Designing Scalable OSPF Design
The ability to scale an OSPF internetwork depends on the overall network structure and addressing scheme. As outlined in the preceding sections in this section concerning network topology and route summarization, adopting a hierarchical addressing environment and a structured address assignment are the most important factors in determining the scalability of your internetwork. Network scalability is affected by operational and technical considerations.
This section discusses designing advanced routing solutions using OSPF. It describes how to obtain scale OSPF designs and what factors can influence convergence in OSPF on a large network. Upon mastering the content, you will be able to describe and use various concepts to perform advanced routing design. This ability includes being able to meet these objectives:
Factors Influencing OSPF Scalability
Scaling is determined by the utilization of three router resources: memory, CPU, and interface bandwidth. The workload that OSPF imposes on a router depends on these factors:
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The number of adjacent neighbors for any one router: OSPF floods all link-state changes to all routers in an area. Routers with many neighbors have the most work to do when link-state changes occur. In general, any one router should have no more than 60 neighbors.
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The number of adjacent routers in an area: OSPF uses a CPU-intensive algorithm. The number of calculations that must be performed given n link-state packets is proportional to n log n. As a result, the larger and more unstable the area, the greater the likelihood for performance problems associated with routing protocol recalculation. Generally, an area should have no more than 50 routers. Areas that suffer with unstable links should be smaller.
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The number of areas supported by any one router: A router must run the link-state algorithm for each link-state change that occurs for every area in which the router resides. Every ABR is in at least two areas (the backbone and one adjacent area). In general, to maximize stability, one router should not be in more than three areas.
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Designated router (DR) selection: In general, the DR and backup designated router (BDR) on a multiaccess link (for example, Ethernet) have the most OSPF work to do. It is a good idea to select routers that are not already heavily loaded with CPU-intensive activities to be the DR and BDR. In addition, it is generally not a good idea to select the same router to be the DR on many multiaccess links simultaneously.
The first and most important decision when designing an OSPF network is to determine which routers and links are to be included in the backbone area and which are to be included in each adjacent area.
Number of Adjacent Neighbors and DRs
One contribution to the OSPF workload on a router is the number of OSPF adjacent routers that it needs to communicate with.
Each OSPF adjacency represents another router whose resources are expended to support these activities:
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Exchanging hellos
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Synchronizing link-state databases
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Reliably flooding LSA changes
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Advertising the router and network LSA
Some design choices can reduce the impact of the OSPF adjacencies. Here are some recommendations:
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On LAN media, choose the most powerful routers or the router with the lightest load as the DR candidates. Set the priority of other routers to zero so they will not be DR candidates.
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When there are many branch or remote routers, spread the workload over enough peers. Practical experience suggests that IPsec VPN peers, for example, running OSPF over GRE tunnels are less stable than non-VPN peers. Volatility or amount of change and other workload need to be considered when determining how many peers a central hub router can support.
Any lab testing needs to consider typical operating conditions. Simultaneous restarts on all peers or flapping connections to all peers are the worst-case situations for OSPF.
Routing Information in the Area and Domain
The workload also depends on how much routing information there is within the area and the OSPF autonomous system. Routing information in OSPF depends on the number of routers and links to adjacent routers in an area.
There are techniques and tools to reduce this information. Stub and totally stubby areas import less information into an area about destinations outside the routing domain or the area then do normal areas. Therefore, using stub and totally stubby areas further reduces the workload on an OSPF router.
Interarea routes and costs are advertised into an area by each ABR. Totally stubby areas keep not only external routes but also this interarea information from having to be flooded into and within an area.
One way to think about Autonomous System Boundary Routers (ASBR) in OSPF is that each is in effect providing a distance vector–like list of destinations and costs. The more external prefixes and the more ASBRs there are, the more the workload for Type 5 or 7 LSAs. Stub areas keep all this information from having to be flooded within an area.
