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.
Spanning Tree Protocol (STP IEEE 802.1d)
; L2 protocol prevents switching loops in networks with redundant switched paths.
; Root switch is the one with lowest STP Priority: if tied, low MAC is the Root
; Root Port has the least-cost path to the Root switch
; STP path cost is determined by the sum of the costs based on bandiwidth.
; Spanning Tree Topology Ports states:
1. Blocking: Sending no data, listening for BPDUs
2. Listening: Sending and recieving BPDUs
3. Forwarding: Normal operation
; Convergence: 50 seconds (20 sec Max Age + 15 sec Fwd Delay + 15 sec Fwd Delay)
Boot Sequence For Router/Switch
1. POST – Device finds hardware and performs hardware-checking routines.
2. Locate IOS.
3. Load IOS.
4. Locate configuration (startup-config).
5. Load configuration (running-config).
Configuration register settings:
; 0x2102 (default): Checks NVRAM for “boot system” commands; if none, loads first valid IOS in Flash.
; 0x2100: Boots into ROM Monitor mode (ROMMON).
; 0x2101: Boots into ROM RxBoot mode. RxBoot can connect to a TFTP server to download an IOS to Flash.
; 0x2102: Ignores startup-configuration in NVRAM when booting (for password recovery). Memory Components of a router/switch:
; ROM: Basic microcode for starting and maintaining device Power On Self Test (POST), bootstrap, ROM Monitor (ROMMON),
; Flash memory: Stores IOS
; NVRAM: Stores startp-config (configuration loaded at bootup)
; RAM: Running IOS and running-config (active configuration after startup)
Securing Your Router
; To configure a password on all five telnet lines, the configuration will be similar to the following:
Router(config)# line vty 0 4
Router(config-line)# password cisco
Default Administrative Distances:
Connected Interface 0
Static Route 1
EIGRP Internal 90
EIGRP External 170
Router(config)#ip route 192.168.1.0 255.255.255.0 10.1.1.1 The default route syntax is
Router(config)#ip route 0.0.0.0 0.0.0.0 192.168.1.1
Distance Vector Routing Protocols
; Advertise the entire routing table to directly connected neighbours, and send the updates regardles of whether a change
has occurred (every x seconds). RIPv1, RIPv2, IGRP.
Link State Routing Protocols
; Sends updates containing the state of their own links to all other routers on the network.
Examples are OSPF, ISIS.
; Triggers exchange of advertisement by a change in the network.
; Builds and maintains topological database from hello packets and LSAs (Link State Advertisements) from other routers.
; Calculates the paths to each destination from the topological database and places the best of them into the routing table
Classful (FLSM) Versus Classless (VLSM)
; Classful (RIPv1, IGRP, EIGRP by default): Does not advertise subnet masks.
; Classless (RIPv2, IS-IS, OSPF, EIGRP): Advertises subnet mask Route Summarisation
Route summarisation/aggregation/supernetting represents several networks/subnets as one large network address, by shortening
the subnet mask to include only the “in-common” bits from all the networks.
Syntax: directly conneced, classful networks:
; IGRP’s compose metric = Bandwidth, Delay by default; Reliability, Load, and MTU optional.
; IGRP max hop count = 255.
; EIGRP: fast convergence, VLSM support. Multiprotocol support: IP, IPX, AppleTalk. EIGRP maintains routing, topology, and
neighbour tables for each protocol.
; EIGRP metric same as IGRP, but 32-bit versus IGRP 24-bit metric.
; The successor route is the best route, loaded in the route table. Feasible successor is the backup route in the Topology table.
; EIGRP max hop count = 224
; Scalable (unlimited hop count), vendor-neutral, link-state, VLSM support. OSPF areas:
; Areas may be assigned any number from 0 to 65535.
; Area 0 is the backbone area.
OSPF RouterID criteria:
; Highest IP address on a loopback (logical) interface.
