Question

Two-way redistribution is highly problematic, but may sometimes be required by a network administrator. How would you implement a two-way redistribution? Explain by giving an example of a specific situation that you would want to rectify.

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Answer 1

Redistribution is a process of passing the routing information from one routing domain to another. The ultimate goal of redistribution is to provide full IP connectivity between different routing domains. Another goal (not always required, though) is to provide redundant connectivity, i.e. backup paths between routing domains. Routing domain is a set of routers running the same routing protocol. Redistribution process is performed by border routers – i.e. routers belonging to more than one routing domain. On the contrary, internal routers belong just to one routing domain. Redistribution may be one-way (from one domain to another but not vice-versa) or two-way (bi-directional). Next, internal routesare the IGP prefixes native to a routing protocol; i.e. they are originated by IGP’s natural method, and their respective subnets belong to the IGP routing domain. External routes are the IGP prefixes injected into IGP routing domain via a border router – they have no corresponding IP subnets in the routing domain. They appear to be “attached” somehow to the border router that has originated them, and detailed information about their reachability is “compressed” and lost during the redistribution. Transit routing domain is the domain used as path to transport packets between two other routing domains. Domain becomes transit when two border routers perform bi-directional redistribution with two other routing domains. Stub routing domain is configured not to transit packets (effectively by blocking transit redistribution) between two other domains.

Let’s look at a picture to clarify the concepts.

$Redistribution_1$

The routing domains on the picture are described in the following table:

Table 1

Domain	Routers
OSPF	R2,R3,R4
EIGRP 123	R1,R2,R3
RIPv2	R4,R5,BB2
EIGRP 356	R3,R5,R6

Examples of internal routers are R1, R6, BB2. The border routers on the picture are reflected in the following table:

Table 2

Protocol	OSPF	EIGRP 356	EIGRP 123	RIPv2
OSPF	N/A	R3	R2,R3	R4
EIGRP 356	R3	N/A	R3	R5
EIGRP 123	R2,R3	R3	N/A	NONE
RIPv2	R4	R5	NONE	N/A

We will further use the figure and the tables for reference. Note that each domain on Fig may be configured to be either transit or stub. For example, if we configure bi-directional redistribution on R3 between RIPv2 and OSPF and also on R2 between EIGRP 123 and OSPF, the OSPF domain will become transit point between two other domains. However, if we take R2 and R3, and configure R2 and R3 to send default routes into EIGRP 123, while redistributing EIGRP 123 into OSPF, we will make EIGRP 123 a stub domain.