administrative distance values to routes learned natively through EIGRP and to routes redistributed in from other sources. By default, EIGRP natively learned routes have an administrative distance of 90, but external routes have an administrative distance of 170. For BGP, use the distance bgp command. BGP assigns different administrative distance values to routes learned through IBGP and routes learned through EBGP.
Content 5.2 Configuring and Verifying Router Redistribution 5.2.1 Configuring Redistribution Configuring route redistribution can be simple or complex, depending upon the mix of routing protocols that you want to redistribute. The commands used to enable redistribution and to assign metrics vary slightly depending upon the routing protocols being redistributed. Before configuring the exchange of routing information between routing protocols, you must understand the procedures for and requirements of each routing protocol. Redistribution must be configured correctly for each routing protocol to obtain proper results. As shown in Figure , redistribution supports all routing protocols. Additionally, static and connected routes can be redistributed to allow the routing protocol to advertise the routes without using a network statement. Exchanging routing information between routing protocols is called route redistribution. Route redistribution can be one-way or two-way. One-way routes are where one protocol receives the routes from another. Two-way routes are where both protocols receive routes from each other. Figure displays an example of two-way and one-way redistribution. Routes are redistributed into a routing protocol, which requires the redistribute command to be configured under the routing process that is to receive the routes. Before implementing redistribution, consider these points: Note
You cannot redistribute between Internetwork Packet Exchange (IPX) RIP and OSPF, because IPX RIP supports the IPX/Sequenced Packet Exchange (SPX) stack and OSPF does not. Although there are different protocol-dependent modules (PDMs) of EIGRP for IP, IPX, and AppleTalk, routes cannot be redistributed between them because each PDM supports a different protocol stack. The following generic steps apply to all routing protocol combinations; however, the commands that are used to implement these steps may vary. For configuration commands, it is important that you review the Cisco IOS documentation for the specific routing protocols that need to be redistributed. Note
In this topic, “core” and “edge” are generic terms that are used to simplify the discussion about redistribution.
  1. Locate the boundary router that requires configuration of redistribution. Selecting a single router for redistribution minimizes the likelihood of creating routing loops that are caused by feedback.
  2. Determine which routing protocol is the core or backbone protocol. Typically, this protocol is OSPF, IS-IS, or EIGRP.
  3. Determine which routing protocol is the edge or short-term (in the case of migration) protocol. Determine whether all routes from the edge protocol need to be propagated into the core. Consider methods that reduce the number of routes.
  4. Select a method for injecting the required edge protocol routes into the core. Simple redistribution using summaries at network boundaries minimizes the number of new entries in the routing table of the core routers.
When you have planned the edge-to-core redistribution, consider how to inject the core routing information into the edge protocol. Your choice depends on your network.
Content 5.2 Configuring and Verifying Router Redistribution 5.2.2 Redistributing Routes into a Classful Routing Protocol Figure displays how to configure for redistribution from OSPF process 1 into RIP. In the figure, the example uses the router rip command to access the routing process into which routes need to be redistributed. In this case, it is the RIP routing process. The example then uses the redistribute command to specify the routing protocol to be redistributed into RIP. In this case, it is the OSPF routing process number 1. Note
The default metric is infinity, except when you are redistributing a connected or static route. By default, a connected route is assigned a metric of 0, and a static route is assigned a metric of 1. However, if the static route is configured to point to the outgoing interface instead of a next-hop address, the static route is considered a connected route. The command syntax of the RIP redistribute command is listed in Figure . Figure displays the command parameters.
Content 5.2 Configuring and Verifying Router Redistribution 5.2.3 Redistributing from Classless to Classful Protocols In Figure , routes from OSPF process number 1 are being redistributed into RIP and given a seed metric of 3. Because no route type is specified, both internal and external OSPF routes are redistributed into RIP. A common problem with redistributing routes between RIP and OSPF is that RIP does not advertise routes out an interface if those routes are on the same major network but have a different mask than that particular interface. The following are two scenarios to describe this problem. OSPF Has a Longer Mask Than RIP
In Figure , router RTB is redistributing between RIP and OSPF. The OSPF domain has a different mask (longer in this case) than the RIP domain, and they are on the same major network. Therefore, RIP does not advertise routes learned from OSPF and redistributed into RIP. The subnet mask of the OSPF domain is difficult to change, so instead, add a static route in router RTB that points to the OSPF domain with a mask of 255.255.255.0, but with a next hop of null0. Then, redistribute static routes into RIP. Here is the configuration to accomplish this task: ip route 128.103.35.0 255.255.255.0 null0
router rip
redistribute static
default metric 1 This allows 128.103.35.0 to be advertised through RIP out the E2/0 interface of router RTB. However, router RTB still has more specific routes learned from OSPF in its routing table, so the best routing decisions are made. RIP Has a Longer Mask Than OSPF
In Figure , the RIP domain has a mask of 255.255.255.248, and the OSPF domain has a mask of 255.255.255.240. RIP does not advertise routes learned from OSPF and redistributed into RIP. We can add a static route in router RTB that points to the OSPF domain with a mask of 255.255.255.248. However, because this is a more specific mask than the original OSPF mask, the next hop must be an actual next hop or interface(s). Also, we need multiple static routes to cover all the addresses in the OSPF domain. This way static routes are redistributed into RIP. In the code below, the first two static routes cover the range 128.103.35.32 255.255.255.240 in the OSPF domain. The second two static routes cover the range 128.103.35.16 255.255.255.240 in the OSPF domain. And the last four static routes cover the range 128.130.35.64 255.255.255.240, which are known via two interfaces in the OSPF domain. ip route 128.103.35.32 255.255.255.248 E0/0
ip route 128.103.35.40 255.255.255.248 E0/0

ip route 128.103.35.16 255.255.255.248 E1/0
ip route 128.103.35.24 255.255.255.248 E1/0

ip route 128.103.35.64 255.255.255.248 128.103.35.34
ip route 128.103.35.64 255.255.255.248 128.103.35.18
ip route 128.103.35.72 255.255.255.248 128.103.35.34
ip route 128.103.35.72