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I am currently working on a scenario to get a better understanding of the RIPv2 routing protocol but am currently stuck. Please see my picture below to get a better understanding of what I am trying to do.

I have PC0 in VLAN 10 on the far left side connected to S3, which is connected to R2 via Trunk(Router on a Stick -VLANs10,20).

On the far right side, I have set up PC5 also in VLAN 10, connected to S2 which is connected to R3 via Trunk(Router on a Stick - Vlans 10,20,30)

R3 and R2 are connected to each other via serial link.

PC0 is not able to ping PC5 on the other side of the network although RIP is configured. PC0 can ping devices that are not connected to R3 though.

I checked the simulation in Packet Tracer and figured out that R3 will drop the ARP request.

The ARP request's target IP address does not match the receiving port's IP address.
The ARP process checks the routing table whether the requested IP address is reachable.
The requested IP address is reachable on the receiving port.
The source IP address is in the ARP table. The ARP process updates the ARP table with received information.
The ARP process drops the frame.

Looking at this, I guess that since the VLAN 10 network is directly connected, ARP guesses that the requested ip - which is PC5 172.16.1.10- should be reachable over the connected link? Although PC5 is on the other side of the network?

I have configured RIPv2 on both routers and added the network statements of the VLANS aswell... My guess is, that it is not working, since when R3 advertises a route for VLAN10 for example to R2, R2 won’t 'accept' it since there is already a route for that network in its routing table with a lower hop count?

Or am I missing something? If not, what routing protocol would fit my needs?

Thank you very much for reading! If you need any other information, please let me know.

image

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  • VLANs are layer-2 constructs that do not extend across layer-3 routers. You can have two VLANs with the same number separated by routers, but they are not the same VLAN.
    – Ron Maupin ♦
    Sep 13, 2019 at 19:27

1 Answer 1

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You have the subnet 172.16.10.0/27 in two discontinuous networks (one on each router). This is an invalid configuration. You can't have a subnet divided in two places. If you were to send a packet to 172.16.10.1, how does the router know which network to route it to?

VLANs do not cross router boundaries. So VLAN 10 on one router is not the same as VLAN 10 on another router.

Each network segment (VLAN) needs to be a separate IP subnet.

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  • Wow... Today I learned... Thank you very much for your help. It makes a lot of sense now that I got this information. I have heard in several lectures that it's possible to have one host of a VLAN on one side of the network and another one on the other side. So I must have overheard it and I never even question anything. But now that it is clear, I am wondering why I thought it would be a good Idea to have one subnet on different routers...It just can't work. Thanks again Mr. Trunk and have a nice day!
    – Constantin M
    Sep 13, 2019 at 19:46

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