Cisco Hierarchical Inter Network Model

8/6/2019 Cisco Hierarchical Inter Network Model

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Cisco Hierarchical Internetworking Model

Cisco characterizes networks by using a three-level hierarchical model. This helps clarify the purpose ofeach device in the network. Cisco calls them the Access, Distribution, and Core levels. They are describedhere:

Access level Where end (user) nodes access the network. These are closet or desktop routers orswitches. If the network is designed well, a good portion of the intra-workgroup traffic will remain at theAccess level. VLANs are implemented at this level. Some basic filtering can happen here, but it is usuallyimplemented at the distribution level. If there are WAN connections, they would connect the Access levelto the Distribution level.

Distribution level Aggregates the Access level connections to the Core level. Oversubscription ofbandwidth happens here (150 10Mbps access-level users do not require a T1 to the Core, because theywill not all use this bandwidth at the same time). Most Access Lists, compression, and encryption should

be implemented here. Devices at this level are almost exclusively routers.

Core level Concentrates all traffic that needs to transverse the network. The focus here is switchingtraffic as fast as possible. ATM, Gigabit, SONET, and other high-speed technologies are usuallyimplemented here (although it is becoming more common to see higher speeds closer to the Access level,

as equipment prices drop).

OSI Layer Model

The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI)Reference Model to define functional communications standards. This reference model is widely used byequipment manufacturers to assure their products will interoperate with products from other vendors. TheOSI model describes the functions that are performed in data communications. The model architecture is

seven layers those are listed below with each layer defining specific functions.

Layer 7 ApplicationThe Application layer provides the interface to the user. Any user application that requires networkcommunication accesses the communication resources through this layer. This layer also is responsible for

finding and determining the availability of communication partners. Typical applications in the TCP/IP


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protocols are Simple Mail Transfer Protocol (SMTP), Telnet, and File Transfer Protocol (FTP).

Layer 6 Presentation

The Presentation layer is responsible for encoding and decoding data that is passed from the applicationlayer to another station on the internetwork. This layer is responsible for encoding data in a format thatthe receiving station can interpret and for decoding data received from other stations. Data compression

and encryption are accomplished at this layer. Typical coding schemes include ASCII, EBCDIC, MPEG, GIF,and JPEG.

Layer 5 SessionThe session layer is responsible for creating, managing and termination sessions that are used by entitiesat the presentation layer. The session layer is responsible for coordinating the service requests and

responses generated and received by a station when it is communication with other entities on theinternetwork.

Layer 4 TransportThe Transport layer implements reliable internetwork data transport services that are transparent toupper-layer protocols. The services include flow control, multiplexing, and error checking and recovery. Ifvirtual circuits are needed for the communication to be accomplished, they are built and maintained atthis layer. Flow control is responsible for making sure that a sending station does not transmit data fasterthan the receiving station can process it. Multiplexing allows multiple applications to share a commonnetwork interface. Error checking is implemented to discover errors on transmission and to provide arecovery mechanism when errors are found. Typical error recovery includes retransmission of the data.

Layer 3 NetworkThe Network layer defines routing services that allow multiple data links to be combined into aninternetwork. The Network layer defines network-addressing schemes that logically identify networkdevices. The logical network addresses are different from the physical addresses defined at the MAC layer,and are used by routing protocols running at this level to transfer packets from one network to another.The most common network addressing protocols are IP, IPX, and AppleTalk. Typical routing protocols thatrun at this level are RIP, OSPF, IGRP, and NLSP.

Layer 2 Data LinkThe Data Link layer provides reliable transit of data across a physical network link. The Data Link layeralso defines the physical network-addressing scheme, such as the MAC Address on network interfacecards in a workstation connected to a LAN. The Data Link layer also defines the topology of the network(bus, star, dual ring, and so on). Flow control at the Data Link layer is defined to ensure receiving stationsare not overrun with data before they can process data already received. The Institute of Electrical andElectronics Engineers (IEEE) has redefined the Data Link layer into two sublayers. The sublayers are theLogical Link Control (LLC) layer and the Media Access Control (MAC) layer. The LLC and MAC sublayers

are defined in the IEEE 802.2 standards. The LLC manages communications between devices over a singlelink of a network. The MAC sublayer manages access to the physical medium from multiple upper-levelprotocols. The MAC layer also defines the MAC address, which uniquely identifies devices at the Data Linklayer.

