The summary-address command summarizes external routes as they are injected into OSPF as an ASBR. DUAL uses the Neighbor and Topology tables to calculate route information. Also, the not-advertise keyword can essentially be used to filter the subnets implied by the summary, as covered in Chapter 10, "OSPF." This Slide is related to routing protocol in detail. Also, the area range command can set the cost for the summary route, instead of using the lowest cost of all component routes. The main advantage of a link state routing protocol like OSPF is that the complete knowledge of topology allows routers to calculate routes that satisfy. First, the area range command specifies an area this area is the area in which the component subnets reside, with the summary being advertised into all other areas. The commands have a couple of important attributes. Routes with lower costs are preferred over routes with higher costs. 325 OSPF LSA Type 5 (6.1.2.6) 326 OSPF Routing Table and Types of Routes (6.1.3) 326 OSPF Routing Table Entries (6.1.3.1) 327 OSPF Route Calculation. The SPF algorithm uses this value to calculate the cost for each route.
To edit the following settings, click Edit: OSPF Metric Set the metric in the router’s link state advertisement. All routers use the same logic to calculate the shortest path for each network. Table 11-9 OSPF Route Summarization Commands KEY POINT Default Route Distribution: The default route distribution settings. OSPF executes Dijkstra’s shortest path first (SPF) algorithm to create a loop-free topology of shortest paths. Route summarization helps solve two major problems: Large routing tables Frequent LSA flooding throughout the autonomous system Every time that a route disappears in one area, routers in other areas also get involved in shortest-path calculation. Both commands are configured under router ospf. Route summarization is a key to scalability in OSPF. OSPF uses two different configuration commands to create the summary routes, depending on whether the summary is for inter-area or external routes. To make that happen, OSPF allows route summarization only as routes are injected into an area, either by an ABR (inter-area routes) or by an ASBR (external routes). As a result, all routers in the same OSPF area must have the same summary routes, and must be missing the same component subnets of each summary. All OSPF routers in the same area must have identical LSDBs after flooding is complete.
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