CCNA -A Short Notes - 1 - Internetworking

Part I – Internetworking

ISO’s(International Organization for Standardization) OSI (Open Systems Interconnection) Model:

Layer	PDUs		Remarks, Examples
7	Application		WWW, E-mail gateways, user interface. Also responsible for understanding the resources needed to communicate between two devices and establish their availability. SMTP, FTP.
6	Presentation		Translates and converts data into a known format such as ASCII, JPEG, MIDI, MPEG, encryption, compression. The only layer that can actually change data.
5	Session	Data	Keeps different applications’ data separate, NFS, SQL, RPC, NetBIOS names, X Window. Offers three modes – full-duplex, half-duplex and simplex. Maintains communication channels and provides dialogue control. Managing, setting up and tearing down sessions.
4	Transport	Segments	Reliable or unreliable delivery, error correction before retransmit, TCP/UDP. Performs flow control, end-to-end connection. Port numbers are used at this layer. Multiplexing, teardown of virtual circuits. Reassemble the data stream.
3	Network	Data and Route Update Packets or Datagrams	Logical addressing, routing, IP and IPX. Route update packets are sent at this layer, in addition to the data packets. Layer 3 devices such as routers break up broadcast domains and collision domains.
2	Data-Link	Frames	Layer 2 devices such as switches or bridges break up collision domains whereas hubs do not. Uniquely identifies each device on a local network. This layer uses service access points, identify network layer protocol used, flow control and sequencing of control bits (LLC – Logical Link Control - sublayer) and deals with the protocol access to the physical medium, network topology and error detection/notification (MAC – Media Access Control - sublayer). A MAC address on a NIC (Network Interface Card) is a 48 bits address formatted in 12 hexa digits grouped in twos as such: AF-98-C0-72-A3-2B
1	Physical	Bits	Moves bits between devices, specify voltages, wire speed and pin-out cables. Hubs are also known as multi-port repeaters and operate at this layer.

A layered model enables different vendors’ products to interoperate (“plug-n-play”), breaks a complex problem into more manageable entities, eases the changing of one layer without changing the other. Realize that breaking up a collision (or broadcast) domain creates more collision domains

Cisco Hierarchical Model:

Layer	Examples
Core	Large amounts of traffic reliably and quickly. Fault tolerance important. Don’t use VLAN, access lists or packet filtering at this layer. Cisco recommends using layer 2 switches at this layer.
Distribution	Provides routing, filtering and WAN access. Place to implement policies on a network (packet filtering, access lists, queuing, security and network policies, address translation, firewalls, redistribution between routing protocols, static routing, routing between VLAN and other workgroup support functions, definition of broadcast and multicast domains. Cisco recommends using routers at this layer.
Access	Continued access control and policies from the distribution layer, creation of separate collision domains (and segmentation of contention networks). Cisco recommends using layer 2 switches at this layer.

IEEE Ethernet (MAC) Standards

IEEE Number Standard 802.3 Ethernet 802.3u Fast Ethernet Uses MII (Media Independent Interface) and transmits using nibbles (4 bits at a time) 802.3z Gigabit Ethernet Uses GMII (Gigabit MII) and transmits 8 bits at a time.

Ethernet Physical Media (created by Digital Equipment, Intel and Xerox):

10Base2	Thinnet 50-ohm coax 185m, 30 hosts per segment Physical and logical bus with AUIs	100BaseFX	Fiber cabling 62.5/125-micron multimode fiber point-to-point, 400m ST or SC connectors
10Base5	Thicknet 50-ohm coax 500m and 208 hosts per segment Physical and logical bus with AUIs	1000BaseCX	Copper shielded twisted-pair 25m
10BaseT	EIA/TIA cat 3,4 or 5, using two-pair unshielded twisted-pair (UTP) wiring. 100m and 1 user per segment Physical star and logical bus with RJ-45	1000BaseT	Cat 5, four-pair UTP wiring, 100m
100BaseTX	EIA/TIA cat 5,6 or 7 UTP two-pair wiring 100m and 1 user per segment physical star and logical bus with RJ-45 MII	1000BaseSX 1000BaseLX	Multi-mode fiber 62.5/50-micron, 260m Single-mode fiber 9-micron core, 10km

Straight-through vs crossover cables (wire 1<-> wire 3, wire 2<-> wire 6): 

Considering the devices grouped in two categories: 1-switches / hubs / bridges, 2-workstations / servers / routers

If changing category, use a straight through cable, else use a crossover cable. (or use straight-through when one of the port is marked with an X)

Ethernet Auto-Negotiation: determines the link speed and duplex status.

Ethernet Frames:

Ethernet II – uses a two-byte type field instead of the length.

802.3 – cannot identify the upper-layer protocol

802.2 (802.3 with LLC information in the data field of the header) – able to identify the upper layer protocol

SNAP – Subnetwork Access Protocol – used in AppleTalk and Cisco Discovery Protocol)

Half-Duplex: contention net using CSMA/CD (Carrier Sense Multiple Access/Collision Detection) and a backoff algorithm when collision occur.

Full-Duplex: two communication paths are required and compatible full-duplex NICs. Loopback and collision detection must be disabled. Sets up a point-to-point connection with the remote device. There are no collisions on a Full-Duplex link.

Ring LAN:

Token Ring: standard created by IBM and reflected in IEEE 802.5 with speeds of 4 or 16 Mbps. Stations cannot transmits until they have the token, which they can reserve using the Reservation Bits.

FDDI (Fiber Distributed Data Interface): token-passing ANSI standard providing LAN speed of up to 200 Mbps if dual rings are active. Only LAN topology that is both physical and logical ring.

MSAU: MultiStation Access Unit, the controller of the token ring LAN, for up to 8 stations.

NAUN: Nearsest Active Upstream Neighbour

Active Monitor: one station on the ring always ensures there is only ever one token on the ring.

Beaconing: process by which a station attempts to determine a network failure.