OSPF ABR Rules

CCIE Routing and Switching Official Guide : Page 505

Let’s restate once again the rules regarding originating and processing type 3 LSAs on ABRs. First, when an ABR originates type 3 LSAs on behalf of known routes, it translates only intra-area routes from a nonbackbone area into type 3 LSAs and floods them into the backbone, and it translates both intra-area and inter-area routes from the backbone area into type 3 LSAs and floods them into nonbackbone areas. Second, when an ABR runs the SPF algorithm, it ignores all type 3 LSAs received over nonbackbone areas. 

For example, without this second rule, in the internetwork of Figure 9-10, router ABR2 would calculate a cost 3 path to subnet 1: from ABR2 to ABR1 inside area 1 and then from ABR1 to ABR3 in area 0. ABR2 would also calculate a cost 101 path to subnet 1, going from ABR2 through area 0 to ABR3. Clearly, the first of these two paths, with cost 3, is the least-cost path. However, ABRs use this additional loop-prevention rule, mean- ing that ABR2 ignores the type 3 LSA advertised by ABR1 for subnet 1. This behavior prevents ABR2 from choosing the path through ABR1, so in actual practice, ABR2 would find only one possible path to subnet 1: the path directly from ABR2 to ABR3.

Cost 3 path Area 0
Area 1
Figure 9-10

Cost 101 path

Area 2

Subnet 1
Cost 1
Cost 1

Cost 1
Cost 1

Cost 100
Cost 1

Effect of ABR2 Ignoring Path to Subnet 1 Through Area 1

It is important to notice that the link between ABR1 and ABR2 is squarely inside non- backbone area 1. If this link were in area 0, ABR2 would pick the best route to reach ABR3 as being ABR2 – ABR1 – ABR3, choosing the lower-cost route.
This loop-prevention rule has some even more interesting side effects for internal routers. Again in Figure 9-10, consider the routes calculated by internal Router R2 to reach subnet 1. R2 learns a type 3 LSA for subnet 1 from ABR1, with the cost listed as 2. To calculate the total cost for using ABR1 to reach subnet 1, R2 adds its cost to reach ABR1 (cost 2), totaling cost 4. Likewise, R2 learns a type 3 LSA for subnet 1 from ABR2, with cost 101.

ABR1

ABR2

R1

R2

ABR3

Key Topic

This section covers the core OSPF configuration commands, along with the OSPF con- figuration topics not already covered previously in the chapter. (If you happened to skip the earlier parts of this chapter, planning to review OSPF configuration, make sure to go back and look at the earlier examples in the chapter. These examples cover OSPF stubby area configuration, OSPF network types, plus OSPF neighbor and priority commands.)
Example 9-8 shows configuration for the routers in Figure 9-5, with the following design goals in mind:

• Proving that OSPF process IDs do not have to match on separate routers, though best practice recommends using the same process IDs across the network
  
• Using the network command to match interfaces, thereby triggering neighbor dis- covery inside network 10.0.0.0
  
• Configuring S1’s RID as 7.7.7.7
  
• Setting priorities on the backbone LAN to favor S1 and S2 to become the DR/BDR
  
• Configuring a minimal dead interval of 1 second, with hello multiplier of 4, yielding a 250-ms hello interval on the backbone LAN
  

R2 calculates its cost to reach ABR2 (cost 1) and adds that to 101 to arrive at cost 102 for this alternative route. As a result, R2 picks the route through ABR1 as the best route.
However, the story gets even more interesting with the topology shown in Figure 9-10. R2’s next-hop router for the R2 – ABR2 – ABR1 – ABR3 path is ABR2. So, R2 forwards packets destined to subnet 1 to ABR2 next. However, as noted just a few paragraphs ago, ABR2’s route to reach subnet 1 points directly to ABR3. As a result, packets sent by R2, destined to subnet 1, actually take the path from R2 – ABR2 – ABR3. As you can see, these decisions can result in arguably suboptimal routes, and even asymmetric routes, as would be the case in this particular example.f