OSPF, Part 7

Yep, we’re up to part seven … there is a lot you can do with OSPF! For this discussion, we’ll use the example topology shown in Figure 1.

From here, we can see that R2, R3 and R5 are ABRs (connecting OSPF areas), and that R5 and R6 are ASBRs (connecting the OSPF autonomous system to other routing domains). Because they are not ABRs, R1, R4 and R6 are referred to as “internal routers”. Also, because R2, R3, R4 and R5 have connections to Area 0 (the OSPF backbone), they are “backbone” routers. Note that a router can play several roles simultaneously, such as R5, which is a backbone router, an ABR, and an ASBR. When assigning a router multiple functions, take care that you don’t run it out of RAM and/or CPU.

Let’s assume that we have configured summarizations on the ABRs for routes being advertised from Area1 and Area 2 into Area 0, and on the ASBRs for routes being redistributed from RIP and EIGRP into OSPF. Instead of summarizing routes from Area 0 into Area 1 or Area 2, though, we’ll make use of another common OSPF feature, the “stub” area.

As you can see from Figure 1, the only way to get from Area 1 to the EIGRP and RIP routing domains is via R2 or R3. We can exploit this by configuring Area 1 as an OSPF stub area. The effect will be that instead of advertising external routes into Area 1, the ABRs will each advertise a default route. R1 will now know:

  • All of the individual subnets within Area 1
  • Summary routes via R2 and R3 for the other areas
  • Default routes via R2 and R3 which allow it to reach the outside world.

R1 will choose the lowest-cost paths between R2 and R3 to the other areas and the outside world, and load-share between them in case of a tie.

So far, so good, but when it comes to conserving RAM, bandwidth and CPU on R1, we can do even better by configuring Area 1 as “totally stubby”. R2 and R3 will now suppress the advertisements of inter-area routes into Area 1, with the result that R1 will now know:

  • All of the individual subnets within Area 1
  • Default routes via R2 and R3 which allow it to reach everything beyond Area 1

Note that “totally-stubby” is a Cisco term, but other vendors’ routers may have a functionally-equivalent mode.

What about Area 2? Can we make that a stub or totally-stubby area? No, because one of the rules for stub and totally-stubby areas is that they cannot contain any ASBRs. We can, however, make Area 2 a “Not-So-Stubby Area” (NSSA), which is a special type of stub area that’s allowed to contain ASBRs! The effect is that R5 (acting as an ASBR) continues to redistribute the EIGRP external routes into Area 0, but R5 (also acting as an ABR for a NSSA) advertises a default route into Area 2 in place of the external routes. Thus, R6 will know:

  • All subnets that lie within Area 2
  • The external prefixes that it’s learning via RIP
  • Summary routes via R5 for the other areas
  • A default route via R5 which allows it to reach the outside world.

We can also make Area 2 into what is variously referred to as “Totally Not-So-Stubby” or “Not-So-Totally-Stubby” (a NSSA into which no inter-area routes are advertised), and at this point R6 will know:

  • All subnets that lie within Area 2
  • The external prefixes that it’s learning via RIP
  • A default route via R5, which allows it to reach everything else

Having now configured summarizations into Area 0 from the outlying areas, Area 1 as totally-stubby, and Area 2 as totally-NSSA, we’ve done about the best that we can do with our example topology from a scalability perspective.

Next time, we’ll discuss the drawbacks of using summarizations and stub areas.

Author: Al Friebe

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