What exactly do OSPF virtual links do?
I've set up a lab on GNS where R4 and R6 have subnets in area 0 (172.16.x.0/24), and area 1 is the backbone across R5 (10.10.10.x/30).
I was suprised to find that R4 still learned about routes on R6 without a virtual link. I thought only area 0 could be used as a transit network?
I then configured a virtual link between R4 and R6 to see what the effect was on the routing table.
I noticed that the only change appeared to be that R4 treated R6's networks as intra area routes (O) instead of inter area routes (O-IA), but there was no effect on the cost of the route (still 21).
So all that virtual links do is allow routers to exchange type 1 LSAs when two routers use the same area ID, but the areas have no direct connection.
The virtual link is not used for routing network traffic. Network traffic is still routed through two hops?
Is that right.
Virtual links are used to connect non-backbone connected areas to the backbone via a transit area, or as in this case, connect two area 0s that are disconnected.
I'm not sure what you mean by Area 1 is the backbone though. Please explain further...
From the JPEG you can see that networks in the orange zones are
configured as area 1, and networks in blue zones were both configured in area 0. So I have successfully used area 1 as a backbone network.
Mike, isn't the described behaviour perfectly normal? Your setup may be a bit abnormal, but in effect the two routers in area 0 are intra-area routers - sharing the same area. The fact that they are separated by router 5 is in my opinion irrelevant. I haven't tried this kind of setup myself but believe that having the virtual link in the middle shouldn't change a thing. I presume that you have auto-summary switched off? Otherwise you could have fun seeing the routes between R4-R6.
Configs attached. The set up is three c2961 as follows
R4 f0/0-------f0/0 R5 f0/1---------f0/0 R6
The problem lies with backbone area being area 0. Even if you use area 1 to connect the 2 areas, area 1 will not be the backbone. It will be area 1 connected to the backbone through two disconnected ABRs...This is not ideal, and in fact is a serious design problem.
So what happens if you have no virtual link and add additional routers in area 0 off of R4 and R6? By design, ABRs will not inject a summary LSA into the backbone area for interarea routes. This would be a problem. The other area 0 routers would not know about the routes of the other area 0 routers.
What if your network was larger than two areas, and connected to additional routers in area 0?
I agree with Del and Brian virtual link is used to connect routers from deffernt areas other than the backbone 0 for example if you wanna connect ABR router in area 34 and othe ABR router in area 60 u have to configure virtual link between the Abr and the backbone router that is in are 0 I hope that will answer the Question.
Area 1 can not be a backbone area. That is reserved for area 0. Every area has to be directly connected to area 0, the backbone area, and every area has to be continious, not segmented. In this example, area 0, the backbone area, is segmented, so you would use virtual links to connect the segmented area 0's together.
In other examples if you have:
Area 2 --> Area 1 --> Area 0
You would use virtual links to connect area 2 directly to area 0.
Hi thanks all,
I've gone and done a bit of reading and built a few different topologies, and can see that the routes don't appear when you build areas without connecting them to area 0 because the router realises it is an illeagal configuration.
So to ask my question again with a different perspective 'What exaclty do virtual links do?' Do they simply allow LSAs to transit through non-backbone areas so that "far away" areas can learn that routes exist throughg a non-backbone area, or do virtual links create a tunnel (like a GRE tunnel) through which all traffic goes?
In OSPF, everything flows through the backbone area, hence the name backbone. So if an area is separated from this area, the domain will not have all of the topology information.
Virtual-links create a tunnel so that neighborships can form between routers, and the topology data can be exchanged appropriately. In OSPF there are two primary scenarios where we use virtual-links:
1. To connect two disconnected backbone areas.
2. To connect an area that has no connection to area 0.
Ahhhh... THIS would explain a strange conversation and whiteboard moment I had with someone last week.
I didn't realize there was a question out here... I guess I've been away too long!
Anyway, in the diagram you've provided, other than connecting the discontiguous area 0's, your VL wouldn't actually accomplish anything. Once a route becomes a summary (type 3), there's no area information. The only thing is that type 3 LSAs don't EXIT regular areas, just the backbone.
So if you expanded your diagram with other routers and other areas outside of each area 0, THEN you'd see a more noticeable difference in life with and without the VL there.