I would like use ECMP for 0.0.0.0/0. This route announced on 10.10.97.33, 10.10.97.72, 10.10.97.73 as LSA5. Below I have given the output of the command on the router 10.10.97.67.
I expected to see a default route through 2 interfaces C3 to D1, but next hops not merge.
Maybe I misunderstand the work "merge external on". I expected that the bird would combine routes from different routers for the same destination although OSPF.metric1 or OSPF.metric2.

bird> show route for 10.10.97.73 table igpcoret4 all

Table igpcoret4:

10.10.97.73/32 unicast [igpcore4 20:05:45] * E2 (150/310/10000) [10.10.97.73]

via 10.112.0.14 on C3 weight 40

via 10.112.0.28 on D1 weight 40

Type: OSPF-E2 univ

OSPF.metric1: 310

OSPF.metric2: 10000

OSPF.tag: 0x00000000

OSPF.router_id: 10.10.97.73

bird> show route for 10.10.97.72 table igpcoret4 all

Table igpcoret4:

10.10.97.72/32 unicast [igpcore4 20:05:45] * E2 (150/2300/10000) [10.10.97.72]

via 10.112.0.14 on C3

Type: OSPF-E2 univ

OSPF.metric1: 2300

OSPF.metric2: 10000

OSPF.tag: 0x00000000

OSPF.router_id: 10.10.97.72

bird> show route for 10.10.97.33 table igpcoret4 all

Table igpcoret4:

10.10.97.33/32 unicast [igpcore4 20:05:45] * E2 (150/300/10000) [10.10.97.33]

via 10.112.0.14 on C3

Type: OSPF-E2 univ

OSPF.metric1: 300

OSPF.metric2: 10000

OSPF.tag: 0x00000000

OSPF.router_id: 10.10.97.33

bird> show route for 0.0.0.0/24 table igpcoret4 all

Table igpcoret4:

0.0.0.0/0 unicast [igpcore4 20:05:45] * E2 (150/300/10000) [10.10.97.33]

via 10.112.0.14 on C3

Type: OSPF-E2 univ

OSPF.metric1: 300

OSPF.metric2: 10000

OSPF.tag: 0x00000000

OSPF.router_id: 10.10.97.33

bird>

> show ospf state all igpcore4

area 0.0.0.0

router 10.10.97.33

distance 300

router 10.10.97.72 metric 2000

router 10.10.97.73 metric 10

router 10.10.97.67 metric 300

router 10.10.97.66 metric 300

router 10.10.97.68 metric 65535

stubnet 10.112.0.0/31 metric 2000

stubnet 10.112.0.2/31 metric 2000

stubnet 10.112.0.4/31 metric 2000

stubnet 10.112.0.6/31 metric 2000

stubnet 10.112.0.12/31 metric 10

stubnet 10.112.0.14/31 metric 300

stubnet 10.112.0.18/31 metric 300

stubnet 10.112.0.22/31 metric 65535

external 0.0.0.0/0 metric2 10000

external 10.112.0.0/31 metric2 10000

external 10.112.0.2/31 metric2 10000

external 10.112.0.4/31 metric2 10000

external 10.112.0.6/31 metric2 10000

external 10.112.0.12/31 metric2 10000

external 10.112.0.14/31 metric2 10000

external 10.112.0.18/31 metric2 10000

external 10.112.0.22/31 metric2 10000

external 87.245.238.204/31 metric2 10000

external 10.10.8.0/21 metric2 10000

external 10.10.97.33/32 metric2 10000

router 10.10.97.66

distance 10

router 10.10.97.73 metric 300

router 10.10.97.33 metric 300

router 10.10.97.69 metric 65535

router 10.10.97.67 metric 10

stubnet 10.112.0.16/31 metric 300

stubnet 10.112.0.18/31 metric 300

stubnet 10.112.0.24/31 metric 65535

stubnet 10.112.0.28/31 metric 10

external 10.112.0.16/31 metric2 10000

external 10.112.0.18/31 metric2 10000

external 10.112.0.24/31 metric2 10000

external 10.112.0.28/31 metric2 10000

external 10.10.0.0/21 metric2 10000

external 10.10.97.66/32 metric2 10000

router 10.10.97.67

distance 0

router 10.10.97.33 metric 300

router 10.10.97.66 metric 10

router 10.10.97.69 metric 65535

stubnet 10.112.0.14/31 metric 300

stubnet 10.112.0.28/31 metric 10

stubnet 10.112.0.26/31 metric 65535

external 10.112.0.14/31 metric2 10000

external 10.112.0.26/31 metric2 10000

external 10.112.0.28/31 metric2 10000

external 10.10.0.0/21 metric2 10000

external 10.10.97.67/32 metric2 10000

router 10.10.97.68

distance 65835

router 10.10.97.73 metric 65535

router 10.10.97.33 metric 65535

stubnet 10.112.0.20/31 metric 65535

stubnet 10.112.0.22/31 metric 65535

external 10.112.0.20/31 metric2 10000

external 10.112.0.22/31 metric2 10000

external 10.10.97.68/32 metric2 10000

router 10.10.97.69

distance 65535

router 10.10.97.66 metric 65535

router 10.10.97.67 metric 65535

stubnet 10.112.0.24/31 metric 65535

stubnet 10.112.0.26/31 metric 65535

external 10.112.0.24/31 metric2 10000

external 10.112.0.26/31 metric2 10000

external 10.10.97.69/32 metric2 10000

router 10.10.97.72

distance 2300

router 10.10.97.33 metric 2000

router 10.10.97.73 metric 2000

stubnet 10.112.0.0/31 metric 2000

stubnet 10.112.0.2/31 metric 2000

stubnet 10.112.0.4/31 metric 2000

stubnet 10.112.0.6/31 metric 2000

stubnet 10.112.0.8/31 metric 2000

stubnet 10.112.0.10/31 metric 2000

external 0.0.0.0/0 metric2 10000

external 10.112.0.0/31 metric2 10000

external 10.112.0.2/31 metric2 10000

external 10.112.0.4/31 metric2 10000

external 10.112.0.6/31 metric2 10000

external 10.112.0.8/31 metric2 10000

external 10.112.0.10/31 metric2 10000

external 10.10.97.72/32 metric2 10000

external 100.105.99.0/30 metric2 10000

router 10.10.97.73

distance 310

router 10.10.97.72 metric 2000

router 10.10.97.33 metric 10

router 10.10.97.66 metric 300

router 10.10.97.68 metric 65535

stubnet 10.112.0.8/31 metric 2000

stubnet 10.112.0.10/31 metric 2000

stubnet 10.112.0.12/31 metric 10

stubnet 10.112.0.16/31 metric 300

stubnet 10.112.0.20/31 metric 65535

external 0.0.0.0/0 metric2 10000

external 10.112.0.8/31 metric2 10000

external 10.112.0.10/31 metric2 10000

external 10.112.0.12/31 metric2 10000

external 10.112.0.16/31 metric2 10000

external 10.112.0.20/31 metric2 10000

external 10.10.8.0/21 metric2 10000

external 10.10.97.73/32 metric2 10000

external 188.170.160.52/30 metric2 10000