Route redistribution is another concept that you will need to be familiar with. Under normal operation, most routing protocols follow a “ships in the night” method of operation – RIP routers talk to RIP routers, OSPF routers talk to OSPF routers, and never the twain shall meet. In other words, RIP and OSPF will not share information about the networks they are individually aware of by default. By the same token, if a router is configured for two different EIGRP autonomous systems (like EIGRP 99 and EIGRP 100) they will not share information either – they are separate and distinct processes. However, there may also be cases where you have a router connected to one network running a routing protocol like EIGRP, while also connected to another network running OSPF. By default, these two routing protocols will not exchange information. However, it is possible to make the two routing protocols share information, if that is your goal. For example, it’s possible to “inject” the routes that OSPF has learned into the EIGRP routing process, or vice versa. So, if I were to redistribute OSPF information into EIGRP, EIGRP would then pass the information learned from OSPF to all other EIGRP routers. It’s worth noting that this process is one-way by default. In other words, redistributing OSPF into EIGRP does not redistribute what EIGRP has learned into OSPF automatically – both types of redistribution would need to be explicitly configured, which ultimately gives a network administrator a higher degree of control over the process.
Some protocols do redistribute between each other automatically. The best example of this is when a router is running both IGRP and EIGRP. If both routing protocols are given the same autonomous system number (for example IGRP 1000 and EIGRP 1000), then the two will redistribute information automatically.