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This LSA describes all of the routers that are attached to that Ethernet segment. We call these intra-area LSAs. Now it is time for the first of our inter-area LSAs. Notice also that in this example we are describing a multi-area OSPF design that is not using any special area types like Stub or Totally Stubby areas. It is the job of this router to bring in external prefix information from another routing domain.
In order to inform routers in different areas about the existence of this special router, the Type 4 LSA is used.
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So once again, the Area Border Router is responsible for shooting this information into the next area and we have another example of an inter-area LSA. The Type 4 LSA describes this device. But what LSA is used for the actual prefixes that are coming in from the other domain? Yes, it is the Type 5 LSA.
Remember, this might change if we are using special area types. This area can act stub, but it can also bring in external prefixes from an ASBR. These prefixes are sent as Type 7 LSAs. The short answer is YES. But we do not often encounter these. E2 is useful if you do not want internal routing to determine the path. E1 is useful when internal routing should be included in path selection. When you look at the routing table by using the show ip route command, the first column indicates the source of the information. Typically, this is just the routing protocol the route was learned from.
Table It is propagated within an area. E1 includes the internal cost to the ASBR added to the external cost. E2 does not compute the internal cost; it reports only the external cost. Now that you understand the components and operation of multi-area OSPF, you should focus on some of the design implications of creating multiple areas, as described in the next section.
The major design consideration in OSPF is how to divide the areas. This is of interest because it impacts the addressing scheme for IP within the network. In designing a network, consider the resources available and make sure that none of these resources are overwhelmed, either initially or in the future. In the creation of areas, OSPF has tried to provide the means by which the network can grow without exceeding the available resources.
You must make summarization part of the initial network design and devise an addressing scheme that supports the use of summarization. With all the interarea traffic disseminated through the backbone, any reduction is beneficial.
The entire network benefits when fewer summary LSAs need to be forwarded into the backbone area. When network overhead is minimized, the network grows more easily. It is also important in a design to allow for transitions or breaks in the network. OSPF provides a device called the virtual link that allows areas disconnected from the backbone area to appear directly connected to the backbone as required.
The following sections consider these topics as they pertain to multi-area OSPF design. Although it is possible to have a router attach to more than three areas, the Cisco Technical Assistance Center TAC recommends that a greater number of areas be created only after careful consideration. The results of having more areas will vary depending on the router memory and CPU , as well as network topology and how many LSAs are generated. Various sources recommend that the number of routers per area not exceed 40 to 80; however, this is a rough guideline and not a strict rule.
Each ABR maintains a complete picture of the topology map for each area it connects. Therefore, it is not strictly the number of routers or areas that is important, but also the number of routes and the stability of the network. You must consider these issues because the number of LSAs in your network is proportional to the amount of router resources required. Major considerations pertinent to capacity planning with OSPF are.
The area type also affects convergence time. CPU resources of member routers— Smaller routers are not designed to manage large databases or to continuously run the SPF algorithm. Link speed— The higher the link speed, the less congestion within the router as it queues the packets for transmission.
To overcome the resources required to maintain a fully meshed network, Cisco suggests that a well-designed partial mesh over low-bandwidth links reduces the number of links and thus the amount of traffic and resources required. External links— If the area has external connections, is there a large number of external LSAs? If the external routes are summarized to a default route, far less memory and CPU are required.
Summarization— Do you have a hierarchical design with summarization? The greater the summarization, the smaller and fewer the LSA packets that need to be propagated. The following sections describe how to determine the appropriate number of neighbors to which a router should be connected, or the number of areas to which an ABR should be connected.
Increasing the number of neighbors increases the resources on the router that are allocated to managing those links. More importantly if there is a designated router DR , the router that performs the DR function might become overloaded if there are a lot of routers on the link.
OSPF Design over MLAG
It's advisable to select the DR through manual configuration to be the router with the most available CPU and memory on the segment. Also, be sure that the router is not selected to be the DR for more than one link. An ABR maintains a full topology table for every area to which it is connected. This not only uses a lot of memory, it also forces the router to recalculate SPF that much more often. The number of areas a router can support depends on the caliber of the router and size of the areas.
