ospf virtual-link

Unanswered Question
Jan 3rd, 2008

We have disconntigious non-backbone area.

Can we configure virtual-link through backbone area

I have this problem too.
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nordick26 Thu, 01/03/2008 - 09:49

Hi Vaishali,

Virtual link is used mainly as a temporary solution in case you have lost you connectivity to backbone area from other than backbone area.

Different areas can comunicate between each other only throught area 0 (backbone), so in case you've lost connectivity between lets say area 1 and 0, you can temporarily solve with virtual links throught non backbone area, if (in our example) area 1 has a possibility to connect to area 2. Area 2 is directly connected to area 0 and will privide direct (virtual) connection for area 1 to area 0.

Does this make sence?

Reffer to:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9ee.shtml

HTH

Regards

Ivan

[pls rate all helpful posts]

bvsnarayana03 Thu, 01/03/2008 - 10:07

Yes, you are right. You need to create virtual-link to connect discontigous area to backbone.

Here is the command:

area "transit area No." virtual-link "opposite router-id"

This has to be applied on both end routers.

hope that helps.

pls rate all helpful posts.

Richard Burts Thu, 01/03/2008 - 10:41

vaishali

I am not clear what you mean when you say that you have discontiguous non-backbone areas. Perhaps you can clarify this. Once I understand what the issue is I will be better able to answer whether virtual link can be used to address it.

If the backbone is discontiguous that is a problem and virtual link is the common way to fix it. But if the discontiguous area is non-backbone it is less clear whether you need virtual links. In many instances a non-backbone area can be discontiguous and does not need virtual links to fix it. (discontiguous may not be a problem for non-backbone areas)

HTH

Rick

dodgerfan78 Thu, 01/03/2008 - 19:17

i don't think you need virtual link. as long as areas connect to backbone it should be fine. i have seen multiple area 1's connected to area 0 without issue but it was a simple network. it should not be recommended at all. why dont you just use a different area #?

ariesc_33 Thu, 01/03/2008 - 20:00

Found this info and could be a solution but has to support by your MPLS provider.

EIGRP Connectivity Between VPN Client Sites over a Service Provider Backbone

In Figure 1, the EIGRP routes in Site 1 are carried through the BGP core network as iBGP routes. The EIGRP routes in "Site 1" and "Site 2" are converted to iBGP routes and EIGRP extended community attributes are appended to the iBGP routes. (See Table 1 for a description of these attributes.) The EIGRP extended community attributes are appended to the EIGRP routes when they are redistributed into BGP as iBGP routes, and VPN routing information is redistributed between the PE routers by multiprotocol BGP.

The routes that originate in "Site 1" travel to the PE router that is connected to the CE router in "Site 2" of the VPN and are then converted back to EIGRP routes using the EIGRP extended community attributes. EIGRP routes are treated the same in "Site 1" and "Site 2." If the route is internal in "Site 1", it will be internal in "Site 2", and if the route is external in "Site 1", it will be external in "Site 2." All EIGRP metrics are preserved, and EIGRP metric information, along with the autonomous system, tag, and external data, is carried across the VPN over the BGP core network.

http://www.cisc.com/en/US/products/sw/iosswrel/ps1829/products_feature_guide09186a008010ead2.html#wp1043433

ditjohns Tue, 11/02/2010 - 05:47

I have query related to this post. I am just learning OSPF help me to understand more clearly.... If multiple area 1's connected to area 0 do we have multiple ABR ? or Whether ABR selection will happen according to router id ?

Richard Burts Tue, 11/02/2010 - 10:49

Ditta

It is a fundamental concept in OSPF that every non-backbone (non area 0) area must connect to the backbone area 0 through at least one ABR. So if you have an OSPF network with an area 0 and you have 4 other areas, and each of the other areas is configured as area 1, then there will be 4 area 1s and there will be at least 4 ABRs since each non-backbone area must connect to the backbone through its own ABR.

I do not understand your question about ABR selection and router ID. Perhaps you can clarify your question?

HTH

Rick

ditjohns Wed, 11/03/2010 - 01:51

Hi Rick,

     Thanks a lot for the explanation !

Let me put some more light on my query. How ospf will work, if we have one area say area 2 which is connected to area 0 by two routers say router A and router B. Whether both A and B will act as ABR or do we have any election process here ? How the routes are exchanged with back bone area in this situation.

Thanks,

Ditta

Richard Burts Wed, 11/03/2010 - 04:37

Ditta

If area 2 is connected to the backbone by A and by B then both A and B will be ABR. There is no election process and they are both ABR.

If there are multiple routers within area 2 then each router chooses the ABR that has the least cost path. So some routers are closer to A and will use A to get to the backbone while other routers are closer to B and will use B to get to the backbone.

Let us look a bit closer into what is an ABR and what it does. In basic terms a router is an ABR when it has interfaces in two or more areas and one of those areas is the backbone. One important function of the ABR is to take LSA type 1 and LSA type 2 from an area (perhaps area 2) and to generate LSA type 3 (representing the data in the type 1 and 2) for the other area (perhaps area 0).

So in your example area 2 is connected by A and by B. Both A and B will function as ABR and will take type 1 and type 2 from area 2 and generate type 3 for the backbone. Note that this means that the backbone will have two copies of the type 3 LSA. So some devices in the backbone will choose A as the best way to get into area 2 and some other devices in the backbone will choose B as the best way to get into area 2.

HTH

Rick

ditjohns Wed, 11/03/2010 - 05:44

Hi Rick,

     Thank you so much ! Actually I was bit confused how it will process the LSA 3 if we have multiple ABRs, your answer really give the clarity.

Thanks,

Ditta

Richard Burts Wed, 11/03/2010 - 05:46

Ditta

I am glad that my answer was helpful. The forum is an excellent place to learn more about Cisco networking and to expand knowledge. I encourage you to continue your participation in the forum.

HTH

Rick

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