Cisco Campus Network Design Basics

Lessons Discussion
23

Hi Sriguruprassad,

Let’s look at just one example why you might want to use L2 between the distribution layer switches. Consider this design:

https://networklessons.com/wp-content/uploads/2015/03/distribution-l3-access-l2.png

In this design, we have VLAN 10 on both access layer switches. All links on the distribution and access layer are layer two links.

Think for a minute about spanning-tree…let’s say that the left distribution switch is the root bridge. What will be the root port on all of our switches? Something like this:

campus-l2-distribution-layer-root-ports

Now imagine we have an L3 link in between the distribution layer switches:

campus-l2-distribution-layer-root-ports-wrong

Since the link in between the distribution layer switches is now L3, the distribution layer switch on the right side will select another interface as the root port for VLAN 10.

Now imagine you configure something like VRRP…this means that all VRRP traffic from the right distribution layer switch goes through the access layer to get to the left distribution layer switch. Not a good idea :smile:

Hope this helps!

Rene

2 Likes