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Chapter 1: Bridging/SwitchingThis first chapter will introduce you to VLANs and how to configure VLANs on Cisco switches using static VLAN number assignments. Understanding frame tagging within a VLAN will also be discussed.
When there is more than one path through the network, data can come back to the original source. In this situation, Spanning-Tree Protocol (STP) is used in Layer 2 switched networks to prevent these network loops, and to block ports that can allow data to return, causing a loop. We will discuss the basics of STP and how to use STP in a Layer 2 switched internetwork.
Cisco switches can operate in three different LAN switch modes: store-and-forward, cut-through, and FragmentFree. This chapter will discuss all three methods.
The chapter will end with a discussion of the Point-to-Point Protocol (PPP). PPP is used to connect either asynchronous serial or synchronous serial links. Configuration of PPP, as well as PPP authentication, will be discussed.
Switches were placed in networks to replace bridges and hubs in flattopology networks (where there is no segmentation of broadcast or collision domains). As networks grow, so does the amount of broadcast traffic. Not all broadcasts are bad; they are a necessity in today's networking protocols, to allow the location of certain devices to be known throughout the network segment. But when devices on the same network segment number in the hundreds, the result of all these devices sending broadcasts as well as data traffic can slow the network and the devices to a crawl.
Excessive broadcasts reduce the bandwidth available to end users and require every node on the network to process every frame,regardless of whether it is the intended recipient of the data. The processor in each machine is responsible for doing this task, taking away from the processing power needed for the end-user applications.
As more and more broadcasts enter your network, the network can actually grind to a halt. This situation is known as a broadcast storm. Broadcast storms occur when broadcasts throughout the LAN use up all available bandwidth, bringing the network completely to a stop. Switches use VLANs to segment the network into smaller broadcast and collision domains. Let's look at the two different types of VLANs, both static and dynamic.
NOTE Token Ring, Fast Ethernet, Gigabit Ethernet, and Fiber Distributed Data Interface (FDDI) are also interfaces that can be found on Cisco switches.
If you want to allow different ports on a switch to be part of different subnetworks, you need to create virtual LANs within the switch. A virtual local area network (VLAN) is a logical grouping of network users and resources connected to defined ports on the switch. A VLAN looks like, and is treated like, its own subnet. By using virtual LANs, you're no longer confined to physical locations. VLANs can be created by location, function, department-even by the application or protocol used-regardless of where the resources or users are located.
VLANs are created by an administrator to break up broadcast domains in a Layer 2 switch. The administrator then assigns switch ports to the VLAN. This objective will discuss how VLAN assignments are created and configured on Cisco switches.
The two types of VLAN assignments are static and dynamic. A static VLAN is the typical means of creating a VLAN. The switch port to which you assign a VLAN association maintains that association until an administrator changes the port assignment. This type of VLAN configuration is easy to set up and monitor, working well in a network where the movement of users within the network is controlled. Using network management software to configure the ports can be helpful but is not mandatory.
Dynamic VLANs determine a node's VLAN assignment automatically. Using intelligent management software, you can enable hardware (MAC) addresses, protocols, or even applications to create dynamic VLANs. For example, suppose MAC addresses have been entered into a centralized VLAN management application. If a node is then attached to an unassigned switch port, the VLAN management database can look up the hardware address and assign and configure the switch port to the correct VLAN. This can make management and configuration easier for the administrator. If a user moves, the switch will automatically assign them to the correct VLAN. However, more administration is needed initially to set up the database.
Cisco administrators can use the VLAN Management Policy Server (VMPS) service to set up a database of MAC addresses that can be used for dynamic addressing of VLANs. VMPS is a MAC address-toVLAN mapping database.
To configure VLANs on a switched internetwork, you need to follow the steps listed below:
- Configure the VLANs.
- Assign switch ports to VLANs.