only for the bored / curious

here’s the day 1 notes. I was going to make omnigraffle diagrams of the diagrams I drew by hand, but that would just be too time consuming; instead I’ll just scan them in when I return.

Day 1 - 4/29

Chapter 1

Basic Concepts
1) Routers create broadcast domains, switches do not. Broadcast packets will be sent to all
ports on a switch (without VLANs), but routers will never forward broadcast packets between

2) Routers make dissimiliar networks talk to each other. Default gateway for a host is
always a router

Cisco's three layer model for networks:
Access Layer = End users, workstations, switches, etc
Distribution Layer = Filtering, routing, WAN access; creates broadcast domains
Core Layer = where major services or servers exist

OSI Reference Model:
 - a way to understand how an internetwork operates
 - guidline for creation or implemenation of standards, devices and networking schemes
 - breaks down a complex system into simpler elements
 - allows for specialized design and development of modular components

Application Layer:
 - User interaction
 - Applications exist here (Word, FTP client, etc)

Presentation Layer:
 - converts data into a specific format
 - ensures that data can be read from the application layer of one system to the application layer of another system
 - jpeg, gif, .doc, encryption

Session Layer:
 - Establish, manage, terminate communications
 - system calls, process management
 - eg database server & client

Transport Layer:
 - Responsible for end to end transport
 - connection oriented (TCP) or connectionless (UDP)
 - Port assignments
 - After adding TCP header, unit is known as a SEGMENT
 - Applications determine which transport protocol is used

TCP = reliability, data will be acknowledged
UDP = unreliable, no acknowledgement

Transport layer adds TCP or UDP headers

Within that header there are logical ports; source port and destination port. The
destination port is used to differentiate between applications. Even if source IP, dest IP,
source MAC and dest MAC are the same, the communications between different applications on
the two hosts is identified by the TCP or UDP port number.

Source port is random; destination port is well known (and less than 1024)

Names for data units are associated with specific layers:
Transport	-->	Segment
Network		-->	Packet
Data Link	-->	Frame
Physical	-->	Bits

PDU = protocol data unit.

PDUs are added as info travels down the stack and removed as it travels up the stack. PDUs
are refered to as headers or trailers. Encapsulation is the process of adding PDUs, de-
encapsulation removes PDUs.

Collision Domains / Broadcast Domains:

Collision Domains:
 - Signaling
 - Ethernet must have rules / standards to communication
 - Allows access to one user at a time
 - Corrects collisions


Collision domain: devices attached to the same physical media such that if two devices
access the media at the same time, a collision occurs (eg hub).

Broadcast domain: a group of devices in a network that receive each others broadcasts

On a switch, each port is its own collision domain.

CSMA / CD = carrier sense multiple access / collision detect:
 - Each host listens on teh line for traffic
 - If in use, back off using algorithm
 - If not in use, send traffic
 - If traffic is not heard on teh line, assume collision and resend; back off algorithm
 - If traffic is heard, assume success.

If two sets of hosts within a collision domain try to use the media at the same time, a
collision occurs. Using CSMA, hosts sense the network to know when to talk.

Back off algorithm - wait to send; round robin style.

Problems with CSMA / CD: more hosts = more collisions
Solution = create more collision domains (replace hubs with switches)

Half duplex = send OR receive; uses CSMA / CD
Full duples = send AND receive; does not use CSMA / CD

Data Link Layer
2 sublayers:
 - MAC = 802.3; physical addressing

 - Logical Link Control - LLC 802.2
 - Identify different protocols with SAP / SNAP and then encapsulate them in order

MAC: 48 bits in a MAC address = hardware address or NIC address

LLC: uses either SAP or SNAP to differentiate between protocols; eg IP, IPX, AppleTalk.

SAP = service access point
SNAP = subnetwork access protocol

Preamble is a combination of 1s and 0s that signify the start of a frame

Preamble	Dest MAC	Source MAC	Type		Data		CRC
64 bits		48 bits		48 bits		16 bits		MTU 1500	32 bits

Data portion of a SAP frame:


DSAP / SSAP = 1 byte
Pointer to upper layer protocols
06 hex = IP
EO hex = IPX

In SAP, your DSAP and SSAP tells dest and source protocol

Data portion of a SNAP frame:

AA		AA		03		ID

DSAP and SSAP fields have a fixed value of AA in SNAP frames. CNTL is always 03. OUI is a
vendor code. Type field contains an EtherType which determines protocol.

Bridges and Switches
 - Operate at layer 2, the data link layer

 - Switching is *hardware* based frame forwarding
 - ASIC = application specific integrated circuits; used to create high performance chips
 - Fast; scales up to gigabit speeds

 - Bridge = 2 or 4 ports
 - Software based; slower

Bridges can be used to increase the number of collision domains, which decreases the number
of collisions.

Bridging / Switching Process:
 - Receives frame
 - Checks forwarding table for the dest MAC address
 - Proceeds with one of three functions:
	- Filter data = when source and dest are on same physical port
	- Floods data = sends to all ports; when dest port is unknown
	- Copies the frame to another segment = from one physical port to another

Network Layer:
 - Defines how to transport traffic to devices not locally attached; not in same broadcast
 - 2 pieces of information required:
	logical address of destination
	path to reach destination

IP address = 32 bits; exists in network layer

Routers have information from all lower layers (due to encapsulation)

IP addresses are assigned to interfaces of routers.

