Tài liệu Networking Theory doc

➤ General routing concepts—Includes reviewing split horizon, poison reverse, rec-ognizing the differences between switching and routing, the importance andtechniques of route summarizat

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Networking Theory

Terms you’ll need to understand:

✓ Open Systems Interconnection

(OSI) model

✓ Peer-to-peer communication

✓ Transmission Control Protocol (TCP)

✓ Internetwork Packet Exchange (IPX)

Techniques you’ll need to master:

✓ Identifying and describing the

functions of each layer of theOSI model

✓ Comparing IP and IPX, and

explaining the protocols’ functions

✓ Comparing TCP and UDP, and

explaining the protocols’ functions

✓ Using common routing commands

on Cisco routers

✓ Understanding frame formats for

IP, TCP, UDP, and IPX

This chapter addresses the CCIE blueprint objectives as laid out by the CiscoSystems CCIE program Specifically, the chapter reviews the following topics:

➤ OSI model—Encompasses understanding the functions of the OSI model’s

layers and how each layer compares to the other layers

➤ General routing concepts—Includes reviewing split horizon, poison reverse,

rec-ognizing the differences between switching and routing, the importance andtechniques of route summarization, comparing link state versus distance vec-tor protocols, discussing routing loops, understanding tunneling, and review-ing IP routing tables

➤ Protocol operation—Includes understanding Windowing/Acknowledgments

(ACK), fragmentation, maximum transmission units (MTU), handshaking,and termination

➤ Protocol descriptions and use—Reviews Internet Protocol (IP), IP

fragmenta-tion, Transmission Control Protocol (TCP), User Datagram Protocol (UDP),and Internetwork Packet Exchange (IPX)

➤ IEEE 802.x standards—Reviews the various 802.x protocol descriptions.

Open Systems Interconnect (OSI) Model

Before discussing any protocols, you need to have a thorough understanding ofthe OSI model This chapter focuses on the function of each layer of the OSImodel By working through this chapter, you will obtain an understanding of thefunctions performed by each layer If you can understand how each layer per-forms, then you will be able to understand how a protocol functions Therefore,this section focuses on the OSI model and what you, as a potential CCIE profes-sional, need to know

OSI Model Structure

The OSI model consists of seven layers and is an international standard thatenables vendors, such as Cisco, to adhere to certain criteria This will enable, forexample, a Windows PC to communicate with a Unix workstation Table 2.1displays the framework of the OSI reference model

Peer-to-Peer Communication

Each layer of the OSI model has its own function and interaction with the layers

above and below it Furthermore, there is also peer-to-peer communication tween end devices through each corresponding layer of the OSI model Peer-to-

be-peer communication means that each layer of the OSI model uses its own protocol

to communicate with its equivalent peer layer in another system For example,

the Transport layer of Device A in Figure 2.1 will communicate with the port layer in Device B, assuming there are no intermediate devices The layersbetween the two end stations communicate via protocol data units (PDUs).

Trans-In other words, each layer communicates to the corresponding layer above andbelow it and also exchanges protocol data units (PDU is an OSI term for a packet)between end systems Figure 2.1 shows how each layer of the OSI model pro-vides services to the layers above and below The PDU exchanges are represented

by the horizontal lines in Figure 2.1

Note: Layers 1 and 2 of the OSI model are implemented with hardware Layers 3

through 7 are implemented in software.

Table 2.1 The OSI reference model.

Application Presentation Session Transport Network Data Link Physical

Peer Communication

Device A Device B

OSI Model Layers

The following sections provide descriptions and typical examples of each OSIlayer Furthermore, examples of communication methods and functions followeach layer description

Layer 1: Physical Layer

The Physical layer consists of standards that describe bit ordering, bit sion rates, connector types, and electrical specifications Information is transmit-ted as binary bits (ones and zeros) Examples of Physical layer standards includethe following:

Layer 2: Data Link Layer

The Data Link layer will focus on getting data reliably across any particular kind

of link Flow control and error notifications are other functions of the Data Linklayer, as well The Data Link layer applies to all access methods whether they areLAN or WAN methods Information being processed at this layer is commonly

known as frames Examples of data link frame types include the following:

Layer 3: Network Layer

The Network layer is used to determine the best path to a destination Deviceaddressing, packet fragmentation, and routing all occur at the Network layer

Information being processed at this layer is commonly known as packets

Ex-amples of Network layer protocols include the following:

➤ Internet Protocol (IP)

➤ Internetwork Packet Exchange (IPX)

At the Network layer, a packet is associated with a connection-orientedprotocol, while a datagram is associated with a connectionless protocol.

