I had a philosophical discussion with a contractor the other day about his desire to use L3 routed links between the core and access. At present, we're using L2 VLAN trunks between the core and access.
Some concerns I have with his argument are:
* We're used to using L2 VLAN trunks
* The L2 design is fairly simple
* The end users are not "sensitive" enough to feel a failover of links from one core switch to another when a trunk fails (i.e. STP topology changes)
* The configuration of the access layer switches is fairly easy
What has made me think about L3 links between the core and access layers are the potential fast convergence (is OSPF convergence faster than STP at layer 2?), and that our current network design is approaching 3+ years old. The downside is that we would have to start using /31 blocks of IP addresses, rather than have unnumbered trunks.
One concern of mine is probably a bit dated now. We have VoIP being slowly rolled out (read over months...) within the building. My thought was that the VoIP traffic over L3 routed links would have some latency added, whereas L2 trunks is purely switched.
With new equipment going into the building as part of the rollout of VoIP, maybe my (dated) thought is moot - if we use CEF, would latency continue to be a factor?
What are your thoughts on L2 VLAN trunks vs L3 routed links between the core and access layers?
Rebecca, I hope you're sitting down because this is gonna be long! :-)
Seriously, though, there is a lot that can be said about this, so let me start by saying that the trend is to migrate to a routed access layer, especially if you have a collapsed backbone, which it seems you have, since you make no mention of a distribution layer.
The reason for migrating to a routed access layer is to enjoy the benfits of L3 isolation and minimizing the reach of the switched domain. This minimizes the possibility of suffering from a L2 spanning-tree loop while maintaining fast convergence through L3 switching and deploying dynamic routing protocols, like OSPF and EIGRP, with adjusted timers and stub router configurations.
Moreover, in a collapsed core, it is definitely desirable to keep it a L3/routed layer and prevent the possibility that a spanning tree loop will actually adversely effect your core.
A routed access layer may not be feasible in a server farm because of L2 adjacency requirements.
There is a lot to take into consideration. Here are two excellent links that you will definitely benefit from: