question about communication between two layer three devices

[pendal8286]

Hello all and thanks for your time and expertise.  Pardon my ignorance as I'm sure this will be an easy question for most of you but I'm building a network map of a building.  The edge device for building A has an IP address of 10.52.X.X.  The Cisco neighbor of this device is the core switch for the entire network.  This core device is in another building and it has an address of 10.255.52.X with a subnet of /24.  

My stupid question is how are these devices communicating as they appear to my novice eye to have different IP configurations.  Appreciate any insight you can provide as I just want to understand the communication process involved.

[Reza Sharifi]

Hi,

It is really hard to tell without a diagram and configuration but It could be that the edge devices are simply layer-2 and they are in a management subnet (10.52.X.X) that terminate at the core and the 10.255.52.X subnet is a user subnet (data, phone, printer, etc.) that also terminate at the core and this 2 subnets communicate at the core. So, as you can see, we can only guess and would need more info in order to help further.

HTH

[pendal8286]

Thanks Reza.  These two devices are both layer three.  The only routing we have in place, however, is static routing and these two devices are directory connected.  

In the sh cdp nei det:

The one edge device has an IP of 10.52.X.X and the Core Switch in the other building (and this Core switch is the device all of the other buildings connect to so it is the Core switch for all of the buildings) has an IP Address of 10.255.52.X.    

The 10.255.52.X/24 vlan is described as the comcast to (School) so the 10.255.52.x core switch is connected to our ISP.  

Hope this helps.

[Reza Sharifi · Accepted solution]

Hi,

Ok, got it. The core device is picking up the SVI IP address for the vlan that connects to Comcast in the "sh cdp nei" and the other device is layer-3 and the IP in the "sh cdp nei" is 10.52.X.X. which could also be an SVI IP address or management IP address

HTH

[Giuseppe Larosa]

Hello pendal8286,

a common IP subnet has to be shared between the access switch and the core switch.

The core switch has likely multiple IP addresses in different IP subnets.

If you can access the access layer switch is enough to type

show cdp neighbor detail

or simply

show ip route

the default-gateway of the access-layer switch is likely in 10.52.x.x subnet and it is also one of the IP addresses of the core switch

Hope to help

Giuseppe

[pendal8286]

Thank you.  Can you expand upon what you mean when you say a common subnet is shared between the access switch and the core.

What you said is correct.  The IP Default Gateway on the Access switch is 10.52.X.X.  The Static route on the Core switch is 10.52.X.X/16.

And then 10.255.52.0/24 is also a connected vlan on the core switch.

I just figured the access switch and the core switch and the interfaces they are using to communicate would be on a /32 subnet.  

[Joseph W. Doherty]

I would guess the core switch is a L3 switch configured to route. If so, it can forward traffic (by routing it) between networks.

[pendal8286]

The core switch is a layer three switch but we use static routing between our buildings.

[Joseph W. Doherty]

Static routing is still routing. Also with a L3 switch enabled for routing or a router, either will route between physically connected networks without the need to define static route statements or use a dynamic routing protocol. You only need to provide the L3 routing device routing destination information (via static routes and/or via a dynamic routing protocol) when the destination network is not physically connected to the device.