Layer 4: Transport Layer

The Transport layer is responsible for segmenting upper-layer applications andestablishing end-to-end connections between devices Other functions of theTransport layer include providing data reliability and error-free delivery mecha-

nisms Information being processed at this layer is commonly known as segments.

Examples of Transport layer protocols include the following:

➤ Transmission Control Protocol (TCP)

➤ Novell’s Sequenced Packet Exchange (SPX)

➤ User Datagram Protocol (UDP)

Layer 5: Session Layer

The Session layer performs several major functions, including managing sessionsbetween devices, and establishing and maintaining sessions Examples of Sessionlayer protocols include the following:

➤ Database SQL

➤ NetBIOS Name Queries

➤ NetBEUI

Layer 6: Presentation Layer

The Presentation layer handles data formats and code formatting The functions

of this layer are normally transparent to the end user, because this layer will takecare of code formats and present them to the Application layer (layer 7) wherethe end user can examine the data Examples of Presentation layer protocols in-clude the following:

➤ GIF

➤ JPEG

➤ ASCII

➤ MPEG

Layer 7: Application Layer

The Application layer is closest to the end user, which means that the application

is being accessed by the end user The major function of this layer is to provideservices to end users Examples of Application layer services include the following:

➤ File Transfer Protocol (FTP)

➤ Telnet

➤ SMTP

➤ HTML browsers

How Data Flows through the OSI Layers

To get a better understanding of how the OSI layers function, it is important toknow how data flows between the layers In this section, we’ll trace the data as itflows through the layers of the OSI model As you will see in this section, each

layer adds (or encapsulates) some form of header or trailer (Layer 2, the Data

Link layer, is responsible for adding a trailer.) Figure 2.2 shows the data flowfrom Device A to Device B

Note: The example in Figure 2.2 demonstrates how end user packets (header and data)

flow through the OSI model The figure assumes there are no intermediate devices.

When the end system receives the unstructured bit stream from the physicalwire, each layer removes the header information applicable to it until the applica-tion receives the data The following depicts what occurs in the OSI model’slayers when an email is sent from Device A to Device B:

1 An application, such as an email program, creates data that will be sent

by an end user, such as an email message The Application layer (layer 7)places a header (encapsulation) field that contains information such asscreen size and fonts, and passes the data to the Presentation layer (layer 6)

2 The Presentation layer places layer 6 header information For example,the text in the message might be converted to ASCII The Presentationlayer will then pass the new data to the Session layer (layer 5)

3 The Session layer follows the same process by adding layer 5 header formation, such as information that the Session layer will manage thedata flow, and passes this data to the Transport layer (layer 4)

in-4 The Transport layer places layer 4 information, such as an ment that the segment was received in the header, and passes it to theNetwork layer (layer 3)

acknowledg-5 The Network layer places layer 3 header information, such as the sourceand destination address so the Network layer can determine the best

delivery path for the packets, and passes this data to the Data Link layer(layer 2).

6 The Data Link layer places layer 2 header and trailer information, such

as a Frame Check Sequence (FCS) to ensure that the information is notcorrupt, and passes this new data to the Physical layer (layer 1) for trans-mission across the media

7 The bit stream is then transmitted as ones and zeros on the Physicallayer It is at this point that the Physical layer ensures bit synchroniza-tion Bit synchronization will ensure the end user data is assembled inthe correct order it was sent

8 Steps 1 through 7 occur in reverse order on the destination device vice B collects the raw bits from the physical wire and passes them up the

Transport header (TH) Session header (SH) Presentation header (PH) Application header (AH)

Path to Device B

Bits received

by Device B

Data link trailer

Figure 2.2 End user header and trailer flow.

