5

enter image description here

Hi,

I wanted to configure Multi area OSPF between these routers. I created test simulation on packet tracer.

Can any one help me how I can do that because I am bit confused about the configuration in Mesh topology.

In single area all things and fail-overs also working fine, but when I tried to configure multi-area network, I am getting confused that where I configure which area. Because every router is connected to each other.

Please help me..


Edit:

Is this work

@John, If I not put my complete thing in Area 0, this could be the possible topology? Can I achieve my goal with this?

  • Nishad, with only 5 routers why not put them all in area 0? Do you have some kind of requirement to split them into different areas? – John K. Jan 5 '17 at 14:05
  • All are not located in same locations some of them are in USA and other are in india but in different locations. and yes we have some kind of requirement to split them into different areas. we have around 8-10 routers. – Nishad Morey Jan 5 '17 at 14:18
  • Ok, but keep in mind that traffic between different areas always travels through area 0 which essentially creates a hub and spoke topology, not a mesh topology. – John K. Jan 5 '17 at 14:26
  • Thanks John, like in the above diagram every router is directly connected to area 0 router so I have confusion because we have more then 6 links are connected to each router so apart from area 0 how I can configure other routers? – Nishad Morey Jan 5 '17 at 14:31
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    I just ran this in a simulator and your new topology won't work. Only the LSA received from area 0 will be installed as routes. Although the other areas will have the LSA in their database, they will not be installed as routes. So the redundant paths will not be used. – John K. Jan 5 '17 at 18:51
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The area design you have specified will not work. The interface and its adjacent interface need to be in the same area. You can't have one interface in area0 and the adjacent router interface in area1.

One of the conditions to create a successful relationship with a neighbour is that the area id must match, which it would never do in your design.

See the picture below which might help you understand it better.

enter image description here

As you can see that the interface and its adjacent interface are both in the same area and not separate. This will allow a neighbour relationship to be established.

  • In that case if I connect 2651-1 & 2651-2 then it will work.??? – Nishad Morey Jan 5 '17 at 12:31
  • If you want to create a neighbour relationship between those two routers, you can place them either in area 3 or use any of the existing areas. Both sides just need to be told to be in the same area so that area-id matches on both. – SleepyMan Jan 5 '17 at 12:34
  • Because we have 8 routers need to configure in mesh. Each router is directly connected to each other. So for intercommunication I wanted to configure OSPF. If consider above case for each direct link I have to specify different area. Consider above example that I send and please guide me. – Nishad Morey Jan 5 '17 at 13:01
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    FYI - the topology in that diagram isn't valid either, unless you've got a virtual link in there somewhere. A router connected to more than one area needs to have at least one connection to area 0. – rnxrx Jan 5 '17 at 13:29
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    @Nishad Morey I think you will need to do more studying up on Multi Area OSPF as there are more consideration to take into as rnxrx mentioned, connections to Area0 as your backbone area, area border routers etc. The scope is too large to explain all of this on this site I am afraid. – SleepyMan Jan 5 '17 at 14:21
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I wasn't able to paste this picture in the comments. Usually what you want is your full mesh connections to all be in the backbone area 0. Then you split off other stub areas that are all attached to area 0. So all of your WAN links would be in area 0, then the links on the router towards the local site LAN would be in its own area.

OSPF areas

1

OSPF doesn't use a mesh topology. All OSPF areas must connect to Area 0 (backbone area) because all traffic from one are to another area must travel through Area 0. This is a loop prevention mechanism.

An OSPF router knows about everything (routes routers, costs, etc.) in its own area. Areas separate and compartmentalize this knowledge. All the router knows about other areas is that it must send traffic destined to another area to Area 0.

If, as sometimes happens, you have an area that cannot physically attach to Area 0, you must create a virtual link between that area and Area 0. Area 0 must also be contiguous, you cannot have two different Area 0s. You will need to implement a virtual link if Area 0 is split.

If you want redundant connections, then you need to have multiple connections from each area into Area 0.

The reason you cannot use a mesh with OSPF is found in RFC 2328, OSPF Version 2:

3.1. The backbone of the Autonomous System

The OSPF backbone is the special OSPF Area 0 (often written as Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP addresses). The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous. However, it need not be physically contiguous; backbone connectivity can be established/maintained through the configuration of virtual links.

Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to-point backbone network. On the graph of the backbone, two such routers are joined by arcs whose costs are the intra-area distances between the two routers. The routing protocol traffic that flows along the virtual link uses intra- area routing only.

3.2. Inter-area routing

When routing a packet between two non-backbone areas the backbone is used. The path that the packet will travel can be broken up into three contiguous pieces: an intra-area path from the source to an area border router, a backbone path between the source and destination areas, and then another intra-area path to the destination. The algorithm finds the set of such paths that have the smallest cost.

Looking at this another way, inter-area routing can be pictured as forcing a star configuration on the Autonomous System, with the backbone as hub and each of the non-backbone areas as spokes.

The topology of the backbone dictates the backbone paths used between areas. The topology of the backbone can be enhanced by adding virtual links. This gives the system administrator some control over the routes taken by inter-area traffic.

The correct area border router to use as the packet exits the source area is chosen in exactly the same way routers advertising external routes are chosen. Each area border router in an area summarizes for the area its cost to all networks external to the area. After the SPF tree is calculated for the area, routes to all inter-area destinations are calculated by examining the summaries of the area border routers.

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