Provides an interface between a host’s communication software and any necessary external applications
Evaluates what resources are necessary and the available resources for communication between two devices
Synchronises client/server applications
Provides error control and data integrity between application
Provides system independent processes to a host
Presents data to the Application layer
Acts as a data format translator
Handles the structuring of data and negotiating data transfer syntax to Layer 7 Processes involved include data encryption, decryption compression and decompression
Handles dialog control among devices
Determines the beginning middle and end of a session or conversation that occurs between applications (intermediary)
Manages end to end connections and data delivery between two hosts
Segments and reassembles data
Provides transparent data transfer by hiding details of the transmissions from the upper layers
Determines best path for delivery across the network
Determines logical addressing, which can identify the destination of a packet or datagram Uses data packets (IP, IPX) and route update packets (RIP, EIGRP, and so on) Uses routed protocols IP, IPX, and AppleTalk DDP
Devices include routers and Layer 3 switches
Ensures reliable data transfer from the Network layer to the Physical layer Overseas physical or hardware addressing
Formats packets into a frame
Provides error notification
Devices include bridges and Layer 2 switches
Moves bits between nodes
Assists with the activation, maintenance, and deactivation of physical connectivity between devices Devices include hubs and repeaters
The CCNA Cram Sheet
This Cram Sheet contains key facts about the CCNA exam. Review this information as the last thing you do before you enter the
testing center, paying special attention to those areas in which you feel that you need the most review. You can transfer any of
these facts from your head onto a blank sheet of paper immediately before you begin the exam.
Layer Name Protocols and Devices PDU
7 Application FTP, Telnet, TFTP, SMTP, POP3, SNMP, DNS, NTR, HTTP, HTTPS, DHCP Data
6 Presentation ASCII, .jpg, .doc Data 5 Session RPC, SQL/Telnet (for login only) Data 4 Transport TCP – Connection-oriented, reliable using PAR Segment
UDP – Connectionless, unreliable, uses upper layer protocols for reliability
3 Network IP, ICMP, RIP, IGRP, EIGRP, OSPF Packet
Routing and Path determination, logical addressing
2 Data Link Ethernet, Frame Relay, PPP, HDLC Frame
Physical (hardware) addressing (MAC addresses)
1 Physical Bits transmitted on media Bits
Hubs, Repeaters, Connectors
TCP and UDP Ports
TCP Ports UDP Ports
FTP 20, 21 DNS 53
Telnet 23 DHCP 67, 68
SMTP 25 TFTP 69
DNS 53 NTP 123
HTTP 80 SNMP 161
TCP utilises Positive Acknowledgement and Retransmission (PAR):
; The source device starts the timer fro each segmentl retransmits if acknowledgment is not recieved before the timer
; The source device records all segments sent and expects and acknowledgment of each.
; The destination device acknowledges receipt of a segment by sending an ask for the next dequence number it expects.
Be able to recognise a TCP header Be able to recognise a UDP header
Source Port Destination Port Source Port Destination Port
Sequence Number Length Checksum
Misc. Flags Window Size
; Proprietary (Cisco only) Data Link (Layer 2) protocol
; L3 protocol and media independent
; Uses L2 multicast to gather hardware and protocol information about directly connected devices.
; Enabled by default; can be disabled globally by no cdp run.
; To learn remote device L3 addresses, hardware platform and IOS ver, use
Show cdp neighbor detail
show cdp entry *
; Ethernet physical addressing = MAC addresses.
o 12 hexadecimal digits
o First six digits are OUI of NIC manufacturer
; PC to switch/hub = straight-through cable
; Hub-hub, switch-switch, PC-PC, router-router, PC-router directly (no switch/hub): use cross-over cable
; Switches, bridges, and routers segment a network. Hubs and repeaters EXTEND a network.
; Switches increase the number of collision domains, do not segment broadcase domains. Routers, L3 switches, and VLANs
segment broadcast domains.
; A switch is a multiport bridge. Switches forward frames using hardware ASIC, making them faster than bridges. Dedicated
bandwidth per port.
; Bridges and switches learn MACs by reading the source MAC of each frame. ; Switches operate in one of three modes:
o Store-and-ForwardL Entire frame is buffered. FCS is run (error checking).
o Cut-throughL Only destination MAC is read, frame is forwarded.
o Fragment-Free: First 64 bytes of frame are buffered, frame is forwarded,. Cisco proprietary. ; Half-duplex: Shared collision domain and lower throughtput
; Full-duplex: Point-to-point and higher throughput
; To remotely manage a switch, you need an IP address, subnet mask, and default gateway. The switch must be reachable on
a port in its maangement VLAN.
; Logically divide a switch into multiple, independent switches at L2 ; Create separate broadcast domains in a switch, increasing the number of broadcast domains ; Span multiple switches using trunks
; Allow logical grouping of users by function
; Simplify adding, moving, and changing hosts in the network
; Enhance security
VLAN configuration steps:
1. The VLAN must be created.
2. The VLAN may be named.
3. The desired ports must be added to the new VLAN.
4. Routing between VLANs requires a router or a Layer 3 switch.
Trunks carry traffic from multiple VLANs over a single connection (cross-over cable). The VLAN ID is tagged using one of two
2. IEEE 802.1q
; A trunk can operate in one of five modes:
o Dynamic Auto
o Dynamic Desirable
VTP (VLAN Trunking Protocol)
VTP simplifies VLAN administration. Configuration of VLANs is distributed to all switches in a VTP domain from a single server-mode
The three VTP modes are as follows:
; Switches must be in the same VTP domain, and must use the same password to exchange VTP information.