Data Link layer The Data Link layer removes the headers and trailersand passes the remaining information to the Network layer and so forthuntil data is received by the Application layer Eventually, Device B willreceive an email notification displaying a message to indicate that a newemail message has been received.

Familiarize yourself with the OSI model and each layer’s responsibility.You should be able to recognize a function of each layer of the OSImodel The seven layers of the OSI reference model are typicallydivided into two categories: upper layers (layers 4 through 7) and lowerlayers (layers 1 through 3)

As you can determine from the example of encapsulation, the OSI model vides a service that allows information to flow smoothly from one layer to an-other Eventually, the information will be presented to the end device in a readableformat Now that we’ve reviewed the OSI model, the next section takes a look athow packets are sent across a network using a routing algorithm

pro-General Routing Concepts

Routing simply means moving a packet from one location to another Routing

uses best-effort delivery and occurs at layer 3 (the Network layer) of the OSI model.

An example of a routing protocol that routes IP is Routing Information Protocol

(RIP) Routing protocols provide the information required to determine the

to-pology of the internetwork and the best path to a destination A routed protocol

is one that is routed by a routing protocol such as RIP IP is an example of arouted protocol The following sections discuss the differences between a routedand routing protocol and provide some common examples

In contrast to routing, switching is the moving of a frame or framesfrom one location to another Switching occurs at layer 2 in the OSImodel An example of a switching protocol is transparent bridging

Note: Chapter 3 describes the available bridging and switching modes available on a

Cisco router.

Routing Vs Routed Protocols

Routing protocols apply a set of rules to a network topology to determine the best

path to a destination from a given reference point They also communicate work topology information to other routers in their networks Routing protocolsbuild routing tables from the gathered information Examples of routing proto-

net-cols are Open Short Path First (OSPF) and IPX’s Routing Information Protocol(IPX RIP).

In contrast, a routed protocol is a protocol that contains layer 3 information that

allows it to be moved from one destination to another Examples of a routedprotocol include IP and IPX

Routing protocols can be divided into three types—distance vector, link state,and hybrid These three routing protocol classifications are discussed in the fol-lowing sections

Distance Vector Protocols

Distance vector protocols, such as RIP, determine a path to a network using hop

count as the metric A hop count is a number that increments each time a packet

traverses a router

Convergence—the process that ensures all routers in a network have the samenetwork information as quickly as possible—of distance vector protocols is con-siderably slower, and periodic updates are sent at set intervals Figure 2.3 showshow networks are discovered when using a distance vector protocol

Each router in Figure 2.3 will have the same IP routing table and will send andreceive periodic updates Not every routing protocol sends out periodic updates

at the same interval The distance vector protocol IP RIP sends a periodic updateevery 30 seconds

Link State Protocols

Link state protocols, such as IS-IS and OSPF, create a topology of the networkwith each router running that protocol as the root of the tree Link state proto-cols implement the shortest path first (SPF) algorithm to determine the path to

a network The metric used by these protocols is cost, which is determined by anadministrator or calculated by the routing protocol based on a mathematical for-mula A network with the lowest cost is chosen as the preferred path to a remotenetwork Link state protocols have no concept of hop count The speed of con-vergence with link state protocols is much faster when a network change occurs.This is because a faster algorithm is used and the CPU is heavily utilized tocompute changes rapidly When using link state protocols, updates are only sentwhen a topological change occurs or at an interval set by an administrator

Link state protocols use hello packets to discover neighbors A hello packet is an

IP packet sent at regular intervals When a topology change occurs, a link statepacket is sent to all neighbors with information regarding any new neighbors,metric changes, or down networks When a router receives a link state packet, itrecords the information in its local database and reconstructs a path to the newnetwork If a remote network goes down, the routing table entry will be removed

Link state packets are used to notify remote neighbors of available networks Theaim is to form a link state database that contains all the available networks Thesteps needed to form the database are as follows (Figure 2.4 depicts these steps):