Layer 1 PhysicalThe Physical layer defines the parameters necessary to build, maintain, and break the physical l inkconnections. It defines the characteristics of the connectors, data transmission rates and distances, andthe interface voltages.


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TCP/IP

IP was created as a way to hide the complexity of physical addressing by creating a virtual addressingscheme that is independent of the underlying network. IP does not ensure that data is delivered to theapplication in the appropriate order; that responsibility is left to upper-layer protocols such as TCP and

UDP.

IP is a connectionless, Network-layer protocol.

An IP address is 32 bits long. The bits can be broken down into four bytes. Each byte is expressed indecimal form and separated from other bytes by a dot (that is, x.x.x.x). This is called dotted-decimalformat. Each bit within a byte carries a binary weight (starting from left to right) of 128, 64, 32, 16, 8, 4,2, 1. If you add up these values, you get a range of 0-255 for each byte.

For example, one byte can be translated from binary format to decimal format as follows:

128 64 32 16 8 4 2 10 1 1 1 0 0 0 10 + 64 + 32 + 16 + 0 + 0 + 0 + 1 = 113

IP addressing has been broken down into five separate classes based on the number of maximum hostsrequired by the network.

IP Address Classes

8 16 24 32

Class A 0 Network Host

Class B 10 Network Host

Class C 110 Network Host

Class D 1110 Multicast Address

Class E 1111 Reserved

You can see from above figure that each address class contains a network portion and a host portion. The

network portion identifies the data link that is in common with all the devices attached to that network.The host portion uniquely identifies an end device connected to the network.

ClassDecimal Value of FirstByte

PurposeMax,Hosts

Class A 0127 Large organizations 16,777,214

Class B 128191 Medium-sized Organizations 65,543

Class C 192223 Small organizations 254

Class D 224247 Multicast addresses n/a

Class E 248255 Experimental n/a

Private IP Addresses

Private address space is not recognized by the Internet and can be used by anyone for use within aprivate network. Public address space, on the other hand, is a unique address that is assigned to a


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Line Console 0 Puts you in the console configuration mode Router(config)#line console 0

Router(config-line)#login

Router(config)# password cisco

Line Vty Puts you in VTY (Telnet) interfaceconfiguration mode

Router(config-line)#line vty 0 4

Router(config-line)#login

Router(config)# password cisco

Service Password-encryption

Encrypts the user mode and enable password Router(config)#service password-encryption

Terminal HistorySize

Changes your history size from the default of10 up to 256

Router#terminal history size 25

Telnet Tests IP connectivity and configures a router Telnet [destination ip]

Router#telnet 172.16.10.2

Ping Tests IP connectivity Ping [destination ip]

Router#ping 172.16.10.2

InterfaceFastethernet 0/0

Puts you in interface configuration mode for afastethernet port

Router(config)#int fa 0/0

Interface Serial 1 Puts you in configuration mode for interfaceserial 1 and can be used for show commands

Router(config)#int serial 1

Ip Address Sets an IP address on an interface Router(config)#ip address172.16.10.2 255.255.255.0

Bandwidth Sets the bandwidth on a serial interface Router(config-if)#bandwidth 64

Copy Run Start Short for copy running-config, startup-config.

Places a configuration into NVRAM

Router#copy run start

Erase Startup Deletes the startup-config Router#erase startup-config

Show Controllers S0

Show the DTE or DCE status of an interface Router#sh controllers s 0

Show History Shows you the last 10 commands entered bydefault

Router#sh history

Show Interface S0

Shows the statistics of interface serial 0 Router#sh int s 0

Show Run Short for show running-config. Shows theconfiguration currently running on the router

