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ScreenOS Cookbook by Sunil Wadhwa, Joe Kelly, Ken Draper, David Delcourt, Vik Davar, Stefan Brunner

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16.13. Configure Virtual Links

Problem

You have an area that is not attached to the backbone area, or you have a partitioned backbone area. This is an invalid OSPF topology and you need to find a workaround.

Solution

Configure a virtual link from the ABR that is not connected to the backbone to an ABR that is connected to the backbone through a transit area. Both ABRs need to be connected to the transit area. The endpoints of a virtual link are the Router IDs. Configure the virtual link on both ABRs:

	FIREWALL-B-> set vr trust-vr protocol ospf vlink area-id 2 router-id 192.168.16.1
	FIREWALL-A->set vr trust-vr protocol ospf vlink area-id 2 router-id 192.168.19.1

Explicitly permit OSPF traffic on all firewalls in the transit area:

	FIREWALL-C-> set policy global any any ospf permit

Discussion

OSPF topologies require a hierarchical design for loop avoidance (see the Introduction to this chapter). All areas are connected to a backbone area. To get from one area to another, traffic has to transit the backbone area, and there can be only one backbone area. Even if an ABR connecting two nonbackbone areas originates a Summary LSA into both areas, another ABR will not pick up those Summary LSAs and flood them into the backbone area. The area will be isolated from the backbone area.

An area has TransitCapability when an ABR, which terminates a virtual link, is in the area. A virtual link is an emulated point-to-point link between an ABR connecting two nonbackbone areas and an ABR connecting to one ...

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