Subnet mask determins which portion of IP is the network portion and which is the host

Router interfaces must be on different networks

Once you configure IP addresses on router interfaces, the routing table will look like this
(see diagram)

Define static routes = manual routing entries
Enable Routing protocols = routers talk to each other and exchange routing tables

Router functions:
 - Does not forward layer 2 broadcasts / multicasts
 - Determines optimal path to destination
 - strips layer 2, forwards on layer 3 addresses - ARP
 - can filter traffic; access lists

multicast = specific group of hosts
broadcast = everybody
unicast = just one host

ARP = Address Resolution Protocol; when you have IP and need MAC. (see diagram) MAC addresses are replaced to allow for return communication without flooding.

 - Can perform both bridging and routing
 - Can provide connectivity between VLANs in switched environment
 - Can provide quality services based on traffic type

Uses bridging to pass non routable protocols.

If VLANs want to talk to each other, they need a router.

Chapter 2

UTP Implementation

3 kinds of UTP:
 - straight through; used to connect PC to router, hub, or switch; dissimiliar OSI layers
 - crossover; switch to switch, hub to hub, etc; used to connect devices on same layer
 - rollover; used for console ports; DB9 connector (com1 on a pc)
	console port = in band (local)
	AUX = out band management (modem dialup)

WAN Connections


CSU / DSU; analog to digital and back; makes signaling at both ends compatible

Supported by Cisco Equipment:
EIA / TIA - 232
EIA / TIA - 449
EIA - 530

Needed on serial links:
DCE = modem = gives clock rate to router = data communications equipment
DTE = accepts clock rate = data terminal equipment

When connecting routers to each other, one end must be made to be a DCE (DTE by default)

Serial Interfaces on a router:
 - Abbreviated differently on different routers
 - s0 = fixed
 - s0/0 = modular

Router commands:

erase start = deletes startup config from NVRAM
reload = reboot router; return to factory defaults

startup config	running config

> = user mode
# = privledged mode

show ip interface brief = brief listing of interfaces & their status

Interfaces are administratively down on Cisco routers by default

Chapter 3

Router Startup Processs:
1) Power On Self Test (POST); finds / checks hardware
2) Run startup routines; locate / load IOS (Internet Operating System)
3) Locate / load software config file

Config locations:
 - console terminal or AUX
 - TFTP connected via ethernet (can hold configurations and IOS backups)

Console Help:
? = list of commands
s? = list commands that start with s
show ? = commands used with show

Abbreviating commands:
show interfaces = sh int

Error Messages of 3 different types:
 - Ambiguous command - more than one possible meaning (for an abbreviation)
 - incomcplete command
 - invalid input detected at ^ marker

show history = history of commands entered
control P = previous command in history buffer
control N = next command in history buffer
control a = beginning of line
control e = end of line
control f = forward one character
control b = back one char
control u = clear line
control r = refresh command buffer
show version = system information; IOS version, hardware info
show running-config = running configuration
show int serial0 = detailed info on interface
show ip = for switches, not routers
configure terminal = glocal configuration mode
(show commands do not work in global config mode)
enable = enter privledged mode
disable = exit priv mode

Setting IP on a switch:
ip address x.x.x.x - enables SNMP management
setup - from exec mode to enter guided setup
control-c exits setup process

At the end of setup, there are three options:
0) go to exec without using or saving config that was just set up
1) restart setup
2) accept / save config, go to exec mode

Routers only:
no terminal editing = disbles terminal editing (control) commands
terminal editing = enables editing commands

Lines greater than 69 characters show a $ at beginning of line

show startup-config = show startup config
copy run start = saves running config to startup config in NVRAM

from global config:
int s0 = interface config for serial0 - prompt changes to: Router(config-if)

sub-interfaces; multiple subinterfaces can be on a single physical interface
int s0.1 = from global config; configures sub-interface 1 on s0

from priv mode:
show line
 - cty = console
 - vty = virtual telnet sessions
 - aux = aux

By default, Cisco routers have 5 VTY lines

from global config:
line console 0 = line config mode
router rip = configure router to use RIP

control-Z returns to priv mode (use from global config or interface config)

setting the hostname:

Setting a console password:
config terminal
line console 0

Setting a VTY password:
configure terminal
line VTY 0 4

Setting an enable (priv mode) password:
enable password 

Setting an encrypted enable password:
enable secret  - does not show with 'show running-config'

To disable passwords, use the no form:
no enable password 

To disable console timeouts:
config t
line console 0
exec-timeout 0 0

By default, logging is sent to console
logging syncronyous = disable logging output to console while entering a command

show ip route = shows routing table

(diagram of pod 5)

Configuring Interfaces (from interface config mode):
address on port
data encapsulation
media type
define bandwidth
set direct memory access buffering parameters

ip address   = assign an IP to an interface
no shut = removes "administratively down" status

show controllers s0 = determine if serial 0 is dce or dte
DCE ends must set clocking from interface config mode
clock rate xxxxx

Entering bandwidth: 
 - Used by routing protocols to determine metric
 - By default, T1 speed

Disabling Interfaces (serial0 for example):
config t
int s0

Chapter 4

CDP = Cisco Discovery Protocol
Enabled by default
Media and protocol independant
Used to aid in managing Cisco devices
Summary of neighbor info
shows only info within a collision domain (directly attached)
show cdp neighbors
show cdp ?
show cdp neighbors detail

Disabling CDP globally:
config t
no cdp run

Disabling CDP on an interface:
config t
int s0
no cdp enable

 - cannot telnet out of catlyst 1900
control-shift-6, x = suspent telnet session
show sessions = shows active sessions
show users = shows connected users
clear line  = kick user
resume  = resume telnet session (can also just press enter if only one session)


About dre

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One Response to only for the bored / curious

  1. sewshi says:

    wow.. certainly a lot of good notes. i will save this at some point… thanks dre!

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