Router#sh run

Show Start Short for show startup-config. Shows the

backup configuration stored in NVRAM

Router#sh start

Show Terminal Shows you your configured history size Router#sh terminal

Show Version Shows you statistic of the router Router#sh ver

Shutdown Puts an interface in administratively-downmode

Router(config-if)#shutdown

IP Routing Commands


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Command Description Syntax

Ip route Creates static and default routes on arouter

Ip route [destination_network] [mask][next_hop_address or exitinterface][administrative_distance] [permanent]

Example: Router(config)#ip route

172.16.20.0 255.255.255.0 172.16.10.2

No ip route Removes a static default rout Router(config)#no ip route 172.16.20.0255.255.255.0 172.16.10.2

Router RIP Turns on ip rip routing on a router Router(config)#router rip

Network Tells the routing protocol what networkto advertise

Router(config-if)#network 172.16.20.0

Router igrp AS Turns on ip igrp routing on a router Router igrp[Autonomous system no.]

Router(config)#router igrp 10

Debug ip rip Sends console messages displayinginformation about rip packets being sent

and received on a router interface

Router#debug ip rip

Debug ip igrpevents

Provides a summary of the igrp routinginformation running on the network

Router#debug ip igrp events

Debug ip igrptransactions

Shows message requests from neighborroutes asking for an update and the

broadcast sent from your router to thatneighbor router

Router#debug ip igrp transactions

Show ip route Displays the ip routing table Router#sh ip route

Show protocols Shows the routed protocols and networkaddresses configured on each interface

Router#sh protocols

Show ipprotocols

Shows the routing protocols and timersassociated with each routing protocol

configured on a router

Router#sh ip protocols

Managing Internetwork

Command Description Syntax

Cdp enable Turns on cdp on an individual interface Router#cdp enable

No cdp enable Turns off cdp on an individual interface Router#no cdp enable

Cdp run Turns on cdp on a router Router#cdp run

No cdp run Turns off cdp completely on router Router#no cdp run

Cdp holdtime Changes the holdtime of cdp packets Router(config)#cdp holdtime 240

Cdp timer Changes the cdp update timer Router(config)#cdp timer 90

Clear line Clears a connection connected via telnet to yourrouter

Router#clear line 2

Config-register Tells the router how to boot and to change theconfiguration register setting

Router(config)#config-register0x0101

Copy flash tftp Copies a file from flash memory to a tftp host Router# copy flash tftp

Copy run start Copies the running-config file to the startup-config Router#copy run start


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file

Copy run tftp Copies the running-config file to a tftp host Router#copy run tftp

Copy tftp flash Copies a file from a tftp host to flash memory Router#copy tftp flash

Copy tftp run Copies a configuration from a tftp host to therunning-config file

Router#copy tftp run

Erase startup-config

Deletes the contents of NVRAM on a router Router#erase startup-config

Ip host Creates a host table on a router Ip host[hostname] [ip address]

Example: Router(config)#ip host

router2 172.16.10.2

No ip host Removes a hostname from a host table Router(config)#no ip host router2172.16.10.2

Show cdp Displays the cdp timer and holdtime frequencies Router#sh cdp

Show cdp entry Same as show cdp neighbor detail, but does notwork on a 1900 switch

Router#sh cdp entry

Show cdpinterface

Shows the individual interfaces enabled with cdp Router#sh cdp interface

Show cdpneighbor

Shows the directly connected neighbors and thedetails about them

Router#sh cdp nei

Show cdpneighbor detail

Shows the ip address and IOS version and type,and includes all of the information from the showcdp neighbor command

Router#sh cdp neighbor detail

Show cdp traffic Shows the cdp packets sent and received on adevice and any errors

Router#sh cdp traffic

Show flash Shows the files in flash memory Router#sh flash

Show hosts Shows the contents of the host table Router#sh hosts

Show sessions Shows your connections via telnet to remote device Router#sh sessions

Cisco Discovery Protocol (CDP)

Cisco Discovery Protocol (CDP) is a Cisco proprietary (layer 2) protocol that provides information aboutdirectly connected Cisco routers and switches (if enabled on the switch). CDP messages are not forwardedand, of course, not routed (layer 2). The protocol operates transparently and is enabled by default on all

Cisco routers. It can be useful if you are trying to troubleshoot a connectivity problem. If the Data Linklayer is operating, you will see CDP information from all directly connected routers (and switches, ifenabled) that also have layer 2 functionality. Here is the kind of information provided:

ROUTER#show cdp neighbors detail-------------------------Device ID: RTR3Entry address(es):IP address: 10.10.10.2Novell address: a1.0090.ab80.5e0f(Other configured network protocols would appear here)Platform: cisco 2500, Capabilities: RouterInterface: Serial0.103, Port ID (outgoing port): Serial0.1Holdtime : 151 sec


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Routing

Routing is the process of forwarding packets from one network to another.All the information needed for a router to forward packets to a hop(router/relay device) can be found in the router's routing table.

