0.0.0.0/0 fib:0 index:0 locks:3
src:CLI refs:1
index:17 locks:2 flags:shared,looped, uPRF-list:16 len:0 itfs:[]
index:17 pl-index:17 ipv4 weight=1 recursive: oper-flags:recursive-loop,
via 1.1.1.1 in fib:0 via-fib:14 via-dpo:[dpo-drop:0]

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when entry(1.1.1.1/32) source cover change(from 0.0.0.0/0 to 1.1.1.0/24), should clear

fib path oper-flags . then in func fib_path_contribute_urpf will update urpf.