A good hierarchical design—where the maintenance of the areas is spread over a few routers—not only shares the burden, but also builds in redundancy. OSPF is valuable because it scales—scalability in a routing protocol comes from summarization. Multiple areas are a great way to limit computation and propagation of routing updates; the hierarchical approach imposed by using multiple areas allows for intelligent summarization on ABRs and ASBRs.
This section applies summarization to the design and implementation of multi-area OSPF. OSPF benefits from the hierarchical design created by using multiple areas connected to the backbone area. It is important to design the hierarchy to carefully take advantage of interarea and external summarization. The ABR is resident in both areas and holds a full topological database for each area. Networks must bend to organizational policies, however. OSPF has provided a solution called a virtual link for the unhappy occasion when a direct connection to the backbone is not possible.
If the new area cannot connect directly to the backbone area, two ABRs are set up to "bridge" the gap and recreate the connectivity. The configuration commands pass area information between ABRs in the intermediary area. The area is critical to the company, and an extra link has been configured for redundancy. Although the virtual link feature is extremely powerful, virtual links are not recommended as part of the design strategy for your network. They should be a temporary solution to a connectivity problem.
You must ensure that you observe the following when creating a virtual link:. Figure illustrates the use of a virtual link to provide a router in Area 10 connectivity to the backbone in Area 0. There are two main ways to approach the inclusion of an NBMA network:. The logic is that if the remote sites are made satellite areas, all traffic will have to traverse the NBMA, so it makes sense to make it the backbone area.
Define a hub network as Area 0 with remote sites and associated links as spoke areas. This is a good design if the satellite areas are stub areas because it means that the routing information—and network overhead—is kept to a minimum over the NBMA cloud. Depending on the design, the rest of the network might constitute one other area or multiple areas. This will depend on the size and growth expectations of the OSPF domain.
The first configuration step is to start OSPF. Many OSPF commands allow "tuning" but the following need to be defined at the startup of the process:. Identification of the area— Identify which interfaces are in which areas. Router ID— Specify the parameter used to uniquely identify the router by a single address.
OSPF Design: 3 – Fundamentals — about Networks!
To configure OSPF as the routing protocol, use the following command:. Recall that process-number is a number local to the router. It is possible to have more than one process running on a router, although this is an unusual and expensive configuration in terms of router resources. The process number does not have to be the same on every router in the area or the autonomous system.
In the interest of sanity, however, many administrators assign the same number to the routers. These are not related in any way. The process ID is simply a mechanism to allow more than one process to be configured on a router. The network command was explained in Chapter 5 in terms of identifying the interfaces that participated in the OSPF routing process. In this chapter, the network command is used to identify not only the interfaces that are sending and receiving OSPF updates, but also the area in which they reside. Defining the areas connected by an ABR is carried out with the network command.
The following is the syntax for the OSPF network command:. The area requested in the preceding syntax is the area in which the interface or interfaces configured with the network address reside. You must take care in the use of the wildcard mask. In a single-area configuration, all the interfaces are in the same area. The network commands identify only the network numbers in use.
Therefore, they can be configured to the classful network address provided by the IANA. The only reason to be more specific would be to exclude some interfaces from the OSPF domain. Figure illustrates the configuration of an ABR. Example shows two interfaces on Router A in distinct areas, where each interface lies within subnets of the same major network. The network number has been subnetted into the last octet, where you can see the granularity of the wildcard mask at work. Figure illustrates this configuration.
The network Command.
Network Engineering Stack Exchange is a question and answer site for network engineers. It only takes a minute to sign up. Since there are no connections between the DataCenter WAN routers - wasn't sure of implications of having 'two' Area0s in this manner? You could use virtual links to connect separate Area 0s in the same AS. Think about it this way, Area 0 is the backbone area.
What is the backbone of your network. That is where you put Area 0. That may be the WAN connections. It may be that you configure and Area 0 at each site, and you connect the sites with BGP, another OSPF instance, static routes, or another routing protocol, and you redistribute routes. You could also have a main site that is Area 0, and all the other sites are in different areas, with the main site WAN connections for the other areas in those areas.