Static Routing

Static routing occurs when you manually add routes in each router'srouting table. Static routes are routes that are administratively configuredin routers. They are typically used when dynamic protocols are eitherunnecessary or unwanted.

Static routing has following benefits:

y There is no overhead on the routers CPUy There is no bandwidth usage between routers, which mean u could

possibly save money on WAN links

Static routing has following disadvantages:

y The administrator must really understand the internetwork andhow each router is connected in order to configure routes properly

y If a network is added to internetwork, the administrator has to adda route to it on all routers

Default Routing

Default routing useto send packets with a remote destination network notin the routing table to the next-hop router. You can only use defaultrouting on stub networks-those with only one exit path out of the network.

Dynamic Routing

Dynamic routing is the process of using protocols to find and updaterouting tables on routers and to maintain a loop-free, single path to eachnetwork. This is easier than using static routing but it will cost u in terms ofrouter CPU processes and bandwidth on the network links.

There are two types of dynamic routing protocols used in

internetwork.Interior Gateway Protocols (IGP) and Exterior GatewayProtocols (EGP). IGP routing protocol are used to exchange routinginformation with routers in the same autonomous system (AS). An AS is acollection of network under a common administrative domain. EGP's are

used to communicate between ASes. BGP is an example of EGP.

Link State versus Distance Vector Routing Protocols

In determining the best route to a destination, different routing protocolsuse a number of different measurements. These measurements are calledmetrics. Each routing protocol uses one or more metric to calculate thebest route to a particular destination. The most common metrics includepath length (hop count), reliability, delay, bandwidth, load, and financialcost of a link.


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Another major difference between routing protocols is how they handleupdating each other with current information. There are many methods ofdoing this. Given these major differences, routing protocols are broken intotwo main categories: Distance Vector and Link State.

Distance Vector protocols include RIP and IGRP. They send their entire

routing tables out in all directions at regularly scheduled intervals.

Link State protocols are more advanced than distance vector protocolsbecause, unlike distance vector, they do not send periodic routing updates.Link State protocols include OSPF, NLSP, BGP, and IS-IS.

They send partial routing tables (of their own networks) to everyone andthen send updates when necessary.

Classful Versus Classless Routing

The basic definition of classful routing is that subnet mask information isnot carried within the routine, periodic routing updates. This means thatevery interface and host on the network must use the same subnet mask.

In other words, a classful routing protocol abides strictly to the bitboundaries of the IP address classes. For example, the 10.0.0.0 network-aClass A network-cannot be advertised as anything Other than a route to10.0.0.0, since the default network mask of a Class A network is255.0.0.0. In other words, VLSMs are effectively useless. This is becausethe routing update packet has no field for subnet mask, so the defaultmask according to the class is assumed. Classful routing protocols includeRIP v1 and IGRP.

Classless routing protocols include the subnet mask information when anupdate is sent. This allows different length subnet masks to be used on thenetwork called Variable Length Subnet Masks (VLSM).

Default Administrative Distances

Administrative distances are used to rate the trustworthiness of routinginformation received on a router from a neighbor router. If a router learnsof different types of routes to the same destination (statically configured oradvertised via a dynamic routing protocol), it must select which route toinclude in its routing table. Typically, only one route to a specificdestination (same address and mask) is in a router's routing table. Onemethod of route "selection" is accomplished by comparing the

administrative distance of all the routes to the same destination.

