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Having completed the Networking Essentials Course from Cisco Networking Academy, I've been practicing on Packet Tracer until I came across this scenario.

Supposing we have two intranets, A and B. Intranet A has three routers (router 1,router 2 and router 3). Intranet B has three routers also (router 4, router 5 and router 6). Intranet A's router 3 connects to intranet B's router 4.

The problem however is that they are using different routing protocols. Intranet A is using RIP while intranet B is using EIGRP. Gateway of last resort has not been set on any of the routers.

What can be done to enable hosts on intranet A communicate with intranet B (through the link between router 3 and router 4)?

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  • The obvious, if tedious, answer is often static routes. This assume either network is not constantly / randomly changing their internal network configuration. (my office is just such a static island, and yes corp IT does add/change networks from time to time.)
    – Ricky
    Commented Aug 1, 2020 at 15:38

2 Answers 2

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First, make sure your IP address ranges are not overlapping between the two intranets.

Second, the connection between router 3 and router 4 should be set up, e.g., as a /31 subnet with one IP address configured on each end. Router 3 and router 4 could set each other as gateway of last resort (this is the most convenient way in this scenario, without having to import routes from the other intranet). However, this is a bad idea because then when a packet is sent to any IP address outside of intranet A and intranet B, there would be a routing loop, with packets going back and forth between router 3 and router 4 until they hit the time-to-live. So it would be better to add a static route in router 3 to cover just intranet B, and a static route in route 4 to cover just intranet A.

Third, set the gateway of last resort to router 3, for all the routers in intranet A. Set the gateway of last resort to router 4, for all the routers in intranet B. Then everything coming from intranet A with an unknown destination (actually, in intranet B) goes to router 3, which then forwards it to router 4. And vice versa in the other direction.

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    Router 3 and router 4 could set each other as gateway of last resort is a bad idea as it creates a routing loop and disables connectivity to anywhere without a specfic route, e.g. to the Internet. Instead, Router 3 and 4 should get static routes for the remote network and redistribute them to its peer routers normally.
    – Zac67 ♦
    Commented Aug 1, 2020 at 7:16
  • @Zac67 Thanks for pointing that out, have edited the answer.
    – auspicious99
    Commented Aug 1, 2020 at 7:51
  • Also, both intranets use an internal routing protocol. It'd be reasonable to inject the routes from Routers 3 and 4 into those to have them redistributed.
    – Zac67 ♦
    Commented Aug 1, 2020 at 7:57
  • @auspicious99 /31 subnet will have a block size of two. This makes it possible to have only two ip addresses. If one of them is for network address and the other for broadcast address, there'll be none for valid hosts. Can they work?
    – Kelvinskell
    Commented Aug 1, 2020 at 8:17
  • I was thinking maybe using a /30 subnet which provides a block size of 4, therfore leaving two valid host addresses to be configured on the router interfaces. Will a /31 subnet do the same job?
    – Kelvinskell
    Commented Aug 1, 2020 at 8:19
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Ensure that your network subnet is not overlapping. To ensure communication between two networks which are configured with different routing protocols as you said RIP and EIGRP

Redistribute the route of EIGRP to RIP and resdistribute route of RIP to EIGRP to ensure communication between intranet network A and intranet network B

Redistribution?

Route redistribution is a technic that ensures a network to use a routing protocol to dynamically route traffic depends on information learned from a different routing protocol. Route redistribution helps increase accessibility within networks when different routing protocols are used or configured on different networks

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