Course Review Series CCENT Review 1-800-COURSES CCENT Review Rick Chapin, Global Knowledge Instructor Introduction CCENT has been created to address the need for providing networking professionals with a solid practical understanding of modern TCP/IP networks built with Cisco hardware, and will certify practical skills required for entry-level network support positions. This certification will serve as the base of Cisco's certification pyramid.It is similar in nature to CompTIA's Network+ Certification and represents a tangible first step in earning your CCNA certification. This document is intended to help students gain an understanding of the basic network fundamentals prior to attending our ICND1 – Interconnecting Cisco Network Devices 1course (and exam 640-822 ICND1) or our CCNA Boot Camp.This review is intended only as a preview and additional training/knowledge may be needed in order to attend the ICND1 course or the CCNA Boot Camp. Please note:This document is not intended to replace hands-on course work. Table of Contents Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 2 OSI Reference Points OSI Layer Upper or Data Network Reference Network Device Flow Layer 7 – Application Upper 6 – Presentation Upper 5 – Session Upper PDU or Message 4 – Transport Data Flow Segment 3 – Network Data Flow Packet or Datagram MultiLayer Switch or Router 2 – Data Link Data Flow Frame Switch or Bridge 1 – Physical Data Flow Bits and Signaling Hub OSI Reference Points Remembered:Please Do Not Throw Sausage Pizza Away. OSI Layers OSI Layer Purpose Examples Application Provides services to network applications.• Simple Mail Transport Protocol (SMTP) This layer is responsible for determining• Telnet resource availability, identifying communi- • File Transfer Protocol (FTP) cations peers, and synchronizing communi-• Trivial File Transfer Protocol (TFTP) cations between the applications. • HyperText transfer Protocol (HTTP) PresentationProvides the coding and conversion func-• ASCII (text) tions that are applied to the data to/from• EBCDIC (text) the Application layer. This layer ensures • JPEG (image) that there is a common scheme used to• GIF (image) bundle the data between the two ends.• TIFF (image) There are various examples and this list is by• MPEG (sound/video) no means complete. Text can be either• Quicktime (sound/video) ASCII or EBCDIC. Images can be JPEG, GIF, or TIFF. Sound can be MPEG or Quicktime. Session Maintains communications sessions • Session Control Protocol (SCP) between upper-layer applications. This• Remote Procedure Call (RPC) from layer is responsible for establishing, main-Unix taining, and terminating such sessions• Zone Information Protocol (ZIP) from AppleTalk Transport Responsible for end-to-end data transmis-• Transmission Control Protocol sion. These communications can be either(TCP) from IP reliable (connection-oriented) or non-reli- • User Datagram Protocol (UDP) able (connectionless). This layer organizesfrom IP data from various upper layer applications into data streams. The transport layer also handles end-to-end flow control, multiplex- ing, virtual circuit management, and error checking and recovery. Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 3 OSI Layers continued Network Uses administrator-defined logical address- • Internet Protocol (IP) ing to combine many data flows into an internetwork. This layer allows both con- nection-oriented and connectionless data flows to access the network. The network layer addresses help define a network hier- archy. Network devices are normally grouped together based on their common Network Layer address. Data Link Provides either reliable or non-reliableLAN: transmission of data across a physical medi-• Ethernet/IEEE 802.3 (include Fast um. Most networks use a non-reliable dataEthernet) link layer, such as; Ethernet or Token Ring.• 802.3z (Gigabit Ethernet) The data Link Layer provides a physical• Token Ring /IEEE 802.5 address to each device called a Media• FDDI (from ANSI) Access Control (MAC) address. MAC addresses are typically burned into the net-WAN: work interface card (NIC). The Data Link• High-Level Data-link Control Layer also uses a Logical Link Control (LLC)(HDLC) to determine the type of Network Layer• Point-to-Point Protocol (PPP) data is traveling inside the frame. • Frame Relay Physical Defines the electrical, mechanical, and func-LAN: tional specifications for maintaining a physi- • Category 3 cabling (LAN) cal link between network devices. This • Category 5 cabling (LAN) layer is responsible for such characteristics as voltage levels, timing and clock rates,WAN: maximum transmission distances, and the • EIA/TIA-232 physical connectors used. • EIA/TIA-449 • V.35 Network Hierarchy Layer Purpose Network Device Core To move network traffic as fast as possible.