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Module 10Routing Fundamentals

and Subnetting

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Routable Protocols

• A protocol is a set of rules that determines how computers communicate with each other across networks

• A protocol describes the following: – The format that a message must conform to – The way in which computers must exchange a

message within the context of a particular activity

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Routed Protocols

• A routed protocol allows the router to forward data between nodes on different networks. – it must have the ability to assign a network number and

a host number to each individual device

• These protocols also require a network mask (subnet mask) in order to differentiate between the network and host portion of the address.

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Routable Protocols

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Encapsulation Process

Segments

Packets

Frames

Bits

Data

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Transport Layer

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Internet Protocol (IP)

• The Internet Protocol (IP) is the most widely used network-addressing scheme

• IP is a connectionless, unreliable, best-effort delivery protocol

• IP does not verify that the data reaches its destination; this function is handled by the upper layer protocols

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IP

• Data is encapsulated into packets, also known as datagrams at the network layer of the OSI model.

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Connection-Oriented vs. ConnectionlessNetworks

• Connectionless Networks– Often referred to as packet-switched processes – IP Protocol– Internet

• Connection-Oriented Networks– Often referred to as circuit-switched processes – A connection with the recipient is first established, and

then data transfer begins – TCP adds Layer 4, connection-oriented reliability

services to IP

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IP Header

• IP header length (HLEN) – Indicates the datagram header length in 32-bit words.

• Padding – extra zeros are added to this field to ensure that the IP header is always a multiple of 32 bits.

• Flags – A three-bit field in which the two low-order bits control fragmentation

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Network Layer

• Routing is an OSI Layer 3 function

• Routing is the process of finding the most efficient path from one device to another

• The primary device that performs the routing process is the router

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Routers

• A router is a network layer device that uses one or more routing metrics to determine the optimal path along which network traffic should be forwarded

• Routers make logical decisions regarding the best path for the delivery of data

Routing metrics are values used in determining the advantage of one route over another

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Routers• The encapsulation and de-encapsulation process occurs each time a

packet transfers through a router

• This process breaks up the data stream into segments, adds the appropriate headers and trailers then transmits the data. The de-encapsulation process is the opposite process, removing the headers and trailers, then recombining the data into a seamless stream

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Routing versus Switching

• Switching occurs at Layer 2 of the OSI model

• Routing occurs at Layer 3

• The Layer 2 switch can only recognize its own local MAC addresses and cannot handle Layer 3 IP addresses

• A Layer 2 switch interconnects segments belonging to same logical network or subnetwork

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Routing versus Switching

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Routing versus Switching

• Each computer and router interface maintains an ARP table for Layer 2 communication. The ARP table is only effective for the broadcast domain (or LAN) that it is connected to.

• The router also maintains a routing table that allows it to route data outside of the broadcast domain

• Each ARP table contains an IP-MAC address pair

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Routing versus Switching

• The Layer 2 switch can only recognize its own local MAC addresses and cannot handle Layer 3 IP addresses

• When a host has data for a non-local IP address, it sends the frame to the closest router also known as its default gateway

• The host uses the MAC address of the router as the destination MAC address

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Routing versus Switching

• Another difference between switched and routed networks is switched networks do not block broadcasts

• Routers block LAN broadcasts

• Because routers block broadcasts, routers also provide a higher level of security and bandwidth control than switches

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Routed versus Routing Protocols• Routed protocols transport data

across a network

• Examples of routed protocols include:– Internet Protocol (IP)– Novell's Internetwork

Packet Exchange (IPX)– DECnet– AppleTalk– Xerox Network Systems

(XNS)

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Routed versus Routing Protocols• Routing protocols allow routers to

choose the best path for data from source to destination

• Routers use routing protocols to exchange routing tables and share routing information

• Routing protocols enable routers to route routed protocols

• Examples of routing protocols include:– Routing Information Protocol (RIP),

Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and Enhanced IGRP (EIGRP).

