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Chapter 5

ETHERNET

802.3 başlığında tanımlıdır.
local are network teknolojisidir.
Ethernet OSI de ilk 2 katmanda çalışır.
mac address kullanılır.

mac address 
48 bitliktir. hexadecimal (16 lı sayı sistemi) ile gösterilir.
physical address = mac address = hardware address = bia (burned-in address)

0 = 0000
1 = 0001
2 = 0010
3 = 0011
4 = 0100
5 = 0101
6 = 0110
7 = 0111
8 = 1000
9 = 1001
10 a = 1010
11 b = 1011
12 c = 1100
13 d = 1101
14 e = 1110
15 f = 1111

00-21-9B-E3-B9-1D = 0021.9BE3.B91D
0000 0000 0010 0001 1001 1011 1110 1011 1001 0001 1101

ethernet / wireless cihazların herbirinin eşsiz (unique) mac address leri olmalıdır.

OUI (organizationally unique identifier) mac adreslerini yönetir.

switch L2 de çalışır (mac address lerinden anlar).

ipconfig/all = ipconfig -all ip configuration ve mac address bilgisi görülebilir.

A unicast MAC address is the unique address used when a frame is sent from a single transmitting device to a single destination device.

broadcast mac address : the destination MAC address is the broadcast MAC address of FF-FF-FF-FF-FF-FF in hexadecimal (48 ones in binary).

Multicast addresses allow a source device to send a packet to a group of devices. The multicast MAC address is a special value that begins with 01-00-5E in hexadecimal.

 SWITCH FUNDEMENTALS

The MAC address table is sometimes referred to as a content addressable memory (CAM) table.

switch mac address table oluşturur. switch açıldığında mac address table boştur.

switch e bağlı host lar trafik yaptıkça switch cihazların mac address lerini öğrenir.

If the source MAC address does exist in the table but on a different port, the switch treats this as a new entry. The entry is replaced using the same MAC address but with the more current port number.

If the destination MAC address is in the table, it will forward the frame out the specified port. switch gelen frame deki hedef mac adresini biliyorsa, frame i o mac adresinin olduğu porta anahtarlar.

Switch#show mac-address-table komutu ile switch in öğrendiği mac address leri görülebilir.

* switch aging yapar. öğrendiği mac address lerini ram de (mac address table da) default olarak 300 sn tutar.

* switch kendisine gelen frame lerin source mac address field a bakarak mac address table oluşturur.

* switch kendisine gelen frame lerine destination mac address field a bakarak anahtarlama (switching) yapar.

* switch filtering yapar. bozuk frame ler anahtarlanmaz. hedef hangi interface de ise o interface e frame i anahtarlar. konfigürasyon ile switch te mac address bazlı filtreleme yapılabilir.

* eğer switch kendisine gelen frame deki destination mac address i bilmiyorsa frame in geldiği port haricindeki tüm aktif portlarına frame i gönderir (flooding).

* eğer switch e gelen frame in hedef mac address i broadcast ise (FFFF.FFFF.FFFF) switch frame in geldiği port haricindeki tüm aktif portlarına frame i gönderir (flooding).

* switch osi ye göre 2. katmanda çalışır. mac addreslerine göre anahtarlama yapar.

 

Frame Forwarding Methods on Cisco Switches

Store-and-forward switching : In store-and-forward switching, when the switch receives the frame, it stores the data in buffers until the complete frame has been received. During the storage process, the switch analyzes the frame for information about its destination. In this process, the switch also performs an error check using the Cyclic Redundancy Check (CRC) trailer portion of the Ethernet frame.

Cut-through switching : In cut-through switching, the switch acts upon the data as soon as it is received, even if the transmission is not complete.

Fast-forward switching - Fast-forward switching offers the lowest level of latency. Fast-forward switching immediately forwards a packet after reading the destination address.

Fragment-free switching - In fragment-free switching, the switch stores the first 64 bytes of the frame before forwarding.

Memory Buffering on Switches

Port-based Memory Buffering : In port-based memory buffering, frames are stored in queues that are linked to specific incoming and outgoing ports.

Shared Memory Buffering : Shared memory buffering deposits all frames into a common memory buffer that all the ports on the switch share. The amount of buffer memory required by a port is dynamically allocated. The frames in the buffer are linked dynamically to the destination port. This allows the packet to be received on one port and then transmitted on another port, without moving it to a different queue.

There are two types of duplex settings used for communications on an Ethernet network: half duplex and full duplex.

Full-duplex – Both ends of the connection can send and receive simultaneously.

Half-duplex – Only one end of the connection can send at a time.

Switch(config)#interface fa 0/24
Switch(config-if)#duplex ?
auto Enable AUTO duplex configuration
full Force full duplex operation
half Force half-duplex operation

Default auto dur. Bu sayede bir konfigürasyon yapmaya gerek kalmadan auto negotiation adı verilen process ile switch ve ona bağlı end device anlaşır.

When the auto-MDIX feature is enabled, the switch detects the type of cable attached to the port, and configures the interfaces accordingly.

 

Address Resolution Protocol (ARP)

Eğer hedef ip adresi biliniyor fakat bu ip address ine ait mac address i bilinmiyor ise ARP request gönderilir. ARP isteği gönderilirken hedef mac address alanına broadcast mac address bilgisi yazılır (FFFF.FFFF.FFFF)

  • Switch ler broadcast leri flooding yapar. Router lar broadcast leri geçirmezler.
  • Broadcast istekleri local network ün dışına çıkamaz. Router lar broadcast leri geçirmezler.