Need advise for preventing loops in 2 vlans.

[wenqi]

Hi All,

I have a setup of 1 router(cellular) connected to 2 switches. These 2 switches had 1 port connected to router via trunk and another port on the 2 switches connected to each switch via trunk.

The rationale is so that if either of the connection between switch to switch or switch to router is down, there will be a redundant link up to router. This router will connect via cellular to my office.

I need advise on how to configure my routing? Current setup have 2 vlans. vlan1 take up first 12 ports of switch, vlan2 takes up rest of 10 ports of switch and last 2 port is for to router and another switch. I would like to route traffic from switch to router, and router to cellular. My main concern is how can I configure the switch and router so that my office will be able to communicate to these 2 vlans stably without causing any loops?

[wenqi]

Adding a sample network diagram for reference.

[Leo Laohoo]

Because the router handles inter-VLAN routing, if the router is down, no inter-VLAN traffic is going to happen.  

So the redundant link between the two switches are pretty much useless.

Looks and sounds nice on paper but there is a fine line between theories and what happens "outside".

[wenqi]

Hi Leo,

Thank you for your reply. I do not necessarily need inter-vlan routing. I would just need inter-vlan routing when router is up.

[wenqi]

Sorry what I meant is I don't need inter-vlan routing if router is down. My main goal is to see how to get both vlan able to be reachable  and stable from office with such setup if router is up.

[Leo Laohoo]

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@wenqi wrote:

Sorry what I meant is I don't need inter-vlan routing if router is down. My main goal is to see how to get both vlan able to be reachable  and stable from office with such setup if router is up.

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If the router is down, inter-VLAN will stop working.  
If the router is down, only same VLAN will work.  If, for example, the DHCP server is in a separate VLAN and the PCs are in a different VLAN, nothing is going to get a valid IP address.  

If the router is down, internet access is dead. 

Is that what you are trying to accomplish?

[wenqi]

Hi Leo,

My main goal is from office, I need to be able to reach the 2 vlans behind the switches. If any of the link between router or switch is down, it has another path to router and back to office.

If the router is down, I will not be accessible from office and that is understandable and I will have to go and check what's wrong.

Main goal here is for stable connection from office to the 2 vlans as I am constantly monitoring devices behind these 2 vlans from office.

[Leo Laohoo]

Make sure the ports are Trunk and cost out the STP. 

[wenqi]

Thank you leo, My ports are currently trunk. For STP how should I set the cost correctly? Thanks.

[pman]

Hi,
Maybe in this case and if the router supports you can use IRB.
Attaches information:

example1:

https://www.cisco.com/c/en/us/support/docs/lan-switching/integrated-routing-bridging-irb/17054-741-10.html

example2:

https://gregsowell.com/?p=495

example3:

https://www.timigate.com/2017/03/how-to-bridge-ports-on-cisco-routers.html

example4:

https://www.cisco.com/c/en/us/support/docs/lan-switching/integrated-routing-bridging-irb/200650-Understanding-Bridge-Virtual-Interface.html