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Path Determination

• Path determination occurs at the network layer

• It enables a router to compare the destination address to the available routes in its routing table, and to select the best path

• The routers learn of these available routes through static routing or dynamic routing – Routes configured manually by the network administrator are

static routes – Routes learned by others routers using a routing protocol are

dynamic routes

• Each router that the packet encounters along the way is called a hop. The hop count is the distanced traveled

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Routing Tables

• Routers use routing protocols to build and maintain routing tables that contain route information

• Routers keep track of important information in their routing tables, including the following: – Protocol type – The type of routing protocol that created the

routing table entry

– Destination/next-hop associations

– Routing metric

– Outbound interfaces

• Routers communicate with one another to maintain their routing tables through the transmission of routing update messages

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Routing algorithm and metrics

• Routing protocols often have one or more of the following design goals: – Optimization – Simplicity and low overhead – Robustness and stability – Flexibility

• Routing algorithms use different metrics to determine the best route. The most commonly used metrics include:– Bandwidth Delay – Load Reliability – Hop count – Ticks – Cost

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IGPs and EGPs

• Two types of routing protocols are Interior Gateway Protocols

(IGPs) and Exterior Gateway Protocols (EGPs)

• IGPs route data within an autonomous system.

• EGPs route data between autonomous systems

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IGPs and EGPs

• Examples of IGPs include:

– Routing Information Protocol (RIP) and (RIPv2)

– Interior Gateway Routing Protocol (IGRP)

– Enhanced Interior Gateway Routing Protocol (EIGRP)

– Open Shortest Path First (OSPF)

– Intermediate System-to-Intermediate System Protocol (IS-IS)

• An example of EGP is:

– Border Gateway Protocol (BGP)

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Link State and Distance Vector

• IGPs can be further categorized as either distance-vector or link-state protocols

• Distance-vector routing determines the distance and direction to any link in the internetwork

– Routers using distance-vector algorithms send all or part of their routing table entries to adjacent routers on a periodic basis

• Link-state algorithms typically use their databases to create routing table entries that prefer the shortest path

– Link-state routing protocols respond quickly to network changes sending trigger updates only when a network change has occurred

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Distance-vector Routing Protocols• Examples of distance-vector protocols include:

– Routing Information Protocol (RIP) – The most common IGP in the Internet, RIP uses hop count as its only routing metric. RIP cannot route a packet beyond 15 hops.

– Interior Gateway Routing Protocol (IGRP) – This IGP was developed by Cisco to address issues associated with routing in large, heterogeneous networks. IGRP can select the fastest available path based on delay, bandwidth, load, and reliability. IGRP also has a much higher maximum hop count limit than RIP.

– Enhanced IGRP (EIGRP) – This Cisco-proprietary IGP includes many of the features of a link-state routing protocol. EIGRP provides superior operating efficiency such as fast convergence and low overhead bandwidth Because of this, it has been called a balanced-hybrid protocol, but it is really an advanced distance-vector routing protocol

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Link-State Routing Protocols

• Examples of link-state protocols include:

– Open Shortest Path First (OSPF)

– Intermediate System-to-Intermediate System (IS-IS). used

for routed protocols other than IP

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Subnetting• Subnetting provides manageability, enables the network

administrator to provide broadcast containment, and low-level security on the LAN.

• Subnet addresses include the Class A, Class B, and Class C network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion of the major IP address.

• A LAN is seen as a single network with no knowledge of the internal network structure. This view of the network keeps the routing tables small and efficient.

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Subnet Mask

• The subnet mask gives the router the information required to determine in which network and subnet a particular host resides

• The subnet octet or octets are determined by adding the position value of the bits that were borrowed. If three bits were borrowed, the mask for a Class C address would be 255.255.255.224.

• This mask may also be represented, in the slash format, as /27. The number following the slash is the total number of bits that were used for the network and subnetwork portion.

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Subnetting Formula

• Number of usable subnets equals two to the power of the assigned subnet bits or borrowed bits, minus two (reserved addresses for subnetwork id and subnetwork broadcast)

2 power of borrowed bits –2 = usable subnets23 = 8 - 2 = 6 usable subnets

• Number of usable hosts equals two to the power of the bits remaining, minus two (reserved addresses for subnet id and subnet broadcast)

2 power of remaining host bits –2 = usable hosts25 = 32 – 2 = 30 usable hosts per subnet

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ANDing Process

• Routers use subnet masks to determine the home subnetwork for individual nodes. This process is referred to as logical ANDing.

• ANDing is a binary process by which the router calculates the subnetwork ID for an incoming packet.