A virtual link can connect an area to the backbone area, or another non-backbone area. In the OSPF
routing domain, all areas must connect to the backbone area. If an area disconnects from the
backbone area, a virtual link to the backbone area is required. Otherwise, the network communication
will become abnormal. The virtual link is created between two ABRs. The area that belongs to both
ABRs is called the transition area, which can never be a stub area or NSSA.
The router-id parameter indicates the ID of OSPF neighbor router and can be displayed with the show
ip ospf neighbor command. You can configure the loopback address as the router ID.
The area virtual-link command defines only the authentication key for a virtual link. You can use the
area authentication command to enable the OSPF packet authentication in areas connected over the
virtual link in routing process configuration mode.
OSPF supports the Fast Hello function.
If the Fast Hello function is enabled, the OSPF can discover neighbors and detects invalid neighbors
quickly. You can enable the OSPF Fast Hello function by specifying the keywords minimal and
hello-multiplier, and the multiplier parameter. You can set the death clock to 1 second in minimal
and hello-multiplier to a value equal to or greater than 2. In this case, the Hello packet sending
interval is less than 1 second.
The hello-interval field of a Hello packet received by a virtual link is omitted if the Fast Hello function is
enabled on the virtual link and the hello-interval field is set to 0 for Hello packets advertised from the
virtual link.
No matter the Fast Hello function is enabled or not, the values of dead-interval must be consistent on
both ends of a virtual link. The values of hello-multiplier on both ends can be different if at least one
Hello packet can be received within dead-interval. You can use the show ip ospf virtual-links
command to monitor dead-interval and hello-interval configured for a virtual link.
For the Fast Hello function, you can only configure either the dead-interval minimal hello-multiplier
parameter or the hello-interval parameter.
The following example sets area 1 as the transition area to establish virtual link with neighbor 2.2.2.2.
Ruijie(config)# router ospf 1
Ruijie(config-router)# network 172.16.0.0 0.0.15.255 area0
Ruijie(config-router)# network 172.16.17.0 0.0.15.255 area1
Ruijie(config-router)#area1 virtual-link2.2.2.2
The following example sets area 1 as the transition area to establish a virtual link with neighbor
1.1.1.1. This virtual link connects area 10 and the backbone area, and works with the OSPF packet
authentication inMD5 mode.
Ruijie(config)# routerospf1
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