I try to simulate the case of using downbit check to prevent routing loop in OSPF/VPN environment. In my lab, I create a vrf Gi with two ospf area: one of 14 and another of 44. Now in R2 and R3 routing table of vrf Gi, we see route 192.168.0.0 is learnt through BGP with next-hop 172.16.0.6. It is what we expected.
R2#sh ip route vrf Gi
100.0.0.0/32 is subnetted, 1 subnets
O 100.100.100.100 [110/11] via 10.0.0.5, 00:59:13, Ethernet1/0
10.0.0.0/30 is subnetted, 2 subnets
O 10.0.0.0 [110/20] via 10.0.0.5, 00:59:13, Ethernet1/0
C 10.0.0.4 is directly connected, Ethernet1/0
192.168.0.0/30 is subnetted, 1 subnets
B 192.168.0.0 [200/0] via 172.16.0.6, 01:19:43
But when I try to ping from R2 to address 192.168.0.2, loop seems to happen. Packets go to R3, but after that R3 forwards them back to R1 while according to the theory, it should forward to R4 through BGP
R2#traceroute vrf Gi ip 192.168.0.2
1 10.0.0.2 [MPLS: Label 18 Exp 0] 64 msec 68 msec 52 msec
2 10.0.0.1 128 msec 44 msec 48 msec
3 10.0.0.2 128 msec 80 msec 128 msec
4 192.168.0.1 144 msec 108 msec 268 msec
5 192.168.0.2 216 msec * 208 msec
R2 routing table and ospf database both show correct result. So we don’t understand why although downbit is set in R2, R3, routing loop still happens?
R3#sh ip route vrf Gi 192.168.0.2
Routing entry for 192.168.0.0/30
Known via "bgp 65535", distance 200, metric 0, type internal
Redistributing via ospf 2
Advertised by ospf 2 subnets
Last update from 172.16.0.6 01:26:16 ago
Routing Descriptor Blocks:
* 172.16.0.6 (Default-IP-Routing-Table), from 172.16.0.6, 01:26:16 ago
Route metric is 0, traffic share count is 1
AS Hops 0
R3#sh ip ospf 2 data sum
OSPF Router with ID (10.0.0.2) (Process ID 2)
Summary Net Link States (Area 14)
LS age: 1279
Options: (No TOS-capability, DC, Downward)
LS Type: Summary Links(Network)
Link State ID: 192.168.0.0 (summary Network Number)
Advertising Router: 10.0.0.2
LS Seq Number: 80000003
Checksum: 0xDD69
Length: 28
Network Mask: /30
TOS: 0 Metric: 1
LS age: 293
Options: (No TOS-capability, DC, Downward)
LS Type: Summary Links(Network)
Link State ID: 192.168.0.0 (summary Network Number)
Advertising Router: 10.0.0.6
LS Seq Number: 80000003
Checksum: 0xC57D
Length: 28
Network Mask: /30
TOS: 0 Metric: 1
