OSPF protocol Zone

One of the key reasons for the rapid convergence of o s p f is that it uses a zone. I E T F uses O S P F to achieve two main goals: to improve network scalability and fast convergence.
The key to achieving two goals is to divide the network into smaller areas. A zone is a collection of Network-end systems, routers, and transmission lines. Each zone is defined by a unique zone number, which is configured in each vro. A vro interface with the same area number is defined as an integral part of the same area. Ideally, these areas are not arbitrarily defined. On the contrary, you should select the boundary of the Zone to minimize the traffic between different zones. Each zone should reflect actual traffic patterns rather than geographical or political boundaries. Of course, this is a theoretical ideal and may not be practical in a specific environment.
The number of zones supported in the o s p f network is limited by the size of Zone I D. This field is a three-two-digit binary number. Therefore, the theoretical maximum value of a three-two-digit binary number is 1 at each position, and the corresponding decimal number is 4 294 967 295. Obviously, the actual maximum number supported is much smaller than the theoretical maximum number. In fact, the network design determines the maximum number of zones that can be supported. Figure 1 shows a fairly simple o s p f network with only three zones numbered 0, 1, and 2.
1. Router type

Note that o s p f is a link-state protocol. Therefore, the link and the router port connected to the link are defined as a zone number. Based on the zone member relationship, there are three different types of routers in the o s p f Network: Internal routers, regional border routers, and backbone routers. Figure 2 uses the network diagram shown in figure 1 to identify three different types of routers.

Figure 1 small OSPF network with three zones


Figure 2 VBR, internal router, and backbone router in OSPF

As shown in figure 2, a vro with multiple interfaces can belong to two or more zones. Such a vro becomes a VBR. That is to say, they connect their region numbers with the backbone. A backbone router has at least one interface defined as a vro0 in Zone 0. A vbr in one zone may also be a backbone router. Any VBR interconnected with Zone 0 will also become a backbone router. The internal router makes all its defined interfaces belong to the same zone, but this zone is not Zone 0. With these three basic routers, you can build an efficient and scalable o s p f network.
2. Route type

Consider the three different types of o s p f routers shown in Figure 2. Note that o s p f supports two different types of routes: intra-zone routes and intra-range routes.
Their names have obvious meanings. Intra-zone routes are self-contained and only limited to routes between routers in one zone. Use the example network shown in Figure 1 3-1, and Figure 3 shows the intra-zone communication in the o s p f network.
Interval routing requires data exchange between different zones. All the route segments must pass through Zone 0, and non-Zone 0 cannot directly communicate with other zones. This hierarchy limit ensures the o s p f has good scalability, without causing confusion between links and routers. Figure 4 shows that the proper use of Zone 0 makes inter-range communication in the o s p f network easy.

Figure 3 intra-region communication in OSPF

Figure 4 interval routing in OSPF

The previous example shows how communication in an o s p f network works at a high level. However, o s p f can also be used to exchange routing information between o s p f networks, not just between a network zone. The usage of o s p f will be discussed below.

3. routes between networks

O s p f can be used to interconnect different networks. This network can be another complete o s p f network or a network that implements completely different routing protocols. It is a complex task to interconnect the o s p f network with other different routing protocols, and a technology called route redistribution should be used. This term describes the summary and redistribution of route information from one network to another. Route information from the non-o s p f network is summarized and re-distributed to the o s p f network.
The o s p f network marks all Routes learned in this way as external routes. It is easier to interconnect two different o s p f networks, because there is no need to change the consumption of one route information into a form that can be understood by another protocol. Moreover, o s p f makes it possible to build an autonomous system. An autonomous system (a s) is an autonomous network. Literally, a s is A network administrator or A group of network administrators who use A routing protocol.
The actual definition of a s is not fixed. This actually does not matter. What really matters is that o s p f allows allocation of autonomous system numbers for a network. A very large o s p f network can be divided into two or more autonomous systems. These systems can be interconnected through the fourth type of o s p f router-autonomous system border router (autonomous system border router ASBR. A s B r summarizes all the routing information of its own a s and forwards the summary to the corresponding adjacent a s B R. The role of a s B R is very similar to that of A VBR. Obviously, the difference between the two is that they constitute the boundaries between autonomous systems rather than the boundaries of autonomous systems or networks. Figure 5 shows the use of a s B R for autonomous system interconnection.

Figure 5 interconnected OSPF Autonomous System