Connecting a layer 3 switch to a router over ISP

[skylarfriden]

Good morning. I have a Cisco C3650 (layer3 switch) that I want to connect with fiber to a Cisco 2951. I have the same sfps on both sides and they green up. However the fiber connection will be going through our ISP. They have layer 2 devices in-between (Adtran). So they either need to know my Vlan for tagging purposes or I can do 802.1q. I'm not sure what I need to do. Currently they connect over a T1. What is the best approach to get these to talk? BVI?

[Carlos Villagran]

Hi!

They are probably using dot1q Tunneling, they are going to change your tag to another tag in their own ISP network.

You will have a Vlan in your switch where everything from your local Vlans are going to be routed to.

Therefore for simplicity, I would say 802.1q (like a trunk between your remote switches).

Hope it helps, best regards!

JC

[Rajeev Sharma]

Hey,

I guess you need not to worry being customer and let ISP figure out this. Additionally I believe you are just extending your layer 2 broadcast domain so you need not to worry.

Regards,

RS.

[skylarfriden]

I guess what I'm looking for is

What should the config be on my layer 3 switch sfp gig interface?  

What should the config on the router gig interface be?

Currently no Vlan exist that links them. Over the T1 link they are talking Eigrp with an ip on both sides.

-Thanks

[Rajeev Sharma · Accepted solution]

Hey,

There can be two approaches:

1. Configure the switch Gig interface as trunk and subinterfaces on the router. This configuration is called router on a stick. E.g. http://www.firewall.cx/cisco-technical-knowledgebase/cisco-routers/336-cisco-router-8021q-router-stick.html

2. Configure same subnet IP on the switch and router interfaces and the default gateway on the switch should points to router IP. Intervlan routing is handled by switch while router will be used for traffic destined for outer world.

HTH.

Regards,

RS.