1 Send link state packets to describe the links in a network

2 Combine link state packets to form a link state database

3 Run the shortest path first (SPF) algorithm

4 Create a link tree with the router running the SPF algorithm as the root

5 Insert networks into the routing table

Hybrid Routing Protocols

Cisco has created a routing protocol called Enhanced Interior Gateway Routing

Protocol (EIGRP) EIGRP combines the characteristics of both link state and

distance vector routing protocols This protocol is called a hybrid protocol because

of this combination A hybrid routing protocol uses distance vector tics for choosing a routing path and link state characteristics for changes EIGRPmaintains neighbor and topology tables instead of a link state database

Updates sent and received

Updates sent and received

Updates sent

and received

Figure 2.3 Learning networks using distance vector protocols.

Now that you have a general appreciation for routing protocols, let’s talk aboutsome common routing protocol characteristics.

Common Routing Characteristics

Routing protocols use certain features to ensure that valid routing information isgathered as accurately as possible and without corruption This section discusses

IP RIP as an example

IP RIP is widely used in the Unix industry and is relatively easy to configure and

maintain To enable RIP on a Cisco router, you simply type the command router

rip and then enable the networks you wish to be advertised with the network

<address> command.

Note: Chapter 6 provides a detailed example of IP RIP operation and

configurations.

One of the main functions of any routing protocol is to discover remote networks

that are reachable via neighboring routers and to converge as quickly as possible.

This ensures that all routers in a network have the same network information

1 Router R2 sends a link state packet advertising the new Ethernet network.

2 Router R1 receives the link state packet

and installs the new netork into the link

state database.

3 Router R1 runs shortest path algorithm

to determine shortest cost path to the

When a change occurs, flash or triggered updates are sent, which takes time topropagate to all routers The better the convergence time, the more likely net-work devices will have correct information about all available networks RIP has

a poor convergence time compared to other protocols, such as OSPF

IP RIP uses holddowns, triggered updates, split horizon, and poison reverse dates to ensure valid routing updates are sent When using holddowns, triggeredupdates, split horizon, and poison reverse updates, routing protocols (such as IPRIP) can avoid routing loops, which helps to speed up convergence

up-Let’s take a closer look at holddowns, triggered updates, split horizon, and son reverse updates, as well as summarization

poi-Holddowns

Holddowns prevent updates about networks that have been altered, disappeared,

or broken from being inserted into a routing table This, of course, is not the mostaccurate routing information, and it should be prevented from being placed inthe routing table Holddowns ensure that invalid routes are not relearned thuspreventing problems, such as routing loops, within a network, unless the newroute metric is smaller than the original

Most routing protocols, such as RIP, will base a decision to place a routeinto the IP routing table on a metric In the case of RIP, the network withthe lowest hop count will be chosen

Triggered Updates

A triggered update is a method used by routing protocols to send updates to

neigh-boring routers outside the normal update interval Triggered updates are used toprevent routing loops in networks by sending an update whenever a networkevent triggers it An example of this would be a link going down, which causes atriggered update

Distance vector protocols send their full routing table of all active links at setintervals In the case of RIP, updates are sent every 30 seconds What happens if

a network becomes unreachable in between the update interval? In this instance,

a triggered update is used to notify other routers of the network event This speeds

up convergence time

Let’s view an example of a triggered update by turning on the following debug

options: debug ip rip and debug ip rip events Let’s say you have three networks

learned via RIP Listing 2.1 shows what happens when RIP receives an update

from another IP RIP router The IOS command show debug in Listing 2.1 also

displays that IP RIP events and protocols updates have been enabled

Listing 2.1 Debug IP RIP display.