; If no loopback, then highest IP address on physical interface. DR/DBR elections only in the following topologies:
1. Broadcast multi-access (for example, Ethernet)
2. Non-broadcast multi-access (for example, Frame-Relay)
To configure OSPF for network 192.168.16.0/24 in area 0
Router(config)#router ospf 7
Router(config-router)#network 192.168.16.0 0.0.0.255 area 0 8The OSPF Metric = cost. Cost = 10/bandwidth in bps.
; Implicit deny any at end: Every access list must have at least one permit, or it denies all traffic.
; Standard IP access lists filter the entire IP protocol based on the source IP address/network.
Number range 1-99. Places as close to destination as possible.
; Extended IP access lists filter based on the source IP address/network, destination IP address/network, specific protocols
(TCP, UDP, ICMP...) and port number. Places as close to source as possible.
; One access list per direction per protocol per interface.
; Wildcard mask: 0s match; 1s ignore corresponding bit in address Extended access-list syntax:
access-list [list#] [permit | deny] [protocol] [source ip] [WCmask] [dest. Ip] [WCmask] [operator] [operand]
The common WAN serial encapsulations are
; Frame Relay
; Encapsulation type must match on both ends of a link
Network Address Translation
; NAT maps private IP addresses to public registered addresses.
; Static: ip nat inside source static [inside ip] [outside ip]
; Inside local: A private IP address assigned to a host on the inside network
; Inside global: A registered Internet address that represents an inside host to an outside network
; Outside global: The registered address of an Internet host
; Outside local: The address of the Internet host as it appears on the inside network Sample PAT configuration, using a pool of addresses to translate to (named MyPool, starting with 220.127.116.11 and ending with
access-list 1 permit 192.168.1.0 0.0.0.255
ip nat pool MyPool 18.104.22.168 22.214.171.124 netmask 255.255.255.240 ip nat inside source list 1 pool MyPool overload
interface Ethernet 0
ip nat inside
interface serial 0
ip nat outside
Vendor-neutral: Cisco to Non-Cisco
Can encapsulate multiple L3 protocols on single L2 link
Configuration of PPP
RtrA(config)#username RtrB password same pass
RtrA(config)#interface bri 0
RtrA(config-if)#ppp authentication chap
RtrB(config)#username RtrA password samepass
RtrB(config)#interface bri 0
RtrB(config-if)#ppp authentication chap
; Two data link layer encapsulation, one for data and one for signaling:
o D channel (16/64 kbps) – LAPD (0.921) for out-of-band signaling
o B channel (64kbps each) – PPP (common), HDLC, SLIP for data
ISDN Reference Points and Interfaces:
; TE1 = native ISDN device
; TE2 = non-ISDN device
; R interface (similar to a serial interface) is for TE2 to TA
; S/T interface: 4 wire
; U interface: 2 wire, connects to Telco (North America/Japan).
; DLCIs identify the circuit (PVC) between the router and the frame switch, DLCI is the L2 address in frame relay.
; LMI is signalling between the router and the local frame relay switch. LMI types are as follows:
o Cisco (the DEFAULT)
; Two Frame Relay encapsulations (must match on both routers):
1. Cisco (default)
; Point-to-point subinterfaces solve Spilt Horizon issues, and map a single subnet to a single DLCI.
; Removes IP address on physical interface if using subinterfaces.
; Must specify sub-if as point-to-point or multipoint – no default since IOS 12.0.
; In order to map Layer 3 IP addresses to Layer 2 DLCIs, Frame Relay uses inverse ARP, or static map:
(config-if)#frame-relay map ip [next-hop-address] [local DLCI] broadcast
; Broadcast keyword allows routing updates over the PVC.
Troubleshooting Commands And Outputs
show interface serial 0
; Serial 0 is Up/Line protocol is up: Interface is working.
; Serial 0 is Up/Line protocol is down: Layer 1 is up. Layer 2 is down (clocking or mismatching frame types).
; Serial 0 is Down/Line protocol is down: Layer 1 down. (Fault or remote end is shut down.)
; Administratively down/Line protocol is down: Interface is shut down and must be no shut.