The conclusion is that area size and layout design, area types, route types, redistribution, and summarization all affect the size of the LSA database in an area.
Designing Areas
Area design can be used to reduce routing information in an area. Area design requires considering your network topology and addressing. Ideally, the network topology and addressing should be designed initially with division of areas in mind. Whereas EIGRP will tolerate more arbitrary network topologies, OSPF requires a cleaner hierarchy with a more clear backbone and area topology.
Geographic and functional boundaries should be considered in determining OSPF area placement.
As discussed previously, to improve performance minimize the routing information advertised into and out of areas. Bear in mind that anything in the LSA database must be propagated to all routers within the area. With OSPF, note that all changes to the LSA database need to be propagated; this in turn consumes bandwidth and CPU for links and routers within the area. Rapid changes or flapping only exacerbate this effect because the routers have to repeatedly propagate changes. Stub areas, totally stubby areas, and summary routes not only reduce the size of the LSA database, but they also insulate the area from external changes.
Experience shows that you should be conservative about adding routers to the backbone area 0. The first time people do an OSPF design, they end up with almost everything in area 0. Some organizations find that over time, too many routers ended up in area 0. A recommended practice is to put only the essential backbone and ABRs into area 0.
Some general advice about OSPF design is this:
Area Size: How Many Routers in an Area?
Cisco experience suggests that the number of adjacent neighbors has more impact than the total number of routers in the area. In addition, the biggest consideration is the amount of information that has to be flooded within the area. Therefore, one network might have, for example, 200 WAN routers with one Fast Ethernet subnet in one area. Another might have fewer routers and more subnets.
It is a good idea to keep the OSPF router LSAs under the IP maximum transmission unit (MTU) size. When the MTU is exceeded, the result is IP fragmentation. IP fragmentation is, at best, a less-efficient way to transmit information and requires extra router processing. A large number of router LSAs also implies that there are many interfaces (and perhaps neighbors). This is an indirect indication that the area may have become too large.
Stability and redundancy are the most important criteria for the backbone. Stability is increased by keeping the size of the backbone reasonable.
| Note | As a general rule, each area, including the backbone, should contain no more than 50 routers. |
If link quality is high and the number of routes is small, the number of routers can be increased. Redundancy is important in the backbone to prevent partition when a link fails. Good backbones are designed so that no that single link failure can cause a partition.
Current ISP experience and Cisco testing suggest that it is unwise to have more than about 300 routers in OSPF backbone area 0, depending on all the other complexity factors that have been discussed.
| Note | This number is intended as an appropriate indication that an OSPF design is getting into trouble and should be reconsidered, focusing on a smaller area 0. |
OSPF Hierarchy
OSPF requires two levels of hierarchy in your network (see Figure 3-13).
Route summarization is extremely desirable for a reliable and scalable OSPF network. Summarization in OSPF naturally fits at area boundaries, when there is a backbone area 0 and areas off the backbone, with one or a few routers interconnecting the other areas to area 0. If you want three levels of hierarchy for a large network, BGP can be used to interconnect different OSPF routing domains.
One difficult question in OSPF design is whether distribution or core routers should be ABRs. General design advice is to separate complexity from complexity, and put complex parts of the network into separate areas. A part of the network might be considered complex when it has a lot of routing information, such as a full-mesh, a large hub-and-spoke, or a highly redundant topology such as a redundant campus or data center.
ABRs provide opportunities to support route summarization or create stub or totally stubby areas. A structured IP addressing scheme needs to align with the areas for effective route summarization. One of the simplest ways to allocate addresses in OSPF is to assign a separate network number for each area.
Stub areas cannot distinguish among ABRs for destinations external to the OSPF domain (redistributed routes). Unless the ABRs are geographically far apart, this should not matter. Totally stubby areas cannot distinguish one ABR from another, in terms of the best route to destinations outside the area. Unless the ABRs are geographically far apart, this should not matter.
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