Pros and Cons of L2 versus L3 on access switches?

Question

What are the pros and cons of running layer 2 versus layer 3 in my closet switches? In terms eliminating of spanning tree in L3 (possible pro) but then introducing a more complex routing protocol (possible con), etc... What's the main pros/cons of each solution?

Answer 1

Adding to cliffinutah's answer, you're basically considering an alternative to the traditional campus design of core, distribution and access, where the distribution block separates layers 2 and 3. The alternative you speak of is having the access switch act as a full L3 routing node, and the access to distribution L2 uplink trunks are replaced with L3 point to point routed links. So you're moving the L2/L3 demarc from the distribution to the access switches.

So the default gateway and root bridge for each VLAN is now moved to the access switch. The addressing scheme, VLAN and port specific config info stays the same. There are several advantages of using a L3 access design if your network is well suited for it:

- Improved convergence
- High availability
- Dynamic load balancing
- Simpler multicast configs
- Easier troubleshooting

Answer 2

L2 and L3 switches perform different functions. L3 switches perform fast switching of packets between networks using hardware based switching. Since it is hardware based, it operates much faster than a router would.

An L2 switch can only switch packets based on layer 2 information.

As far as spanning tree, if you're doing L3 switching, VLANs are usually involved. That means spanning tree can potentially become MORE complex, not less. But you can disable certain spanning tree functions on a managed switch provided you are certain that the ports you are disabling it on aren't going to be involved in a situation where a loop can happen, you could turn it off.

If you run a small flat network, L3 switches will be useless to you. However, if you run a campus wide network, you'll really want to have some. You don't use them in every place you need a switch, but they are needed and should be strategically placed.

Answer 3

The big concern for me would be cost difference, as it is in most decisions.