| 33.11. Deploying OSPF | ||
|---|---|---|
 | Chapter 33. OSPF - Open Shortest Path First |  |
| 33.11. Deploying OSPF | ||
|---|---|---|
| | Chapter 33. OSPF - Open Shortest Path First | |
Deploying OSPF requires careful planning and configuration at four levels:
The Autonomous System
The Area(s)
The Network(s)
The Host(s)
For the OSPF autonomous system (AS), you need to:
Designate the backbone area.
Identify the autonomous system boundary routers (ASBRs).
Subdivide the OSPF AS into areas that can be easily summarized by using summary routes. (if need)
Assign area IDs.
Identify virtual links. (if need)
Identify area border routers (ABRs). (if need)
Identify stub areas. (if need)
Figure 1 shows a sample map of an Anonymous System. Router 5 has EGP connection to the other Anonymous System. Is it an ASBR.
Figure 2 shows a sample OSPF area configuration.
The first area consists of networks N1-N4, along with their attached routers RT1-RT4.
The second area consists of networks N6-N8, along with their attached routers RT7, RT8, RT10 and RT11.
The third area consists of networks N9-N11 and Host, along with their attached routers RT9, RT11 and RT12.
In Figure 2, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are internal routers.
Routers RT3, RT4, RT7, RT10 and RT11 are area border routers.
Finally, as before, Router RT5 is AS boundary router.
Router RT11 is a backbone router because it belongs to two areas. In order to make the backbone connected, a virtual link has been configured between Routers R10 and R11. Again, Routers RT3, RT4, RT7, RT10 and RT11 are area border routers. As Routers RT3 and RT4 did above, they have condensed the routing information of their attached areas for distribution via the backbone.
The backbone enables the exchange of summary information between area border routers. Every area border router hears the area summaries from all other area border routers. It then forms a picture of the distance to all networks outside of its area by examining the collected advertisements and adding in the backbone distance to each advertising router.
Note that a failure of the line between Routers RT6 and RT10 will cause the backbone to become disconnected. Configuring a virtual link between Routers RT7 and RT10 will give the backbone more connectivity and more resistance to such failures. Also, a virtual link between RT7 and RT10 would allow a much shorter path between the third area (containing N9) and the router RT7, which is advertising a good route to external network N12.
For each router, you need to:
Define the areas to which the router is connected.
If the area is a stub area, define the area as a stub area.
If the router is an ABR, optionally configure the ranges that summarize the IP networks within the area.
If the router is an ABR that uses a virtual link, define the virtual interface.
If the router is an ASBR, enable ASBR and configure optional external route filters.
For each IP address for each router interface that uses OSPF, you need to:
Enable OSPF on the interface.
Configure the interface for the appropriate area ID.
Configure the interface for the appropriate router priority.
Configure the interface for the appropriate link cost.
Configure the interface for the appropriate password.
Configure the interface for the appropriate network type.
If the interface is a single-adapter Frame Relay (or X.25 or ATM) interface, configure the nonbroadcast multiple access (NBMA) neighbors.
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| 33.10. OSPF Virtual links table |  | 33.12. OSPF configuration examples |
