OSPF: Difference between revisions
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# router ospf 1 |
# router ospf 1 |
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# area 2 nssa no-summary |
# area 2 nssa no-summary |
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== Backbone Area == |
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;Why do we use Backbone Area? |
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Source: [https://networkengineering.stackexchange.com/questions/6329/why-must-all-areas-connect-to-an-area-0-backbone-in-ospf stackexchange.com] |
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The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the "counting to infinity" problem. |
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OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, |
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and limiting ABRs' SPF calculation to consider only Summary-LSAs in the backbone area's link-state database. |
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*Simple example of OSPF's distance-vector behavior: |
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<-- Area 5 --><-- Area 0 --><-- Area 4 --> |
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R5-----------R1-----------R2------------R3---------------------R4 |
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Cost 3 Cost 5 Cost 7 Cost 12 |
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LSA--> LSA--> |
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Type3 LSA Type3 LSA |
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{From R1} {From R2} |
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R5 cost is 3 R5 cost is 8 |
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*Consider what happens to a /32 Loopback route for R5: |
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R5 sends a Type1 LSA containing the /32 Loopback |
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R1 (Area 5 ABR), is connected to Area 0; it translates the Type1 LSA into a Type3 LSA with a cost of 3. |
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R2 (Area 4 ABR) receives R1's Type3 LSA (metric 3) and changes the metric to R5's Loopback, based on R2's cost to R1. |
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Now R2's Type3 LSA for R5 has a cost of 8. This is the distance-vector behavior. |
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*Requiring all non-backbone routes to go through the backbone is a loop-prevention mechanism. |
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*Connecting non-backbone OSPF areas at an ABR: |
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If 2 areas aren't connected through area 0 (discontiguous), how does OSPF behaving as a link state protocol increase the possibility of routing loops? |
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As we saw above, OSPF uses distance-vector behavior to send routes through the Area 0 backbone. |
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Distance-vector protocols have well-known limits, such as the count-to-infinity problem. |
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OSPF would be vulnerable to the same issues, if we didn't have boundaries on its behavior. |
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;Can we use OSPF without area 0? |
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There is only one way to use OSPF without an Area 0, and that is to use OSPF with a single area. |
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If you only have one OSPF area, you can number it any way you like, but if you have even two areas, you must have an Area 0. |
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