Layer 3 design

 Layer 3 design 
5.1.6 This page will describe some Layer 3 design considerations.
A router is a Layer 3 device and is considered one of the most powerful devices in the network topology.
Layer 3 devices can be used to create unique LAN segments. Layer 3 devices allow communication between segments based on Layer 3 addresses, such as IP addresses. Implementation of Layer 3 devices allows for segmentation of the LAN into unique physical and logical networks. Routers also allow for connectivity to WANs, such as the Internet. 
Layer 3 routing determines traffic flow between unique physical network segments based on Layer 3 addresses. A router forwards data packets based on destination addresses. A router does not forward LAN-based broadcasts such as ARP requests. Therefore, the router interface is considered the entry and exit point of a broadcast domain and stops broadcasts to other LAN segments.
Routers provide scalability because they serve as firewalls for broadcasts and they can divide networks into subnetworks, or subnets, based on Layer 3 addresses. 
In order to decide whether to use routers or switches, it is important to determine the problem that needs to be solved. If the problem is related to protocol rather than issues of contention, then routers are the appropriate solution. Routers solve problems with excessive broadcasts, protocols that do not scale well, security issues, and network layer addresses. Routers are more expensive and more difficult to configure than switches.
Figure shows an example of an implementation that has multiple networks. All data traffic from Network 1 destined for Network 2 has to go through the router. In this implementation, there are two broadcast domains. The two networks have unique Layer 3 network address schemes. Multiple physical networks can be created if the horizontal cabling and vertical cabling are patched into the appropriate Layer 2 switch. This can be done with patch cables. This implementation also provides robust security because all traffic in and out of the LAN must pass through the router.
Once an IP address scheme is developed for a client, it should be clearly documented. A standard convention should be set for addresses of important hosts on the network. This address scheme should be kept consistent throughout the entire network. Address maps provide a snapshot of the network.   Physical maps of the network helps to troubleshoot the network. 
VLAN implementation combines Layer 2 switching and Layer 3 routing technologies to limit both collision domains and broadcast domains. VLANs also provide security with the creation of VLAN groups that communicate with other VLANs through routers. 
A physical port association is used to implement VLAN assignment. Ports P1, P4, and P6 have been assigned to VLAN 1. VLAN 2 has ports P2, P3, and P5. Communication between VLAN 1 and VLAN 2 can occur only through the router. This limits the size of the broadcast domains and uses the router to determine whether VLAN 1 can talk to VLAN 2. 
This page concludes this lesson. The next lesson will describe LAN switches. The first page describes the hierarchical design model.