Useful Tech Tips, Tutorials and guides

Cisco CCNA exam Cram Sheet

Cisco CCNA exam Cram Sheet

General Networking

Port Numbers
Port Number Layer 4 Protocol Suite Application
20 TCP FTP Data
21 TCP FTP Program
23 TCP Telnet

Ping Responses

  • ! – Reply from target address
  • . – Timeout
  • U – Destination unreachable
  • C – Congestion experienced
  • I – User interrupt
  • ? – Unknown packet type
  • & – Packet lifetime exceeded

Ethernet 5-4-3 rule – Between any 2 nodes there can only be a maximum of 5 segments, 4 repeaters and only 3 of those segments can have hosts.

Loop back address –, tests link between PC and NIC card.

Repeaters – 100BASE-T LANs
Type Max delay Max number you can use
Class 1 repeater (Translational) 140 1
Class 2 repeater (Transparent) 92 2



  • Increase security and ease administration and relocation
  • Break up broadcast domains
  • VLANS work at layer 2 and 3 of OSI model
  • Communication between VLANs uses layer 3 routing

3 ways of assigning a switched port to VLANs

  • Port-Centric – All nodes connected to ports in the same VLAN are assigned to the same VLAN id
  • Static VLANs – Statically assign ports to a VLAN
  • Dynamic VLANs – Ports that can automatically determine their VLAN assignments based on MAC addresses

Switches segment LANs into individual collision domains, VLANs break up broadcast domains as well.



  • Ethernet is half duplex – only one station can send or receive at any one time
  • Full duplex Ethernet allows transmission and reception at the same time. Two pairs of wires, full duplex NIC cards and a switched connection are required.
  • Switches segment collision domains but forward broadcast packets to all ports.

Switch Memory Buffering –  2 Types (Port Based and Shared Memory)

  • Port Based can cause problems holding up transmission of all other packets if there is a delay on one port
  • Shared Memory Buffering is not linked to a specific port but dynamically allocated within a shared pool of memory

Switching Methods

  • Store and Forward – Entire frame is received before forwarding is commenced. Error detection is high, latency low.
  • Cut through – Comes in 2 types (Fast Forward and Fragment Free)
    1. Fast Forward – Forwards frame after receiving destination address, error prone
    2. Fragment Free – Filters out collision packets (If packet data > 64bytes ok)

SPANNING TREE PROTOCOL – 802.1d – Algorithm by Radia Perlman

  • Prevents loops, loops cause broadcast storms
  • Allows redundant links
  • Resilient to topology changes
  • STA (Spanning Tree Algorithm) – Used to calculate loop-free path
  • BPDUs (Bridge Protocol Data Units) are sent and received by switches in the network every 2 seconds (default) to determine spanning tree topology.

Bridge Priority – Numerical value held by switches. All Catalyst switches are 32768

Bridge ID = MAC Address

Spanning Tree States
Blocking No frames forwarded, BPDUs heard
Listening No frames forwarded, listening for frames
Learning No frames forwarded, learning addresses
Forwarding Frames forwarded, learning addresses
Disabled No frames forwarded, no BPDUs heard


All switch ports are in blocking mode to begin with. It takes approx 30 seconds until packets can be forwarded.

Step 1 : Elect Root Bridge – Lowest bridge priority, if there is a tie then switch with lowest bridge ID Step 2 : Elect Root Ports – Locate redundant paths to root bridge; block all but on root. Root Path Cost is cumulative cost of path to root bridge. Ports directly connected to Root Bridge will be root ports, otherwise lowest root path cost used. Step 3 : Elect Designated Ports – Single port that sends and receives traffic from a switch to and from Root Bridge – Lowest cost path to Root Bridge.

Port Fast Mode – Immediately brings a port from blocking to forwarding state by eliminating forward delays.


ISDN – Integrated services Digital Network

  • Carries a variety of traffic. Video, Circuit Switched Data And Telephone Services using normal phone network.
  • Call set up and data rates faster than modem
  • TE1 – Device compatible with ISDN network, connects to NT1 or NT2
  • TE2 – Device not compatible with ISDN requires TA
  • TA – (terminal Adapter) Converts signals so non-ISDN devices can use ISDN
  • NT1 – Connects 4-wire ISDN to 2-wire local loop
  • NT2 – Directs traffic to and from different subscriber devices and NT1

ISDN Reference Points

  • R – References points between non-ISDN device and TA
  • S – References points that connect into NT2 (Inbound)
  • T – References Outbound connections from NT2 into ISDN network or NT1
  • U – References connection between NT1 and ISDN network (North America only)

ISDN Protocols

  • E – Telephone network standards
  • I – Concepts, terminology, general methods
  • Q – How switching should operate

SPID – Service Profile Identifiers

For proper ISDN configuration you have to configure the correct switch type. Most common type in US is At&T’s 5ESS and Nortel DMS-100. In UK it’s Net3 and Net5.

You also need to know your assigned SPID to identify your configuration at the CO.


Novell IPX

IPX address is made up of 32 bits network and 48 bits MAC address.

Novell 5.0 onwards uses IP.

Netware History
Netware 2 – 3.11 (1983-1985) Ethernet_802.3 (default encapsulation on Cisco routers) Cisco name novell-ether. Also called Ethernet raw. Netware 3.12 – 4.x (1985 – ) Ethernet_802.2 Cisco name sap (service access points) This is standard IEEE format and includes LLC header.

Ethernet_II uses TCP/IP Cisco name arpa Ethernet_SNAP Cisco name snap (Subnetwork Access Protocol)

Novell RIP

  • 2 Metrics (Hops, Ticks)
  • Max hop count 15
  • Updates every 60 sec

IPX Configuration

Router(config)# ipx routing Router(config)# ipx maximum paths x (default 1, max 512) Router(config)# int e0.1 Router(config-if)# encapsulation novell-ether Router(config-if)# ipx network 9e

IPX Troubleshooting

Show ipx interface (status / parameters) Show ipx route (routing table) Show ipx servers (server list) Show ipx traffic (number / type of packets) Debug ipx routing activity (info on RIP updates) Debug ipx sap (info on Sap updates) Ping

SAP Service Advertising Protocols
4 Netware File Server
7 Print Server
24 Remote bridge server (router)

SAP facts

  • SAP packets sent every 60 seconds.
  • Routers build SAP tables.
  • Routers only respond to GNS requests when servers are not local.