How to Connect 2 Routers and 3 Switches Correctly?

July 2 2014 , Written by Cisco & Cisco Router, Network Switch Published on #Networking

How to connect 2 routers and 3 switches correctly? It’s a common question for users to set or reset their network. What’s your experience and suggestions about this? One of Cisco champions called Hamza shared his problem of connecting 2 routers and 3 switches. And more Cisco champions discussed it together. Let’s check it. Any idea? Welcome…

The problem is: “I know how to connect a router with two switches and they're able to do successful communication. But If I extend the scenario to two routers and three switches, they don't communicate…

Consider the scenario details in the image

The thing is router 1 is unable to communicate with the third network. Similarly third network is unable to communicate with the first one.”

Adam Loveless: My guess would be that you need to configure some routing. Please either post up your sanitized configs or the Packer Tracer file.

Kev Santillan: Hello, you need some form of routing. A router will make its decision based on the information that it has in its own routing table. If Router 1 or Router 0 does not have a route to each remote network, then you will not be able to establish communication. Either use static routes or a routing protocol.

I also noticed that you have the same address (172.16.1.1) for both routers. Is this just a typo?

…Hamza uploaded the pkt file

Attachments:

Test.pkt.zip (13.3 K)

Kev Santillan (again): The reason why the routes are not learned by each node is because you have assigned a duplicate address (172.16.1.1) to the interfaces that are in the same segment. Change the address of one of the routers to any other address within the 172.16.1.0 /24 subnet and things will work as expected.

Hamza: So basically they should be of the same class in order to work but not the same id i.e the gateway?

Chandan Singh Takuli: Router conencts 2 different networks of any class or classless. If you know a & i know a too why would i ask you. same happens in routers too.

Every router must have all of its interface in different networks and every port must have a unique ip in a single topology or network. so that it can be identified accurately and reached.

Gateway is just like a door of a room. a single network/subnet is like a room. So when you wanna go to another network or room, you need to go through gateway or door in the room.

Gateway ip must be a part/ip of the source network/subnet.

More than this problem

Rick raised his question: to Kev and Chandan: I tried that out-change one of the router interfaces from 172.16.1.1 to 172.16.1.4. But ping from far left hosts still can't reach far right hosts. And for the two hosts in the middle, what default gateway can they use?

Kev Santillan: Hi Rick, ‘I tried that out-change one of the router interfaces from 172.16.1.1 to 172.16.1.4. But ping from far left hosts still can't reach far right hosts.’ I believe modifying either of the addresses in Hamza's file should make things work immediately. Note that he is using /16 for the 172.16.X.X network. Check the mask that you have specified. Otherwise, please share the PT file.

‘And for the two hosts in the middle, what default gateway can they use?’ They can use either of the routers' addresses. If the "middle LAN" uses Router 2's address as the DG and tries to reach non-local networks, it will always pass through Router 2 first before being routed elsewhere. You can also use HSRP to have one dedicated gateway address but the logic will be the same. The active router will be the one to route traffic accordingly.

What’s your idea? Share here…

Discussion from https://learningnetwork.cisco.com/thread/72202?tstart=0

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