Redistribution Using Default Settings
The first example configuration meets the following design goals:
■ R1 redistributes between each pair of IGPs—RIP, EIGRP, and OSPF.
■ Default metrics are used whenever possible; when required, the metrics are configured on the redistribute command.
■ Redistribution into OSPF uses the non-default subnets parameter, which causes subnets to be advertised into OSPF.
■ All other settings use default values. Example 11-1 shows R1’s configuration for each routing protocol, along with show commands from all four routers to highlight the results of the redistribution.
Example 11-1 Route Redistribution with Minimal Options
Metrics must be set via configuration when redistributing into RIP and EIGRP, whereas OSPF uses default values. In the example, the two redistribute commands under router rip used hop counts of 2 and 3 just so the metrics could be easily seen in the show ip route command output on R3. The EIGRP metric in the redistribute command must include all five metric components, even if the last three are ignored by EIGRP’s metric calculation (as they are by default). The command redistribute rip metric 1544 5 255 1 1500 lists EIGRP metric components of bandwidth, delay, reliability, load, and MTU, in order. OSPF defaults to cost 20 when redistributing from an IGP, and 1 when redistributing from BGP. The redistribute command redistributes only routes in that router’s current IP routing table. When redistributing from a given routing protocol, the redistribute command takes routes listed in the IP routing table as being learned from that routing protocol. Interestingly, the redistribute command can also pick up connected routes. For example, R1 has an OSPF route to 15.1.2.0/24, and a connected route to 15.1.1.0/24. However, R3 (RIP) and R4 (EIGRP) redistribute both of these routes—the OSPF-learned route and one connected route—as a result of their respective redistribute ospf commands. As it turns out, the redistribute command causes the router to use the following logic to choose which routes to redistribute from a particular IGP protocol:
1. Take all routes in my routing table that were learned by the routing protocol from which routes are being redistributed.
2. Take all connected subnets matched by that routing protocol’s network commands. Example 11-1 shows several instances of exactly how this two-part logic works. For instance, R3 (RIP) learns about connected subnet 14.1.1.0/24, because RIP redistributes from EIGRP, and R1’s EIGRP network 14.0.0.0 command matches that subnet.
The redistribute command includes a subnets option, but only OSPF needs to use it. By default, when redistributing into OSPF, OSPF redistributes only routes for classful networks, ignoring subnets. By including the subnets option, OSPF redistributes subnets as well. The other IGPs redistribute subnets automatically; however, if at a network boundary, the RIP or EIGRP autosummary setting would still cause summarization to use the classful network. In Example 11-1, if either RIP or EIGRP had used auto-summary, each redistributed network would show just the classful networks. For example, if RIP had configured auto-summary in Example 11-1, R3 would have a route to networks 14.0.0.0/8 and 15.0.0.0/8, but no routes to subnets inside those class A networks.
■ R1 redistributes between each pair of IGPs—RIP, EIGRP, and OSPF.
■ Default metrics are used whenever possible; when required, the metrics are configured on the redistribute command.
■ Redistribution into OSPF uses the non-default subnets parameter, which causes subnets to be advertised into OSPF.
■ All other settings use default values. Example 11-1 shows R1’s configuration for each routing protocol, along with show commands from all four routers to highlight the results of the redistribution.
Example 11-1 Route Redistribution with Minimal Options
Metrics must be set via configuration when redistributing into RIP and EIGRP, whereas OSPF uses default values. In the example, the two redistribute commands under router rip used hop counts of 2 and 3 just so the metrics could be easily seen in the show ip route command output on R3. The EIGRP metric in the redistribute command must include all five metric components, even if the last three are ignored by EIGRP’s metric calculation (as they are by default). The command redistribute rip metric 1544 5 255 1 1500 lists EIGRP metric components of bandwidth, delay, reliability, load, and MTU, in order. OSPF defaults to cost 20 when redistributing from an IGP, and 1 when redistributing from BGP. The redistribute command redistributes only routes in that router’s current IP routing table. When redistributing from a given routing protocol, the redistribute command takes routes listed in the IP routing table as being learned from that routing protocol. Interestingly, the redistribute command can also pick up connected routes. For example, R1 has an OSPF route to 15.1.2.0/24, and a connected route to 15.1.1.0/24. However, R3 (RIP) and R4 (EIGRP) redistribute both of these routes—the OSPF-learned route and one connected route—as a result of their respective redistribute ospf commands. As it turns out, the redistribute command causes the router to use the following logic to choose which routes to redistribute from a particular IGP protocol:
1. Take all routes in my routing table that were learned by the routing protocol from which routes are being redistributed.
2. Take all connected subnets matched by that routing protocol’s network commands. Example 11-1 shows several instances of exactly how this two-part logic works. For instance, R3 (RIP) learns about connected subnet 14.1.1.0/24, because RIP redistributes from EIGRP, and R1’s EIGRP network 14.0.0.0 command matches that subnet.
The redistribute command includes a subnets option, but only OSPF needs to use it. By default, when redistributing into OSPF, OSPF redistributes only routes for classful networks, ignoring subnets. By including the subnets option, OSPF redistributes subnets as well. The other IGPs redistribute subnets automatically; however, if at a network boundary, the RIP or EIGRP autosummary setting would still cause summarization to use the classful network. In Example 11-1, if either RIP or EIGRP had used auto-summary, each redistributed network would show just the classful networks. For example, if RIP had configured auto-summary in Example 11-1, R3 would have a route to networks 14.0.0.0/8 and 15.0.0.0/8, but no routes to subnets inside those class A networks.
