Cisco Campus Network Design Basics

[lagapidis (Lazarus Agapidis)]

Hello Marcel

@marcel.schmid2

• One VLAN spanning both access switches = Use L2 between the distribution switches. STP would avoid a loop, FHRP communication can be sent/received on the L2 link between the distribution switches.
• One VLAN confining to each access switch = Use L3 between both distribution switches. No STP needed, FHRP communication would go through the access switches.

Yes this is correct.

@marcel.schmid2

Is there a reason for why not also using a L3 link between the distribution switches in scenario 1)?

Yes there is. If you want to span a single VLAN across two access switches, then you must achieve Layer 2 connectivity between those switches.

Remember, you would typically have one IP subnet assigned to each VLAN. So if you have VLAN 10 on both access switches, and you have two hosts, one on each access switch with IP addresses 192.168.10.10/24 and 192.168.10.11/24, then these hosts must communicate with each other on Layer 2. In other words, those two devices must be on the same broadcast domain. If you have configured Layer 3 connectivity between the access switches and the distribution switch, then you are splitting up that broadcast domain, and you are requiring routing to take place between them. But they’re on the same subnet! So you see, from a design standpoint, this won’t work.

The VLAN assignments, IP address subnets, and the access and distribution network design must line up so that layer 2 connectivity is available between all devices that exist on the same subnet/VLAN/network segment. Does that make sense?

I hope this has been helpful!

Laz