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OSPF question

Hi all,

I have one OSPF area 2. This area has two ABRs. Both are advertising the same summary network. These two ABRs are connected via a T1. However, if this T1 fails, the two ABRs are disconnected and two identical areas are created. Now, no one can get to area 2 because both ABRs are advertising the same summary network. However, if I take out the "area range" command from both ABRs, everyone can get to these sites. How is it possible? I think "area range" command is how the ABR tells other areas about its area. Without "area range" command, ABR advertises every single networks it knows in its area? Using what mechanism? How do I prevent this to happen? Do I need more circuit to connect between ABRs? Thanks you.

1 ACCEPTED SOLUTION

Accepted Solutions

Re: OSPF question

Hello,

an ABR will by default insert every network with the exact mask from area 2 into the backbone area 0 and vice versa. The important thing is that the information about other area´s networks is inserted as OSPF LSA type 3. By using a different type than LSA 1 (router LSA) all routers within the area will know that those networks are found somewhere behind the ABR but are not directly connected to the ABR.

This is the default behaviour and nothing to worry about.

The area range command allows you now to suppress the specific networks and summarize them.

Now in case your T1 goes down and this splits your area 2 in two parts, then let each ABR attached to "his half" announce the networks for "his half" only.

The problem arises if a router claims to have networks reachable it doesn´t really have.

Lets assume the following scenario: N1-R1 - R2 - R3-N3

And lets assume R1 has network N1 = 10.1.0.0/16 attached. R3 should have network N3 10.3.0.0/16 attached. Everything will be reachable in case the exact routes will be announced.

Once I write a summary route on R1 and R3 which sy 10.0.0.0/8 is reachable through me things go wrong. Whom should router R2 believe? Loadsharing (f.e. every second packet) will not help either. The underlying reason is that because of the "area range" both routers R1 and R3 claim to have the address range of the other one as well.

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Martin

3 REPLIES

Re: OSPF question

Hello,

an ABR will by default insert every network with the exact mask from area 2 into the backbone area 0 and vice versa. The important thing is that the information about other area´s networks is inserted as OSPF LSA type 3. By using a different type than LSA 1 (router LSA) all routers within the area will know that those networks are found somewhere behind the ABR but are not directly connected to the ABR.

This is the default behaviour and nothing to worry about.

The area range command allows you now to suppress the specific networks and summarize them.

Now in case your T1 goes down and this splits your area 2 in two parts, then let each ABR attached to "his half" announce the networks for "his half" only.

The problem arises if a router claims to have networks reachable it doesn´t really have.

Lets assume the following scenario: N1-R1 - R2 - R3-N3

And lets assume R1 has network N1 = 10.1.0.0/16 attached. R3 should have network N3 10.3.0.0/16 attached. Everything will be reachable in case the exact routes will be announced.

Once I write a summary route on R1 and R3 which sy 10.0.0.0/8 is reachable through me things go wrong. Whom should router R2 believe? Loadsharing (f.e. every second packet) will not help either. The underlying reason is that because of the "area range" both routers R1 and R3 claim to have the address range of the other one as well.

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Martin

New Member

Re: OSPF question

Martin,

So what do I do? Stop using "area range" command for this kind of area?

Re: OSPF question

Now in case you really have the split after the T1 is down then pay attention to your IP addressing so that f.e. behind one ABR you will find 10.1.0.0/16 and behind the other you find 10.3.0.0/16. Then let them each announce the proper address range found behind it into area 0.

Second option is to remove all area range statements.

Whatever you do, keep in mind that no ABR router should announce the address space behind the other ABR in case the T1 is down. otherwise the scenario in the previous post will occur.

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Martin

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