• High-speed routers Characteristics include fast transport to enterprise• Multi-layer switches services and no packet manipulation. DistributionPerform packet manipulation such as filtering• Routers (security), routing (path determination), and WAN access (frame conversion). The distribution layer collects the various access layers. Security is implemented her, as well as broadcast and multi- cast control. Media translation between LAN and WAN frame types also occurs here. Access Where end-stations are introduced to the net-• Switches work. This is the entry point for virtually all • Bridges workstations. • Hubs Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 4 LAN Switch Functions Function Purpose Address LearningDynamically learns MAC addresses that arrive in the switch by reading the sources MAC address of each arriving frame. If this address is not in the cur- rent MAC table, and there is enough space to store it, the address and the inbound port are stored. Forward/Filter Compare the destination MAC address of the arriving frame to the dynami- cally-learned MAC table. If the address is in the table only forward the frame out the port specified in the table, thus filter it from other ports. If the MAC address is not in the MAC table (unknown MAC address) or it is a broadcast or multicast frame, the frame is flooded out every other port except the one it arrived from. Loop Avoidance Since the default behavior of a switch is to forward unknown unicast, broad- cast, and multicast frames, it is possible for one frame to Loop endlessly through a redundant (multiple path) network. Thus the Spanning tree Protocol (STP) is turned on to discourage loops in a redundant switch network. Sources of Switching/Bridging Loops Source Description Redundant Unknown Frames are flooded out all ports. If there are multiple paths, than Topology a flood would go out all ports, except the originator, and come back in on the other ports thus creating a loop. Multiple Frame Two machines live (connect) on the same wire. They send frames to each Copies other without assistance. If there are two bridges/switches attached to the same wire, who are also connected together, then new frames (unknown) going from one machine (same wire) would go directly to the other machine (same wire) and would also be flooded through the Bridges/switches (connect- ed wire) and be flooded back through the bridges/switches to the original wire. The receiving machine would receive multiple copies of the same frame. MAC Database Thanks to a Bridging/switching loop (senairo above) one bridge/switch learns Instability the same MAC address on different ports. Thus, if a bridge/switch needed to forward a frame to its destination MAC address, it would have two possible destination Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 5 Solutions To Switching/Bridging Loops Source Description 802.1d Spanning Aprotocol that prevents loops from being formed when switches or bridges Tree Protocol (STP) are interconnected via multiple paths. Spanning-Tree Protocol implements the 802.1D IEEE algorithm by exchanging Bridge Protocol Data Unit (BPDU) messages with other switches to detect loops, and then removes the loop by shutting down selected bridge interfaces. The switches that are running STP will elect a Root Switch to use as a comparison point in determining which path will shutdown. To assist in determining which path to use the BPDU carries information such as the Bridge ID, path cost, and the Root ID. This algorithm guarantees that there is one and only one active path between two network devices. 802.1w Rapid Rapid Spanning Tree Protocol (RSTP) is an evolution of the Spanning Tree Spanning Tree Protocol (802.1D standard) and provides for faster spanning tree convergence Protocol (RSTP) after a topology change. The standard also includes features equivalent to Cisco PortFast, UplinkFast and BackboneFast for faster network re-convergence. Comparison of Bridges and Switches Bridges Switches Software-based Hardware-based (port-level ASICs) Relatively slow Comparatively fast One STP per bridge Possibly many STPs per switch (possibly one per VLAN) Typically up to 16 ports Possibly hundreds of ports Forwarding Modes in a Switch Mode Description Latency Store-and-Forward The entire frame is buffered, the CRC isRelatively High. Varies examined for errors and frame is checkeddepending on frame size. for correct sizing (Ethernet 64 – 1518 bytes). Cut-Through The frame is forwarded once the destina-Lowest. Fixed delay based on tion MAC address (first 6 bytes) arrives and6 bytes being buffered. Not is checked against the MAC address table.configurable on a Catalyst Buffer until the 6th byte arrives. 1900. Fragment-Free The frame is forwarded once the first 64Low. Fixed delay based on 64 (Cisco) bytes have arrived. Buffering occurs until bytes being buffered. Default the 64th byte arrives. Ethernet collisionson Catalyst 1900. usually occur within the first 64 bytes, thus if 64 bytes arrive there is no collision. Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 6 Half-Duplex vs.Full Duplex Duplex Type Advantages Defaults Half-Duplex • Network devices use the same pair of wire to both trans-10 Mbps. 100 Mbps mit and receive ports if not config- • Only possible to use 50% of the available bandwidth –ured for full-duplex must use the same bandwidth to send and receiveor cannot be Auto- • Available bandwidth decreases as the number of devicessensed. in the broadcast domain increases • Used through hubs (layer 1 devices) – everyone shares the available bandwidth Full-Duplex • Uses one pair of wire for sending and another pair for100 Mbps ports if receiving. manually configured • Effectively provides double the bandwidth – possible tofor full-duplex or send and receive at the same time. can be Auto-sensed • Must be point-to-point stations, such as pc/server to switch or router to switch. • Everyone has their own collision domain (individual bandwidth) on each switch port. LAN Segmentation = dividing up the size of the collision domains Device Abilities Bridge Examines destination MAC address and makes filtering/forwarding decisions based on it. Unknown, Broadcast, and Multicast frames are flooded out all ports except the originator. Each port of a bridge is a collision domain. Switch (VLANs)Examines destination MAC address and makes filtering/forwarding decisions based on it. Unknown, Broadcast, and Multicast frames are flooded out all ports within that VLAN except the originator. Each port of a switch is a collision domain. Each VLAN is a broadcast domain. Benefits include simplifying moves, adds, and changes, reducing administrative costs, controlling broadcasts, tight- en security, load distribution, and moving servers into a secure location. Router Examines destination network (logical – layer3) address and makes filtering/forwarding decisions based on it. Unknown and broadcast frames are discarded. Each port of a router is both a collision and broadcast domain. Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 7 TCP/IP Layers Protocol OSI Reference Function Transmission Control Transport Layer – Layer 4Reliable, connection-oriented, uses sequence Protocol (TCP) and acknowledgement numbers to provide reli- ability verifies that the remote end is listening prior to sending data (handshake). User Datagram Transport Layer – Layer 4Non-reliable, connectionless, no sequence or Protocol (UDP) acknowledgement numbers, and no far-end verification. Internet Protocol (IP) Network Layer – Layer 3 Provides the logical addressing structure. Offers connectionless, best-effort delivery of packets (datagrams). Port Numbers Well-known port numbers are 1 – 1023 (typically used for well-known applications),random port numbers are 1024 and above (typically random numbers are used by the client in a client/server application). Application Port Transport File Transfer Protocol (FTP) 20/21 TCP Telnet 23 TCP Simple Mail Transfer Protocol (SMTP) 25 TCP Domain Name Services (DNS) 53 TCP Domain Name Services (DNS) 53 UDP Trivial Files transfer Protocol (TFTP) 69 UDP Simple Network Management Protocol (SNMP) 161/162 UDP Routing Information Protocol (RIP) 520 UDP IP Protocols Protocol Purpose Internet Control MessageProvides control and feedback messages between IP devices. Protocol (ICMP) Address Resolution ProtocolUsing a destination IP address, ARP resolves or discovers the (ARP) appropriate destination MAC (layer 2) address to use. Map a Layer 3 address to a Layer 2 address. Reverse Address ResolutionUsing a source MAC address, RARP retrieves an IP address form Protocol (RARP) the RARP Server. Map sources Layer 2 address to a Layer 3 address. RARP is an early form of BOOTP and DHCP. Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 8 IP Addresses Class First Binary Numerical Number of Number of Number of Number of Bits Range Networks Hosts per Network Hosts Network Octets Octets A 0xxx 1 – 126* 126 16.5 million1 (N.H.H.H) 3 B 10xx 128 – 191 16 thousand 65 thousand 2 (N.N.H.H)2 C 110x 192 – 223 2 million 254 3 (N.N.N.H) 1 D** 111x 224 – 239 N/A N/A N/A N/A E** 1111 240 – 255 N/A N/A N/A N/A * 127 is used for the Loopback address ** Class D is used for Multicast Group addressing and Class E is reserved for research use only Subnetting Number of networks: 2s – 2, where s = number of bits in the subnet (masked) field. Number of hosts per subnet: 2r – 2, where r = number of host (non-masked) bits. R + S = 32 (always), since there are 32 bits in an IP address and each bit is either a network or host bit.S is the bit(s) after the standard Class number of bits (Mask – Class Bits = S). Subnet Masks 1s in the subnet mask match the corresponding value of the IP address to be Network bits. 0s in the subnet mask match the corresponding value in the IP address to be Host bits. Default Subnet Masks Default Class A mask – = N.H.H.H Default Class B mask – = N.N.H.H Default Class C mask – = N.N.N.H Possible Subnet Mask Values for One Octet Decimal Mask Binary Mask Network Bits Host Bits 0 00000000 0 8 128 10000000 1 7 192 11000000 2 6 224 11100000 3 5 240 11110000 4 4 248 11111000 5 3 252 11111100 6 2 254 11111110 7 1 255 11111111 8 0 Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 9 Possible Class C Subnet Masks Decimal Mask Network Bits (x) Host Bits (y)Number of Number of s r Subnets 2 – 2 Hosts 2 – 2 0 8 0 254 1 7 N/A N/A 2 6 2 62 3 5 6 30 4 4 14 14 5 3 30 6 6 2 62 2 7 1 N/A N/A 8 0 N/A N/A Routing The process of maintaining a table of destination network addresses.A router will discard packets for unknown networks. Sources of Routing Information Source Description Static • Manually configured by an administrator • Must account for every destination network • Each static route must be configured on each router • No overhead in processing, sending, or receiving updates • Saves bandwidth and router CPU • Routing table maintained by administrator Dynamic • A process that automatically exchanges information about available routes • Uses metrics to determine the best path to a destination network • The routing protocol must be configured on each router • Bandwidth is consumed as routing updates are transmitted between routers • Router CPU is used to process, send, and receive routing information • Routing table maintained by routing process Types of Routing Protocols Type Description Interior • Used within a common administrative domain called an Autonomous System (AS) • Typically a single AS is controlled by a single authority or company • Interior routing protocols are used within a corporate network Exterior • Used to connect Autonomous Systems • Exchanges routing information between different administrative domains • Exterior protocols are used to connect sites within a very large corporate network, or are used to connect to the Internet Copyright ©2007 Global Knowledge Training LLC. All rights reserved. Page 10