Learned routes are not disappearing from from when L3VPN deleted

Topology Used : Dual Stack (N1, S1(IPv4), S2IPV6), two ports on each compute.

1. Create Router
2. Add S1 and S2 to Router and Validate FIB --> PASS
3. Create External network(neutron net-create extnetwork --router:external=True --provider:network_type gre)

1. Create L3VPN and Associate external network to L3VPN
2. Validate DCGW routes advertised in FIB. --> PASS
3. Set Router to external network for SNAT(neutron router-gateway-set R1 extnetwork --enable-snat)

1. Delete L3VPN and check FIB removes learned IPV4 routes from FIB –> FAIL