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Can an OSPF virtual link span multiple routers?

I'm trying to get some specifics on the limitations for OSPF virtual links. I've reviewed RFC 2328 (OSPFv2), the Cisco OSPF Design Guide and other OSPF materials.

Can virtual links span multiple routers? RFC 2328, regarding virtual links, states "The two endpoints of a virtual link are area order routers. The virtual link must be configured in both routers."

I'm assuming that it's not possible to have other intermediary (non-stub areas) between the two ABRs? Since the transit area must be configured on both end-points, unless intermediary routers were aware of the transit area, they would discard the traffic?

The reason why I'm bringing this up is that I'm posed with what appears to me to be an OSPF design problem -- I've got two separate areas that are separated from the backbone by two devices (a router and a firewall). The firewall is participating in OSPF in the network, as well as the other router. The firewall interface that these two areas attach to (via the other router) is on a separate subnet than the backbone area. I was hoping that it would be possible to setup a transit area for the remote router to talk directly to the backbone, so that the two remote areas would be connected to the backbone area.

Any thoughts or ideas would be appreciated.

1 REPLY
Purple

Re: Can an OSPF virtual link span multiple routers?

Hi,

The two end-points of the virtual-link *must* share a common transit area. A single virtual link cannot span two areas.

However, you can string a number of virtual links together to create what I call a 'composite' virtual link. For example, consider the following setup:

area3 -- area2 -- area1 -- area 0

Given the above, you can create one VL between the area3/area2 ABR and the area2/area1 ABR. Then create another VL between the area2/area1 ABR and the area1/area0 ABR. That will work very well and ensure that every area is connected to the backbone.

Hope that helps - pls rate the post if it does.

Paresh

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