From the course: CompTIA Network+ (N10-009) Cert Prep

Configuring switching technologies

From the course: CompTIA Network+ (N10-009) Cert Prep

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Configuring switching technologies

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- Okay, we've covered VLANs in previous videos, but we need to talk about some of the terms and concepts from the switch and VLAN configuration that you may run into on the exam. When you configure a VAN to a switch port on a switch in VAN database mode, the configuration is stored in a VAN database file. This file known as the VLAN.vlan.dat file is active and is stored in the NVAM or flash memory of the switch. N is used by the switch to reference and track the VAN and port assignment configurations. In a previous video, I went ahead and covered inter-VLAN communication and how a router is needed for inter-VLAN communication as well as communication between LANs or WANs. And by now you should be fully aware that a VLAN is its own Layer 2 broadcast domain. However, the hosts on one VAN cannot communicate with hosts on another VAN without the assistance of a Layer 3 device, either a Layer 3 switch or a router. As you learned in that earlier video session, providing Layer 3 interfaces for inter-VLAN communication can become quite complex. One solution is reducing this complexity and the headaches that go along with it. So how do we do that? Well, we want to configure what's called an SVI, otherwise known as a switch virtual interface on the switch itself. An SVI provides a Layer 3 connection and can be associated with one or more VLANs configured on that switch. The SVI acts like a virtual router to route inter-VLAN traffic between different VLANs. Another VLAN communication issue is VLAN identification. A VLAN frame sent out of an interface without VLAN identification is assumed to be the same VLAN that is assigned to the switch port. However, if the frame is sent out to a different VLAN, how will the switch know which VLAN to send it to? There are two ways a destination VLAN can be identified using one of two methods. One method is going to be an inter-switch link or ISL. The other way is going to be an IEEE 802.1q header, otherwise known as .1q. In both cases, a header known as a VLAN tag is added to the header of a frame that identifies the VLAN to which the frame is actually sent. The tag is added by the sending switch and removed by the receiving switch, which then forwards it to the switch port through which the destination VLAN is reached. ISL is a Cisco proprietary method that encapsulates a layer two frame, adding a new header and a frame check sequence, otherwise known as an FCS. The header contains the VLAN tag and the CRC is used for error free checking. The receiving switch removes the header and forwards the frame to the VLAN identified in the header. Because ISL adds a 30 byte header to a frame, it's broken into a 26 byte header to begin with, and a four byte FCS. ISL also creates additional overhead for the frame transfer, which is a reason the .1q method is preferred instead. The IEEE 802.1q frame tagging standard is a vendor neutral method that allows VLAN frames from one manufacturer switch to be passed to another manufacturer switch. Using .1q tagging by the trunking device actually adds a four byte tag immediately after the source MAC address in the original ethernet frame, and recalculates the FCS. Within the four bytes or 16 bits of the .1q tag are 12 bits for identifying the destination VLAN, which can be a value from one to 4094. The receiving switch removes the tag and forwards the frame onto the destination VAN. VLANs are an important part of not only the task assigned to a network technician or administrator, but you should expect that you'll definitely encounter VLAN questions and scenarios on your Network+ Exam. Before we leave the topic of switches and switching, there are a few more topics we need to cover. The first of these are switch port speed and duplex settings. In general, a switch port supports transmission speed and combinations of speeds that the switch can automatically negotiate with the sending or receiving device. The most common switch port speed settings are going to be 10 megs by 100 megs and then even 10, 100 to 1000 megabits per second. Although you can set the speed of a switch port specifically to 10, 100 or 1000 megs, there is an auto setting that allows the switch to adjust the switch port speed to that of a negotiated amount between the two endpoints. Duplex settings can be full, half, or auto as well. Full duplex means that both endpoints can transmit and receive at the same time, such as on a telephone call where two people can actually talk at the same time. Full duplexing simulates the effect of two communication lines between two endpoints. Half duplex allows two parties to communicate, but only one of them at a time over it. Where the full duplex allows the endpoints to communicate simultaneously. Half duplex is a single line that transmits only one direction at a time. An example of half duplex is a two-way radio, like a walkie-talkie on which only one party can talk at a time over. You may wonder why we have half duplex at all. Since full duplex seems so much better. Well, not all communications are two way, and even a lot of these that are only send or receive one way at a time, which is essentially half duplex. Computer systems and their peripherals have many half duplex devices such as keyboards, mouse units, printers, and in many cases, networking. Half duplex tries to avoid collisions on a LAN, especially if a node is downloading streaming media, which flows only in one way. So it depends on the nature and the type of network traffic, whether a port needs to be in full or half duplex.

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