zebra: fix removing default route at vrf enabling

When a routing table (RT) already has a default route before being
assigned to a VRF, the default route vanishes in zebra after the VRF
assignment.

> root@router:~# ip route add blackhole default table 100
> root@router:~# ip route show table 100
> blackhole default
> root@router:~# vtysh -c 'show ip route table 100'
> [...]
> VRF default table 100:
> K>* 0.0.0.0/0 [0/0] unreachable (blackhole), 00:00:05
> root@router:~# ip l add red type vrf table 100
> root@router:~# vtysh -c 'show ip route table 100'
> root@router:~#

Do not override the default route if it exists.

Signed-off-by: Louis Scalbert <[email protected]>

zebra/zebra_vrf.c

@@ -391,6 +391,11 @@ static void zebra_vrf_table_create(struct zebra_vrf *zvrf, afi_t afi,
 	memset(&p, 0, sizeof(p));
 	p.family = afi2family(afi);
 
+	if (route_node_lookup_maynull(zvrf->table[afi][safi], &p))
+		/* do not override the existing default route */
+		return;
+
+	/* create a fake default route */
 	rn = srcdest_rnode_get(zvrf->table[afi][safi], &p, NULL);
 	zebra_rib_create_dest(rn);
 }