Let’s continue with our discussion of techniques involving network statements and OSPF. As we previously discussed, we can get all of the interfaces running OSPF process 1 in Area 0 like this: router ospf 1 network 0.0.0.0 255.255.255.255 area 0 Another option is to assign the interfaces (or subinterfaces) to OSPF areas directly. For example, to get all of our […]

Now let’s examine network statements as used with OSPF. Refer to Figure 1: When using network statements with OSPF, the wildcard masks are required, as are area numbers. To get OSPF Area 0 running on all interfaces, we could do this: router ospf 1 network 192.168.1.1  0.0.0.0 area 0 network 172.16.1.1  0.0.0.0 area 0 network 10.1.1.1  0.0.0.0 area 0 network […]

Last time, we had just finished configuring Area 1 as an OSPF stub area. At this point, the best paths from R5’s perspective are: Subnet A via R3 (cost = 3) Subnet B via R3 (cost = 3) Subnet C via R4 (cost = 3) Subnet D via R4 (cost = 3) RIP and EIGRP clouds via defaults from R3 […]

Continuing with our discussion of OSPF summarization options, refer to the figure below… When we left off, we had reduced the number of Type-5 LSAs being injected into the OSPF cloud from two hundred to two, a summary route from each ASBR. But what if the prefixes that lie within the RIP or EIGRP clouds […]

Having discussed how OSPF works without summarization options, let’s take a look at the various summarization options and their effects. If there are 100 prefixes in the RIP cloud, and another 100 in the EIGRP cloud, each being injected into the OSPF domain by the ASBRs, each OSPF router will be tracking over 200 individual prefixes, […]

Continuing on with our detailed discussion of OSPF, let’s look at how OSPF routers handle prefixes from external routing domains. Refer to Figure 1: Let’s say that there are 100 subnets in the RIP cloud. If we configure R3 to redistribute the RIP routes into OSPF (using the “redistribute” command under OSPF), R3 will be […]

Having discussed OSPF route summarization and the various types of stub areas, let’s look at the potential drawbacks of using those features. We’ll start by reviewing how OSPF acts before any summarization features are configured. Refer to Figure 1: As you can see, R3 connects Area 0 and Area 1 together, making R3 an OSPF […]

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 […]

Carrying on with our discussion of multi-area OSPF, refer to the example topology shown in Figure 1: Recall that we’ve allocated the IP address space as follows: Area 0: 10.0.0.0/24 through 10.0.255.0/24 (256 subnets) Area 1: 10.1.0.0/24 through 10.1.255.0/24 (256 subnets) Area 2: 10.2.0.0/24 through 10.2.255.0/24 (256 subnets) By default, all routers will know about […]

In our previous discussion, we saw that dividing an OSPF autonomous system into multiple areas can improve the scalability. An example of a multi-area OSPF topology is shown in Figure 1: Let’s imagine that we’ve allocated the IP address space as follows: Area 0: 10.0.0.0/24 through 10.0.255.0/24 (256 subnets) Area 1: 10.1.0.0/24 through 10.1.255.0/24 (256 […]