Chapter 17. Connecting Devices and Virtual LANs
Contents
1. Connecting Devices 2. Virtual LANs
Yoon-Seok Nam
Dept. of Information and Communication Engineering Dongguk University 123 Dongdae-Ro, Gyeongju-City, Gyeongsangbuk-Do, 38066, Korea Phone : 054-770-2273(Lab), 054-770-2608(Office), 054-770-2605(fax), 010-7641-5004(CP) Email: [email protected]
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Objectives
The first section discusses connecting devices. It first describes hubs and their features. The section then discusses link-layer switches (or simply switches, as they are called), and shows how they can create loops if they connect LANs with broadcast domains.
The second section discusses virtual LANs or VLANs.
The section first shows how membership in a VLAN can
be defined. The section then discusses the VLAN
configuration. It next shows how switches can
communicate in a VLAN. Finally, the section mentions
the advantages of a VLAN.
Hosts and networks do not normally operate in isolation. We use connecting devices to connect hosts together to make a network or to connect networks together to make an internet. Connecting devices can operate in different layers of the Internet model. We discuss three kinds of connecting devices: hubs, link-layer switches, and routers.
1. Connecting Devices
Figure 17.1: Three categories of connecting devices
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Five categories of connecting devices
Gateway : Application Layer 까지 포함
Router : Network Layer 까지 포함
Bridge : Data Link Layer 까지 포함
Hub/Repeater : Physical Layer 포함
Media
Gateway : 프로토콜이 다른 통신망을 상호 접속하기 위한 장치.
예) IBM의 전송망 SNA와 ISO의 표준인 OSI는 구조가 다르므로 곧바로 연결 할 수 없다. 이때 Gateway를 중간에 삽입하여 연결한다.
(상위 계층인 응용 프로그램 계층에서 두 망을 연결)
Hub/Repeater
Figure 17.2: Hub
A hub is a device that operates only in the physical layer. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates and retimes the original bit pattern.
네트워크에 다수의 시스템을 연결할 때 사용. 활성화된 포트가 있을 경우 다른 포트는 활성화 될 수 없는 단점이 있다. 각각의 포트가 개별적인 동작을 할 수 없다. 10/100 Mbps를 동시에 지원하는 허브가 있는데, 이 경우 10 또는 100Mbps의 어느 한쪽의 속도만 사용이 가능하다.
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Bridge
A link-layer switch (or switch) operates in both the physical and the data- link layers. As a physical-layer device, it regenerates the signal it receives.
As a link-layer device, the link-layer switch can check the MAC addresses (source and destination) contained in the frame.
OSI모델의 데이타링크 계층 중 MAC계층에서 일을 수행, 두 세그먼트 사이에서 데이타링크 계층간의 패킷 전송을 담당하는 장 치 이 다 . 즉 , 2 개 이 상 의 독 립 된 세 그 먼 트 를 결 합 해 서 , 결과적으로 하나의 network인 것처럼 보이게 한다.
Figure 17.4: Learning switch
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Figure 17.5: Loop problem in a learning switch (Part a)
Figure 17.5: Loop problem in a learning switch (Part b)
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Figure 17.5: Loop problem in a learning switch (Part c)
Figure 17.5: Loop problem in a learning switch (part d)
Loop 문제 발생 : 해결 방안은?
Loop를 없애는 것…
어떻게 없애는가?
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Figure 17.6: A system of connected LANs and its graph (Part a)
Loop 있음.
어떻게 없앨 것인가?
Figure 17.6: A system of connected LANs and its graph (Part b)
Loop 있음.
어떻게 없앨 것인가?
전송에 유리한 Node를 계속 유지하도록…
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Figure 17.7: Finding the shortest path and the spanning tree for a switch.
Loop 있음.
어떻게 없앨 것인가?
전송에 유리한 Node를 계속 유지하도록…
Figure 17.8: Forwarding and blocking ports after using
spanning tree algorithm
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Router
We will discuss routers in Part IV of the book when we discuss the network layer. In this section, we mention routers to compare them with a two-layer switch and a hub. A router is a three-layer device; it
operates in the physical, data-link, and network layers.
Figure 17.9: Routing example
2. Virtual LANs
A station is considered part of a LAN if it physically belongs to that LAN. The criterion of membership is geographic. What happens if we need a virtual connection between two stations belonging to two different physical LANs? We can roughly define a virtual local area network (VLAN) as a local area network configured by software, not by physical wiring.
Figure 17.10: A switch connecting three LANs
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Figure 17.11: A switch using VLAN software
가상적인 기능을 가진 LAN 스위치나 비동기 전송 방식(ATM) 스위치를 사용해서 물리적인 배선에 구애받지 않고 Broadcasting 패킷이 전달되는 범위를 임의로 나눈 LAN. 접속구 단위, MAC 주소나 IP 주소 단위, 프로토콜 단위 등으로 가상 LAN을 조립할 때의 제품이다. 다만, 다른 가상 LAN에 속한 단말 간의 통신에서는 라우터가 필요하다. 또 복수의 LAN 스위치를 거쳐 가상 LAN을 구성하기 위해서는 IEEE 802.1Q라는 규격이 표준화되어 있다.
Figure 17.12: Two switches in a backbone using VLAN software
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Membership : What characteristic can be used to group stations in a VLAN? Vendors use different characteristics such as interface numbers, port numbers, MAC addresses, IP addresses, IP multicast addresses, or a combination of two or more of these.
Configuration : How are the stations grouped into different VLANs?
Stations are configured in one of three ways: manually, semi-automatically, and automatically.
Communication between Switches : In a multi-switched backbone, each switch must know not only which station belongs to which VLAN, but also the membership of stations connected to other switches. For example, in Figure 17.12, switch A must know the membership status of stations connected to switch B, and switch B must know the same about switch A.
Three methods have been devised for this purpose: table maintenance, frame tagging, and time-division multiplexing.
참고문헌
1. 데이타통신과 네트워킹 (Data Communications and Networking),
Behrouz A. Forouzan 저, 이재광, 김중규, 박동선, 홍충선 공역, 교보문고, 2013