R1#show debug

IP routing:

RIP protocol debugging is on

RIP event debugging is on

RIP: sending general request on Ethernet0 to 255.255.255.255

RIP: received v1 update from 150.100.1.1 on Ethernet0

Listing 2.2 Triggered update debug output

RIP: received v1 update from 150.100.1.1 on Ethernet0

0.0.0.0 in 1 hops

199.172.3.0 in 1 hops

199.172.2.0 in 16 hops (inaccessible)

199.172.4.0 in 1 hops

RIP: Update contains 4 routes

Notice that you still get the full routing table, but the network 199.172.2.0 ismarked as inaccessible or with a hop count of 16 This value of 16 tells the routerthat the remote destination is no longer reachable and to drop any packets des-tined for this network This specification prevents the router from installing thenetwork in its routing table or sending an update to another router The routerdrops any packets that are received for the network Therefore, any chance of arouting loop occurring is prevented

Distance vector protocols primarily use periodic updates that send theentire routing table to neighboring routers Triggered updates are usedwhen an event occurs outside the normal periodic update interval

Split Horizon

Split horizon is when a router that has learned of a network (via a route

advertise-ment) from another router and that network will not be re-advertised back to thesending router Split horizon is enabled by default on Cisco routers Split horizonhelps to prevent routing loops by ensuring all routing information is accurate,which enables information to be properly routed from source to destination

Poision Reverse Updates

Poison reverse updates are used in conjunction with split horizon to prevent

rout-ing loops on a larger scale Poison reverse updates occur when a network is marked

as invalid For example, when a router receives a route through an interface, itadvertises the same route back out the interface as a poisoned reversed update.The receiving router receives the invalid entry in the routing table, but with a hopcount of 16, so it removes the network from the routing table Then, when therouters converge, the holddown timer expires In large networks, convergencetakes longer for all routers due to the size of the network Poison reverse placesnetworks that have disappeared into an “unreachable state” for a period of timesufficient enough so that all routers in the network will have the same routingtable through normal convergence

Other Network Occurences

Some of the more advanced topics of routing include loops and tunneling Arouting loop is detrimental to an IP network, because the IP packet will never

end up at the correct location A field in the IP frame called the Time-To-Live

field will prevent IP packets from traversing the networks forever, but the resultwill be a disgruntled end user A tunnel, on the other hand, is a software interface

on a Cisco router that is used to transport non-routable protocols across an IPnetwork You may for instance have clients running the native AppleTalk proto-col (Note that you can tunnel both routable and non-routable protocols.) In-stead of enabling AppleTalk along the entire path across your Wide Area Networkyou can create a tunnel interface at both remote points and enable AppleTalkover the IP network Tunneling AppleTalk over an IP network involves encapsu-lating AppleTalk in IP and then sending “through” the tunnel to the destinationwhere it is de-encapsulated

Route Summarization

Route summarization is used to reduce the number of entries in a routing table

A routing table consists of entries that define how a remote network can be reached.The larger the routing table, the more memory required This is because eachentry takes up available memory Therefore, if you can reduce the number ofnetworks to be advertised, you can increase performance and the delivery of packetsacross the network because you have now reduced the IP routing table size thatleads to less bandwidth and time required to advertise the network to remotelocations Summarization is typically used in very large networks, such as theWorld Wide Web

Note: Chapter 6 provides some common commands used on Cisco routers regarding

summarization on IP networks.

Routing tables can be as large as the memory installed on the router.For example, an IP RIP table consisting of 1,000 networks will consume20,000 bytes of memory Each IP RIP entry consumes 20 bytes ofmemory If your router does not contain enough memory, some of theremote networks will not be inserted into the IP routing table.

Examining the Cisco IP Routing Table

Routing tables are generated by devices learning new remote networks usingsome form of a routing protocol Routing tables are used by routers, for example,

to make intelligent decisions regarding where packets should be sent so that userdata is sent as efficiently as possible Hence, one of the most common IOS com-mands used on a Cisco router is to display a routing table The command todisplay the IP routing table on a Cisco router is:

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,

B – BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external

type 2

E1 - OSPF external type 1, E2 - OSPF external type 2,

E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2,

* - candidate defaultU - per-user static route, o - ODR

Gateway of last resort is not set

10.0.0.0/32 is subnetted, 1 subnets

C 10.1.1.1 is directly connected, Loopback0

137.10.0.0/16 is variably subnetted, 4 subnets, 3 masks

D 137.10.255.0/24 [90/2681856] via 137.10.253.2, 1w1d,S0

C 137.10.253.0/24 is directly connected, Serial0

D 137.10.17.0/28 [90/688128] via 137.10.253.2, 1w1d, S0

D 137.10.16.0/27 [90/793600] via 137.10.253.2, 1w1d, S0

The first half of Listing 2.3 summarizes the codes used to identify how networkshave been learned dynamically, statically, or from directly connected networks(for example, those networks assigned directly to a router’s interface).

