Historically, LAN switching typically involves Layer 2 switching at the Access Layer and sometimes at the Distribution Layer. Layer 2 switches only forward information based on the MAC address (the Layer 2 frame address). Layer 3 switching, however, will use the MAC address but also adds the Layer 3 address (e.g., an IP address).
Three options exist when designing a switched environment:
- Layer 2 switching throughout the network
- A combination of Layer 2 and Layer 3 switching; this requires a higher degree of planning
- Layer 3 switching throughout the network
When trying to decide between Layer 2 and Layer 3 switching, you must understand the impact this decision has on five main areas, which is illustrated in Figure 4.22 below:
- How the policies are implemented
- How intelligent the load sharing accomplished is
- How network failures are dealt with
- Convergence issues
- The cost factor
Figure 4.22 – Layer 2 vs. Layer 3 Switching
| Note: Using Layer 2 switching, Layer 3 switching, or a combination of both also depends on the switching platforms available. Not all switches support Layer 3 technologies. Layer 3 switches are also called multilayer switches or routing switches.
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At the heart of switched networking is the concept of virtual LANs (VLANs). This basically represents the process of creating logical broadcast domains by grouping particular nodes attached to different switches within a switched environment.
When designing a full Layer 2 environment using VLANs, a router might be used to provide routing between VLANs. This technique is called “router on a stick” because only one router interface is used, which carries all the VLANs.
The advantage of having a combination of Layer 2 and Layer 3 switches at the Distribution Layer or Core Layer, or Layer 3 switches throughout the network, is that a routing process or router switch module is built into the switch itself.
When using Layer 2 switches and VLANs exclusively throughout the network, all the policies, access control lists, and QoS rules are managed at the Data Link Layer. The policy capabilities are very limited at the Data Link Layer but they are greatly enhanced in Layer 3 switches.
Another area in which Layer 2 switches are limited is when considering load sharing capabilities used to ensure redundant links (i.e., multiple paths) throughout the network. This is because Layer 2 switches only know about MAC addresses. They cannot perform intelligent load sharing, for example, based on the destination network like Layer 3 switches that support dynamic routing protocols can. With Layer 2 switching, the load can only be shared on a per-VLAN basis.
In addition, with only Layer 2 switches, the basis of all failures or the failure domain will be isolated to the VLAN only. On the other hand, in a multilayer environment, the failures can be better isolated to the Access Layer, to the Core Layer, and even to particular network segments.
Convergence and loop control are offered only by the Spanning Tree Protocol (STP) in a Layer 2-only switched environment. However, when using Layer 3 switching, this feature can also be implemented at the Distribution and Core Layers using routing protocol technologies (e.g., OSPF, EIGRP, and others).
When considering the cost, using Layer 2 switches everywhere is the cheapest solution but this is also the least flexible and manageable option. Using Layer 3 switches throughout the network is the most expensive option but it is very powerful and flexible. A compromise would be to implement Layer 3 switches first only in the Distribution Layer, and then eventually, as the budget allows and the network scales, extend the Layer 3 switches into the Core Layer, which will give you full Layer 3 switching at the Distribution and Core Layers.