For example, suppose you have two routers (cost 10 and 80, respectively) advertising the same external route, which one do you take? OSPF will determine the link metric going to those external networks. In this case of 10 and 80, because 10 is lower, that is the route that will be chosen. But what if the cost was equal? Then OSPF will use the internal metric to determine the lowest cost, thus breaking the tie.
Controlling Inter-Area Traffic
When an area has only a single ABR, all traffic that does not belong in the area will be sent to the ABR. In areas that have multiple ABRs, two choices are available for traffic that needs to leave the area:
Generally, the latter behavior is desirable because the backbone typically has higher bandwidth lines available. Also, the faster packets get there, the quicker they can be routed to their destination. However, if you want the traffic to use the ABR that is nearest the destination (so that traffic leaves the area as late as possible), the ABRs should inject route summaries into the area instead of just injecting the default route.
Most network designers prefer to avoid asymmetric routing (that is, using a different path for packets that are going from A to B than for those packets that are going from B to A.) It is important to understand how routing occurs between areas so you can avoid asymmetric routing if at all possible.
Routes that are generated from within an area (the destination belongs to the area) are called intra-area routes. These routes are represented by the letter O in the IP routing table. Routes that originate from other areas are called inter-area routes or summary routes. The notation for these routes is O IA in the IP routing table.
Load Balancing in OSPF Internetworks
As part of your design, you will need to consider the traffic flow across the network and whether or not to use load balancing. The use of this OSPF feature can be very helpful to your networks overall health. This section discusses how to best utilize the OSPF load balancing feature with a network.
In routing, load balancing is the capability of a router to distribute traffic over all its network ports that are the same distance from the destination address. Good load-balancing algorithms use both line speed and reliability information. Load balancing increases the utilization of network segments, thus increasing effective network bandwidth.
Internetwork topologies are typically designed to provide redundant routes in order to prevent a partitioned network. Redundancy is also useful to provide additional bandwidth for high traffic areas. If equal-cost paths between nodes exist, Cisco routers automatically load balance in an OSPF environment.
Cisco routers can use up to four equal-cost paths for a given destination. Packets might be distributed either on a per-destination (when fast-switching) or a per-packet basis. Per-destination load balancing is the default behavior. Per-packet load balancing can be enabled by turning off fast-switching using the no ip route-cache interface configuration command.
OSPF IP Addressing & Route Summarization
IP address assignment and route summarization are inextricably linked when designing OSPF networks. To create a scalable OSPF network, you should implement route summarization. To create an environment capable of supporting route summarization, you must implement an effective hierarchical addressing scheme. The addressing structure that you implement can have a profound impact on the performance and scalability of your OSPF network. The ultimate goal is to put as few routes as possible into the routing tables and reduce the number of updates.
Figure 5-13 illustrates the benefits of route summarization on a routing table. Without summarization, only three entries exist in the routing table, and with summarization, only one entry exists in the routing table.
IP Addressing & Route Summarization Design Golden Rules
When planning your OSPF network, consider the following golden rules of design for IP addressing and route summarization:
OSPF Route Summarization Techniques
Route summarization is particularly important in an OSPF environment because it increases the stability and efficiency of the network. Summarizing is the consolidation of multiple routes into one single advertisement. This is normally done at the boundaries of ABRs or ASBRs. Although summarization could be configured between any two areas, it is better to summarize in the direction of the backbone. This way the backbone receives all the aggregate addresses and, in turn, will inject them, already summarized, into other areas. If route summarization is being used, routes within an area that change do not need to be changed in the backbone or in other areas. There are two types of summarization: