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BLADEOS 6.5.2 Application Guide
BMD00220, October 2010 Chapter 20: OSPF 283
Virtual Links
Usually, all areas in an OSPF AS are physically connected to the backbone. In some cases where
this is not possible, you can use a virtual link. Virtual links are created to connect one area to the
backbone through another non-backbone area (see Figure 30 on page 274).
The area which contains a virtual link must be a transit area and have full routing information.
Virtual links cannot be configured inside a stub area or NSSA. The area type must be defined as
transit using the following command:
The virtual link must be configured on the routing devices at each endpoint of the virtual link, though
they may traverse multiple routing devices. To configure a G8124 as one endpoint of a virtual link,
use the following command:
where <link number> is a value between 1 and 3, <area index> is the OSPF area index of the
transit area, and <router ID> is the IP address of the virtual neighbor, the routing device at the
target endpoint. Another router ID is needed when configuring a virtual link in the other direction.
To provide the G8124 with a router ID, see the following section Router ID.
For a detailed configuration example on Virtual Links, see “Example 2: Virtual Links” on page 291.
Router ID
Routing devices in OSPF areas are identified by a router ID. The router ID is expressed in IP
address format. The IP address of the router ID is not required to be included in any IP interface
range or in any OSPF area.
The router ID can be configured in one of the following two ways:
Dynamically—OSPF protocol configures the lowest IP interface IP address as the router ID.
This is the default.
Statically—Use the following command to manually configure the router ID:
To modify the router ID from static to dynamic, set the router ID to 0.0.0.0, save the configura-
tion, and reboot the G8124.
To view the router ID, use the following command:
RS G8124(config-router-ospf)# area <area index> type transit
RS G8124(config-router-ospf)# area-virtual-link <link number>
neighbor-router <router ID>
RS G8124(config-router-ospf)# ip router-id <IPv4 address>
RS G8124(config-router-ospf)# show ip ospf