I. The reason for redistribution is because each protocol has different route advertisement rules.
A. RIP, EIGRP, OSPF, and BGP use different rules so routes will not advertise from one protocol to another protocol without redistribution.
II. Redistribution occurs in the routing table (RIB) with the “show ip route” output. It does not occur in the database.
III. When a route is redistributed from one protocol into another, the routes that are redistributed are all :
A. The routes in the routing table running that protocol.
B. The connected interfaces running that protocol.
IV. Connected Redistribution.
A. It implicitly occurs for connected links running the redistributed protocol.
- Not true for IPv6.
B. Additional connected links can be explicitly included or excluded
- It overrides implicit redistribution
- The command is “redistribute connected [metric] [route-map]
V. How IOS chooses paths.
A. The routing database chooses one or more candidate paths.
- Eigrp uses DUAL
- OSPF uses SPF
- etc.
- Within a protocol, load balancing is possible via the maximum-paths command
B. If there are multiple equal matches for the same route between protocols, IOS chooses the protocol with the lower administrative distance
- The default administrative distances are:
- 0-Connected
- 1-Static
- 5-EIGRP Summary
- 20-External BGP
- 90-Internal EIGRP
- 110-OSPF
- 115-IS-IS
- 120-RIP
- 160-ODR
- 170-External EIGRP
- 200-Internal BGP
- 250-NHRP
- 255-Infinite
2. The protocol installs the results into the RIB and/or the FIB.
VI. Same route, same protocol, in two different Autonomous Systems, or Process IDs.
A. EIGRP uses the lowest autonomous system, or AS.
- Installs the route in the RIB and/or FIB
B. OSPF uses the lowest process ID, or PID.
- Installs the route in the RIB and/or FIB
Ken Hagen CCIE #56460 