Redistribution is used when a router is receiving information about remote networks from various sources. Although all the networks are entered into the routing table and routing decisions will be made on all the networks present in the table, a routing protocol propagates only those networks that it learned through its own process. When there is no sharing of network information between the routing processes, this is referred to as ships in the night, or SIN, routing. Redistribution can occur only between processes routing the same Layer 3 protocol. So, for example OSPF, RIP, IGRP, and EIGRP can redistribute routing updates between them because they all support the same TCP/IP stack and share the same routing table. How one redistributes between the routing protocols differs between protocols. However, there can be no network redistribution between AppleTalk and IPX.
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