Chapter 1: Ethernet Basics

Cisco Press

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Type Field

Description

Protocol Type

DIX V2 Type field; 2 bytes; registered values now administered by the IEEE

DSAP

802.2 LLC; 1 byte, with 2 high-order bits reserved for other purposes; registered values now administered by the IEEE

SNAP

SNAP header; 2 bytes; uses same values as Ethernet Protocol Type; signified by an 802.2 DSAP of 0xAA

Switching and Bridging Logic

In this chapter so far, you have been reminded about the cabling details for Ethernet along with the formats and meanings of the fields inside Ethernet frames. A switch's ultimate goal is to deliver those frames to the appropriate destination(s) based on the destination MAC address in the frame header. Table 1-7 summarizes the logic used by switches when forwarding frames, which differs based on the type of destination Ethernet address and on whether the destination address has been added to its MAC address table.

Table 1-7 LAN Switch Forwarding Behavior

Type of Address

Switch Action

Known unicast

Forwards frame out the single interface associated with the destination address

Unknown unicast

Floods frame out all interfaces, except the interface on which the frame was received

Broadcast

Floods frame identically to unknown unicasts

Multicast

Floods frame identically to unknown unicasts, unless multicast optimizations are configured

For unicast forwarding to work most efficiently, switches need to know about all the unicast MAC addresses and out which interface the switch should forward frames sent to each MAC address. Switches learn MAC addresses, and the port to associate with them, by reading the source MAC address of received frames. You can see the learning process in Example 1-2, along with several other details of switch operation. Figure 1-6 lists the devices in the network associated with Example 1-2, along with their MAC addresses.

Figure 1-6

Sample Network with MAC Addresses Shown

Example 1-2 Command Output Showing MAC Address Table Learning

Switch1# show mac-address-table dynamic
          Mac Address Table
------------------------------------------

Vlan    Mac Address       Type       Ports
----    -----------       ----       -----
   1    000f.2343.87cd    DYNAMIC    Fa0/13
   1    0200.3333.3333    DYNAMIC    Fa0/3
   1    0200.4444.4444    DYNAMIC    Fa0/13
Total Mac Addresses for this criterion: 3
! Above, Switch1's MAC address table lists three dynamically learned addresses, 
! including Switch4's FA 0/13 MAC.
! Below, Switch1 pings Switch4's management IP address.
Switch1# ping 10.1.1.4

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
! Below Switch1 now knows the MAC address associated with Switch4's management IP
! address. Each switch has a range of reserved MAC addresses, with the first MAC 
! being used by the switch IP address, and the rest being assigned in sequence to
! the switch interfaces – note 0xcd (last byte of 2nd address in the table above) 
! is for Switch4's FA 0/13 interface, and is 13 (decimal) larger than Switch4's 
! base MAC address. 
Switch1# show mac-address-table dynamic
          Mac Address Table
------------------------------------------

Vlan    Mac Address       Type       Ports
----    -----------       ----       -----
   1    000f.2343.87c0    DYNAMIC    Fa0/13
   1    000f.2343.87cd    DYNAMIC    Fa0/13
   1    0200.3333.3333    DYNAMIC    Fa0/3
   1    0200.4444.4444    DYNAMIC    Fa0/13
Total Mac Addresses for this criterion: 4
! Not shown: PC1 ping 10.1.1.23 (R3) PC1's MAC in its MAC address table 
------------------------------------------

Vlan    Mac Address       Type       Ports
----    -----------       ----       -----
   1    000f.2343.87c0    DYNAMIC    Fa0/13
   1    000f.2343.87cd    DYNAMIC    Fa0/13
   1    0010.a49b.6111    DYNAMIC    Fa0/13
   1    0200.3333.3333    DYNAMIC    Fa0/3
   1    0200.4444.4444    DYNAMIC    Fa0/13
Total Mac Addresses for this criterion: 5
! Above, Switch1 learned the PC's MAC address, associated with FA 0/13, 
! because the frames sent by the PC came into Switch1 over its FA 0/13.  
! Below, Switch4's MAC address table shows PC1's MAC off its FA 0/6
switch4# show mac-address-table dynamic
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    000a.b7dc.b780    DYNAMIC     Fa0/13
   1    000a.b7dc.b78d    DYNAMIC     Fa0/13
   1    0010.a49b.6111    DYNAMIC     Fa0/6
   1    0200.3333.3333    DYNAMIC     Fa0/13
   1    0200.4444.4444    DYNAMIC     Fa0/4
Total Mac Addresses for this criterion: 5
! Below, for example, the aging timeout (default 300 seconds) is shown, followed 
! by a command just listing the mac address table entry for a single address.  
switch4# show mac-address-table aging-time
Vlan    Aging Time
----    ----------
   1     300
switch4# show mac-address-table address 0200.3333.3333
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    0200.3333.3333    DYNAMIC     Fa0/13
Total Mac Addresses for this criterion: 1

Foundation Summary

This section lists additional details and facts to round out the coverage of the topics in this chapter. Unlike most of the Cisco Press Exam Certification Guides, this "Foundation Summary" does not repeat information presented in the "Foundation Topics" section of the chapter. Please take the time to read and study the details in the "Foundation Topics" section of the chapter, as well as review items noted with a Key Topic icon.

Table 1-8 lists the different types of Ethernet and some distinguishing characteristics of each type.

