Using Metrics and Metric Types to Influence Redistributed Routes
A different set of issues can occur for a router that is internal to a single routing domain, like R4and R5 in Figure 11-4. The issue is simple—with multiple redistributing routers, an internal routerlearns multiple routes to the same subnet, so it must pick the best route. As covered earlier in thechapter, the redistributing routers can set the metrics; by setting those metrics with meaningfulvalues, the internal routers can be influenced to use a particular redistribution point.
Interestingly, internal routers may not use metric as their first consideration when choosing thebest route. For instance, an OSPF internal router will first take an intra-area route over an interarearoute, regardless of their metrics. Table 11-8 lists the criteria an internal router will use whenpicking the best route, before considering the metrics of the different routes.
Table 11-8 IGP Order of Precedence for Choosing Routes Before Considering the Metric
* For E2 routes whose metric ties, OSPF also checks the cost to the advertising ASBR.
To illustrate some of these details, Example 11-7 focuses on R4 and its routes to 10.1.2.0/24 and 10.1.5.0/24 from Figure 11-4. The example shows the following, in order:
1. R1 and R3 advertise 10.1.2.0/24 as an E2 route, metric 20. R4 uses the route through R3,because R4’s cost to reach ASBR R3 is lower than its cost to reach ASBR R1.
2. After changing R1 to advertise redistributed routes into OSPF as E1 routes, R4 uses the E1routes through R1, even though the metric is larger than the E2 route through R3.
3. R4 uses it higher-metric intra-area route to 10.1.5.0/24 through R5. Then, the R4-R5 link fails,causing R4 to use the OSPF external E2 route to 10.1.5.0/24—the route that leads through theRIP domain and back into OSPF via the R3-R2-R1-R5 path.
Table 11-8 IGP Order of Precedence for Choosing Routes Before Considering the MetricIGP Order of Precedence of Metric RIP No other considerations EIGRP Internal, then externalOSPF Intra-area, inter-area, E1, then E2*IS-IS L1, L2, external* For E2 routes whose metric ties, OSPF also checks the cost to the advertising ASBR.
Example 11-7 Demonstration of the Other Decision Criteria for Choosing the Best Routes
Interestingly, internal routers may not use metric as their first consideration when choosing thebest route. For instance, an OSPF internal router will first take an intra-area route over an interarearoute, regardless of their metrics. Table 11-8 lists the criteria an internal router will use whenpicking the best route, before considering the metrics of the different routes.
Table 11-8 IGP Order of Precedence for Choosing Routes Before Considering the Metric
* For E2 routes whose metric ties, OSPF also checks the cost to the advertising ASBR.
To illustrate some of these details, Example 11-7 focuses on R4 and its routes to 10.1.2.0/24 and 10.1.5.0/24 from Figure 11-4. The example shows the following, in order:
1. R1 and R3 advertise 10.1.2.0/24 as an E2 route, metric 20. R4 uses the route through R3,because R4’s cost to reach ASBR R3 is lower than its cost to reach ASBR R1.
2. After changing R1 to advertise redistributed routes into OSPF as E1 routes, R4 uses the E1routes through R1, even though the metric is larger than the E2 route through R3.
3. R4 uses it higher-metric intra-area route to 10.1.5.0/24 through R5. Then, the R4-R5 link fails,causing R4 to use the OSPF external E2 route to 10.1.5.0/24—the route that leads through theRIP domain and back into OSPF via the R3-R2-R1-R5 path.
Table 11-8 IGP Order of Precedence for Choosing Routes Before Considering the MetricIGP Order of Precedence of Metric RIP No other considerations EIGRP Internal, then externalOSPF Intra-area, inter-area, E1, then E2*IS-IS L1, L2, external* For E2 routes whose metric ties, OSPF also checks the cost to the advertising ASBR.
Example 11-7 Demonstration of the Other Decision Criteria for Choosing the Best Routes
