OSPF Virtual Link

The backbone area is a critical component of the OSPF routing domain. It’s designated as Area 0 and serves as the core area through which all other OSPF areas must connect. The backbone area interconnects all other OSPF areas, facilitating the exchange of routing information between them. There can be situations where it is not possible to connect an area to the backbone and this is where a virtual link comes in. An example topology is below where we need to connect area 67 to the backbone and the configuration us pretty straightforward.

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/virtual-link-edit-protocols-ospf.html

vMX1

set interfaces ge-0/0/0 unit 0 family inet address 1.1.1.1/30
set interfaces lo0 unit 0 family inet address 8.8.8.8/32
set protocols ospf area 0.0.0.0 interface ge-0/0/0.0
set protocols ospf area 0.0.0.0 interface lo0.0 passive

vMX2

set interfaces ge-0/0/0 unit 0 family inet address 1.1.1.2/30
set interfaces ge-0/0/1 unit 0 family inet address 2.2.2.1/30
set interfaces lo0 unit 0 family inet address 9.9.9.9/32
set protocols ospf area 0.0.0.0 virtual-link neighbor-id 11.11.11.11 transit-area 0.0.0.52
set protocols ospf area 0.0.0.0 interface ge-0/0/0.0
set protocols ospf area 0.0.0.0 interface lo0.0 passive
set protocols ospf area 0.0.0.52 interface ge-0/0/1.0

vMX3

set interfaces ge-0/0/0 unit 0 family inet address 2.2.2.2/30
set interfaces ge-0/0/1 unit 0 family inet address 3.3.3.1/30
set interfaces lo0 unit 0 family inet address 10.10.10.10/32
set protocols ospf area 0.0.0.52 interface ge-0/0/0.0
set protocols ospf area 0.0.0.52 interface lo0.0 passive
set protocols ospf area 0.0.0.52 interface ge-0/0/1.0

vMX4

set interfaces ge-0/0/0 unit 0 family inet address 3.3.3.2/30
set interfaces ge-0/0/1 unit 0 family inet address 4.4.4.1/30
set interfaces lo0 unit 0 family inet address 11.11.11.11/32
set protocols ospf area 0.0.0.52 interface ge-0/0/0.0
set protocols ospf area 0.0.0.52 interface lo0.0 passive
set protocols ospf area 0.0.0.67 interface ge-0/0/1.0
set protocols ospf area 0.0.0.0 virtual-link neighbor-id 9.9.9.9 transit-area 0.0.0.52

vMX5

set interfaces ge-0/0/0 unit 0 family inet address 4.4.4.2/30
set interfaces lo0 unit 0 family inet address 12.12.12.12/32
set protocols ospf area 0.0.0.67 interface ge-0/0/0.0
set protocols ospf area 0.0.0.67 interface lo0.0 passive

The virtual link will now be up and area 67 will now have routes to the backbone.