Note that directly connected networks are identified on the left side as C, and Drepresents EIGRP discovered networks IPX and AppleTalk maintain similartables The IP table lists the remote network, the next hop and metric, and howlong the route has been valid No layer 2 information, like MAC addresses, islisted in the routing table

You must familiarize yourself with IP routing tables An IP routing tabledisplays how remote networks are reachable A switch or bridge willmaintain a layer 2 table called a bridge table or content addressablememory (CAM) table, which lists layer 2 information only, such asMAC addresses

Protocol Operation

You must have a good understanding of Windowing/Acknowledgments (ACK),fragmentation, maximum transmission unit (MTU), handshaking, and termina-tion This section deals with common networking concepts so that when we re-view TCP/IP and other protocol suites in detail in Chapters 4 and 5, you’ll have

a good understanding of how the protocol suites operate from layer 1 of the OSImodel through layer 7

This section starts by covering connection and connectionless protocol ics Then, the discussion moves on to windowing and acknowledgments as well

mechan-as other pertinent protocol operational mechanics

Connection-Oriented and Connectionless Services

A connection-oriented service is a service that guarantees delivery of tion to that service whether it is FTP or HTTP A service that is guaranteed willprovide reliability, ensure segments are delivered and reassembled in order, andare error free When data cannot be sent reliably or in order, an error is sent to theuser’s application layer These connection-orientated protocols, such as TCP, es-tablish a connection to a destination before any form of data is transferred

informa-A telephone service is a good example of a connection-oriented service Beforeyou can start a conversation, the call setup and data transfer phases must be com-pleted After those stages have completed, you can start talking When you finishthe conversation, the call termination phase takes place Each of these phases in

a telephone call are characteristics of connection-oriented services oriented services consist of:

Table 2.2 summarizes the main characteristics of connection-oriented andconnectionless services and presents examples for each.

Windowing and Acknowledgments (ACK) Services

Windowing and acknowledgment services are used to indicate that packets havebeen received (ACK) and how many packets are expected before any acknowl-edgment is required (Windowing) The window size (amount of data that can besent without an acknowledgment) is negotiated at connection time by connec-tion-oriented protocols, such as TCP

There must be mechanisms to tell any end device how many packets you canreceive without overflowing your buffer; otherwise, packets will be lost, and ses-sions will time out The window size can be adjusted during a connection if bothend systems have more buffer memory available or if memory is decreasing Toillustrate, Figure 2.5 shows a simple windowing flow

Let’s say that you have two end systems that have negotiated that only one packetwill be sent before any acknowledgment (ACK) is required This session would

be inefficient, because acknowledgment packets would traverse the link sarily This form of acknowledgment is basically a form of flow control so thatend systems do not become overwhelmed with data

unneces-Now, look at Figure 2.6 Figure 2.6 shows the same flow as discussed earlierexcept that the window size is set to three packets

In Figure 2.6, the session will perform better than the earlier one-packet ACKscenario, because only one acknowledgment is required for every three packets

Table 2.2 Characteristics of connection-oriented and conectionless services.

Connection-Oriented Path setup, path connection, TCP, SPX, X25

information transfer, teardown connection Connectionless Data packaged and sent IP, Ethernet, Token Ring,

frame relay

sent Notice that Device B sends an acknowledgment for the next expected packet.TCP uses this same model for Telnet sessions, for example This form of ac-

knowledgment is known as a sliding window or advanced windowing.

Fragmentation

Another service provided by various protocols, such as TCP, is fragmentation.

Fragmentation gives you the ability to send user information across a networkregardless of what the minimum frame size between intermediate devices, such

Device B (receiver)

Receive Packets

4, 5, 6 Send ACK 6

Device B (receiver)

Figure 2.5 Simple windowing.

