LAN Switching - Layer 2, Layer 3, Light Layer 3
Without going too deep in to technical terms, this factoid is designed to give an overview of basic Layer 2 and 3 LAN switching.
If you aren’t familiar with the OSI model you can research it on your own. http://en.wikipedia.org/wiki/OSI_model
Layer 2, and 3 refer to the layers of the OSI model, but it doesn’t fully explain the functional differences.
Layer 2 LAN switching (L2)
The term layer 2 switch is a switch that forwards packets based on MAC addresses. Layer 2 switching uses the (MAC address) of the host (NIC) network interface card to decide where to forward the frames. Layer 2 switching is hardware based switching.
A layer 2 switch has (ASICs) application-specific integrated circuits, used to build and maintain MAC address tables for forwarding.
One way to compare a layer 2 switch is a multiport bridge.
Layer 2 switching provides
- Hardware based bridging (MAC)
- Wire speed
- High speed
- Low latency
- Low cost
Layer 2 switches have limitations, such as the switch remains as one large broadcast domain, and only switches MAC addresses.
A layer 2 switch is a switch that forwards packets based on MAC addresses. This is often called a dumb switch as it maintains little intelligence. It simple keeps track of which MAC addresses are on which ports and forwards the packets appropriately.
It doesn’t know of an IP address.
Layer 3 LAN switching (L3) (adds intelligence)
A layer 3 switch is a switch that makes routing decisions based on Layer 3 addresses, also known as IP addresses, and simple routing protocols like RIP and IGRP. This switch can create VLANs and route VLANs as well as changing routing paths based on protocol metrics like cost (distance) and load (traffic).
Layer 3 switches have all the functionality of Layer 2 switches with the addition of full Layer 3.
Layer 3 switches can be placed anywhere in the network because they handle high-performance LAN traffic
Layer 3 switching provides
- Determine paths based on logical addressing
- Run layer 3 checksums (on header only)
- Use Time to Live (TTL)
- Process and respond to any option information
- Update Simple Network Management Protocol (SNMP) managers with Management Information Base (MIB) information
- Provide Security
The benefits of layer 3 switching include the following
- Hardware-based packet forwarding
- High-performance packet switching
- High-speed scalability
- Low latency
- Lower per-port cost
- Flow accounting
- Quality of service (QoS)
Light Layer 3 LAN Switching (LL3)
Light Layer 3 switches also make decisions based on IP addressing and static policies similar to the layer 3 switch. However the difference lies in the fact that Light Layer 3 switch does not support routing protocols, only static routing.
The Light Layer 3 switch allows for VLAN creation, VLAN routing, and IP routing based off static routes, but it cannot dynamically route packets based on dynamic metrics like load and cost.
A Light Layer 3 switch adds capabilities over a Layer 2 switch and is well suited in a VoIP environment
Simply put, a Layer 3 switch is similar to a router with the exception of (BGP) Border Gateway Protocol. Layer 3 switches support RIP and OSPF routing protocols.
VoIP applications work best running on a Light Layer 3 or Layer 3 LAN switching environment. While Layer 2 switches can be used, layer 2 switches would have to be uplink connected to a layer 3 switch. This would allow for the layer 3 switch to act as the traffic cop for the layer 2 switch. (Not the best solution, but functional)
Even in that environment, all layer 2 traffic would have to be trunked to the layer 3 switch and this is not as efficient as a LL3 or L3 switch would be. When connecting switches together in a VoIP environment (uplink), you do not want a layer 2 switch uplinked to another layer 2 switch then to a layer 3 switch. The layer 2 switch would have to directly uplink to a layer 3 switch.
RIP – Routing Information Protocol is a distance-vector routing protocol, which employs a hop count as a routing metric. (Least hops equals shortest route path)
OSPF – Open Shortest Path First is an adaptive routing protocol. It use a link state routing algorithm for IP networks and is widely used (IGP) interior gateway protocol in larger enterprise networks.
BGP - Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reach-ability among autonomous systems. BGP makes routing decisions based on path, network policies and/or rule-sets