Table 1-8 Ethernet Standards

Type of Ethernet

General Description

10BASE5

Commonly called "thick-net"; uses coaxial cabling

10BASE2

Commonly called "thin-net"; uses coaxial cabling

10BASE-T

First type of Ethernet to use twisted-pair cabling

DIX Ethernet Version 2

Layer 1 and Layer 2 specifications for original Ethernet, from Digital/Intel/Xerox; typically called DIX V2

IEEE 802.3

Called MAC due to the name of the IEEE committee (Media Access Control); original Layer 1 and 2 specifications, standardized using DIX V2 as a basis

IEEE 802.2

Called LLC due to the name of the IEEE committee (Logical Link Control); Layer 2 specification for header common to multiple IEEE LAN specifications

IEEE 802.3u

IEEE standard for Fast Ethernet (100 Mbps) over copper and optical cabling; typically called FastE

IEEE 802.3z

Gigabit Ethernet over optical cabling; typically called GigE

IEEE 802.3ab

Gigabit Ethernet over copper cabling

Switches forward frames when necessary, and do not forward when there is no need to do so, thus reducing overhead. To accomplish this, switches perform three actions:

  • Learn MAC addresses by examining the source MAC address of each received frame

  • Decide when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address

  • Create a loop-free environment with other bridges by using the Spanning Tree Protocol

The internal processing algorithms used by switches vary among models and vendors; regardless, the internal processing can be categorized as one of the methods listed in Table 1-9.

Table 1-9 Switch Internal Processing

Switching Method

Description

Store-and-forward

The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame, thus ensuring that errored frames are not forwarded.

Cut-through

The switch performs the address table lookup as soon as the Destination Address field in the header is received. The first bits in the frame can be sent out the outbound port before the final bits in the incoming frame are received. This does not allow the switch to discard frames that fail the FCS check, but the forwarding action is faster, resulting in lower latency.

Fragment-free

This performs like cut-through switching, but the switch waits for 64 bytes to be received before forwarding the first bytes of the outgoing frame. According to Ethernet specifications, collisions should be detected during the first 64 bytes of the frame, so frames that are in error because of a collision will not be forwarded.

Table 1-10 lists some of the most popular Cisco IOS commands related to the topics in this chapter.

Table 1-10 Catalyst IOS Commands for Catalyst Switch Configuration

Command

Description

interface vlan 1

Global command; moves user to interface configuration mode for a VLAN interface

interface fastethernet 0/x

Puts user in interface configuration mode for that interface

duplex {auto | full | half}

Used in interface configuration mode; sets duplex mode for the interface

speed {10 | 100 | 1000 | auto | nonegotiate}

Used in interface configuration mode; sets speed for the interface

show mac address-table [aging-time | count | dynamic | static] [address hw-addr] [interface interface-id] [vlan vlan-id]

Displays the MAC address table; the security option displays information about the restricted or static settings

show interface fastethernet 0/x

Displays interface status for a physical 10/100 interface

show interface vlan 1

Displays IP address configuration for VLAN

Table 1-11 outlines the types of UTP cabling.

Table 1-11 UTP Cabling Reference

UTP Category

Max Speed Rating

Description

1

Used for telephones, and not for data

2

4 Mbps

Originally intended to support Token Ring over UTP

3

10 Mbps

Can be used for telephones as well; popular option for Ethernet in years past, if Cat 3 cabling for phones was already in place

4

16 Mbps

Intended for the fast Token Ring speed option

5

1 Gbps

Very popular for cabling to the desktop

5e

1 Gbps

Added mainly for the support of copper cabling for Gigabit Ethernet

6

1 Gbps+

Intended as a replacement for Cat 5e, with capabilities to support multigigabit speeds

Table 1-12 lists the pertinent details of the Ethernet standards and the related cabling.

Table 1-12 Ethernet Types and Cabling Standards

Standard

Cabling

Maximum Single Cable Length

10BASE5

Thick coaxial

500 m

10BASE2

Thin coaxial

185 m

10BASE-T

UTP Cat 3, 4, 5, 5e, 6

100 m

100BASE-FX

Two strands, multimode

400 m

100BASE-T

UTP Cat 3, 4, 5, 5e, 6, 2 pair

100 m

100BASE-T4

UTP Cat 3, 4, 5, 5e, 6, 4 pair

100 m

100BASE-TX

UTP Cat 3, 4, 5, 5e, 6, or STP, 2 pair

100 m

1000BASE-LX

Long-wavelength laser, MM or SM fiber

10 km (SM)

3 km (MM)

1000BASE-SX

Short-wavelength laser, MM fiber

220 m with 62.5-micron fiber;

550 m with 50-micron fiber

1000BASE-ZX

Extended wavelength, SM fiber

100 km

1000BASE-CS

STP, 2 pair

25 m

1000BASE-T

UTP Cat 5, 5e, 6, 4 pair

100 m

Memory Builders

The CCIE Routing and Switching written exam, like all Cisco CCIE written exams, covers a fairly broad set of topics. This section provides some basic tools to help you exercise your memory about some of the broader topics covered in this chapter.

Fill in Key Tables from Memory

Appendix E, "Key Tables for CCIE Study," on the CD in the back of this book contains empty sets of some of the key summary tables in each chapter. Print Appendix E, refer to this chapter's tables in it, and fill in the tables from memory. Refer to Appendix F, "Solutions for Key Tables for CCIE Study," on the CD to check your answers.

Definitions

Next, take a few moments to write down the definitions for the following terms:

Auto-negotiation, half duplex, full duplex, cross-over cable, straight-through cable, unicast address, multicast address, broadcast address, loopback circuitry, I/G bit, U/L bit, CSMA/CD

Refer to the glossary to check your answers.

Further Reading

For a good reference for more information on the actual FLPs used by auto-negotiation, refer to the Fast Ethernet web page of the University of New Hampshire Research Computing Center's InterOperability Laboratory, at http://www.iol.unh.edu/services/testing/fe/training/.

Copyright © 2007 Pearson Education. All rights reserved.

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