Figure 2.6 Advanced windowing In TCP this is called a sliding window.

as routers, might be Sometimes, a data frame might be larger than the allowablesize to the outside world, or a packet might be sent from a Token Ring LAN to

an Ethernet LAN In such cases, fragmentation is used Fragmentation allowsdata to be broken up into allowable sizes by creating smaller frames and reassem-bling the packet at the destination

The Network layer (layer 3) is typically handled by software; hence, the layerfragments packets as required Fragmented packets are then reassembled by theend device TCP is an excellent example of a protocol that will fragment andreassemble packets as required

Maximum Transmission Unit (MTU)

MTU specifies the maximum frame size allowed across a medium For ample, on an Ethernet interface on a Cisco router, the default MTU size is set to1,500 bytes X.25 can go as low as 128 bytes Listing 2.4 displays the Ethernetstatistics taken from a Cisco router’s Ethernet interface, note the MTU size isreadily visible

ex-Listing 2.4 Show interface Ethernet0 command output

R1>sh interface ethernet0

Ethernet0 is up, line protocol is up

Hardware is Lance, address is 0060.7015.5e4d (bia 0060.7015.5e4d) Internet address is 150.100.1.4/24

MTU 1500 bytes,BW 10000 Kbit,DLY 1000 usec,rely 255/255

Encapsulation ARPA, loopback not set, keepalive set (10 sec)

ARP type: ARPA, ARP Timeout 04:00:00

.

The preceding code displays an MTU set to 1,500 bytes On a Token Ring face, the MTU is 4,464 bytes, as shown in the Listing 2.5, another commandoutput from a Cisco router

inter-Listing 2.5 Show interface tokenring0 command output

R1>sh interface tokenring0

TokenRing0 is up, line protocol is up

Hardware is TMS380, address is 0000.308f.3655 (bia 0000.308f.3655) Internet address is 137.10.9.1/24

MTU 4464 bytes, BW 16000 Kbit, DLY 630 usec, rely 255/255

Encapsulation SNAP, loopback not set, keepalive set (10 sec)

ARP type: SNAP, ARP Timeout 04:00:00

Ring speed: 16 Mbps

Single ring node, Source Route Transparent Bridge capable

Source bridging enabled, srn 2 bn 1 trn 200 (ring group)

proxy explorers disabled, spanning explorer enabled

The MTU parameter can be negotiated between end systems when setting up aconnection However, this negotiation will need to be configured in order for it

to take place

Make sure you are familiar with the Ethernet and Token Ring interfacedisplay, as shown in Listings 2.4 and 2.5 You should know what eachfield means and what is relevant and irrelevant For example, collisions

on Token Ring interfaces are meaningless

Handshaking

The handshaking service provides a mechanism where end systems can negotiatecertain parameters, such as link speed, that will be used during data transfer Forexample, the WAN Point-to-Point Protocol (PPP) can negotiate which layer 3protocols it can carry, like IP or IPX This handshaking, or negotiation, is done atthe start of the communication session If the session or any similar session ter-minates and reestablishes, the same parameters that were originally negotiatedwill need to be renegotiated

Termination

Termination refers to the closure of an active session Some protocols behavedifferently when a session is terminated, like a TCP session It takes three seg-ments (known as the three-way handshake) to start a TCP session and four seg-ments to close it

Now, let’s begin to focus on the specific frame formats for IP, IPX, TCP, andUDP as required by the CCIE R/S blueprint

Protocol Descriptions and Use

Many protocols are used in today’s networks This section looks at the frameformats for Internet Protocol (Chapter 5 covers IP in depth), Internetwork PacketExchange (IPX), Transmission Control Protocol (TCP), and User DatagramProtocol (UDP) Following the descriptions of each protocol, we’ll look at thedifferences between IP and IPX, and then we’ll compare TCP to UDP

Internet Protocol (IP)

The Internet Protocol suite is a Network layer protocol that involves logical dressing The Internet Protocol is a connectionless protocol that defines a net-work portion and a host portion, like any layer 3 protocol An IP address is 32

ad-bits in length, and the subnet mask is used to identify the network and host

por-tion Typically, an IP client might be a PC or router An example of an IP addressand a subnet mask is:

131.108.1.1 255.255.255.0