Administrative Distance is a value, which rates the reliability of the sourceof the route. If the source that provides a route to a router is considered tobe less reliable-less trustworthy-it receives a higher administrative distance

value. The lowest administrative distance becomes the preferred routeentered in the routing table. Administrative distance values range from 0

to 255. If desired, the administrator can configure administrative distancesso that the default administrative distance is not used.


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Routing Protocols

In determining the best route to a destination, different routing protocols use a number of differentmeasurements. These measurements are called metrics. Each routing protocol uses one or more metric tocalculate the best route to a particular destination. The most common metrics include path length (hop

count), reliability, delay, bandwidth, load, and financial cost of a link.

Another major difference between routing protocols is how they handle updating each other with currentinformation. There are many methods of doing this. Given these major differences, routing protocols arebroken into two main categories: Distance Vector and Link State.

Distance Vector protocols include RIP and IGRP. They send their entire routing tables out in all directionsat regularly scheduled intervals.

Link State protocols are more advanced than distance vector protocols because, unlike distance vector,

they do not send periodic routing updates. Link State protocols include OSPF, NLSP, BGP, and IS-IS.

They send partial routing tables (of their own networks) to everyone and then send updates whennecessary.

RIP (Routing Information Protocol)

RIP is a true distance vector routing protocol. It sends the complete routing table out to all other activeinterfaces every 30 seconds. RIP uses Hop Count as it's only metric. The maximum number of hops in aRIP network is 15, one hop is a directly connected network, and 16 hops is an unreachable network.

RIP v1 uses only classful routing. RIP v2 uses classless routing.

RIP uses three different kinds of timers to regulate its performance:

Route Update timer sets the interval (30 seconds) between periodic routing updates in which the routers

sends a complete copy of its routing table out to all neighbors.

Route Invalid timer determines the length of time that must expire (90 seconds) before a routerdetermines that a route has become invalid.

Router Flush timer sets that time between a route becoming invalid and its remove from the routing table(240 seconds).

RIP Configurations

RIP is very simple to configure. All you need to do is enable RIP and add each network that uses RIP.However, RIPv2 has a few more possible commands; you can use two of them: version and no auto-

summary.

Because the router will by default use RIPv1, you must use the version command to tell the router to useRIPv2. In addition, by default RIPv2 will summarize major networks across boundaries. Use the no auto-summary command to stop summarization.

RIPv1 Configuration Example


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Router A (config)#router ripRouter A (config_router)#network 208.28.3.0

Router A (config_router)#network 192.38.56.0

Router B (config)#router ripRouter B (config-router)#network 134.80.0.0Router B (config-router)#network 192.38.56.0Router B (config-router)#network 192.38.57.0Router B (config-router)#network 192.38.58.0

Router C (config)#router rip

Router C (config-router)#network 192.38.58.0Router C (config-router)#network 208.28.1.0

Router D (config)#router ripRouter D (config-router)#network 192.38.57.0Router D (config-router)#network 208.28.2.0

Monitoring and Verifying RIP

Command Description

Show ip protocols Shows the entire routing table

Show ip route Shows routing protocol parameters and current timer values

Debug ip rip Issues log message for each RIP update

Ping Sends and receive ICMP echo messages to verify connectivity

trace Sends a series of ICMP echoes with increasing TTL value


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IGRP (Interior Gateway Routing Protocol)

The Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary protocol. Like RIP, IGRP is a distance-vector interior routing protocol. However, unlike RIP, IGRP can be used in larger autonomous systems dueto its large maximum hop-count limit of 255, compared to RIP's maximum hop count of 16.

IGRP uses bandwidth and delay of the line by default as metric for determining the best route to aninternetwork. This is called a composite metric. Reliability, load and maximum transmission unit (MTU)can also be used, although they are not used by default.

To control performance IGRP uses different kind of timers:

Update Timers specifies how frequently IGRP routing messages will be sent. The default is 90 seconds.

Invalid Timer specifies how long a router should wait in the absence of a routing-update message of aspecific route before declaring it invalid. The default is three times the Update timer, 270 seconds.

Holddown Timer specifies the holddown period. The default is three times the update timer plus 10

seconds, 280 seconds.

Flush Timer indicates how much time should pass before an IGRP route is flushed from the routing table.The default is seven times the routing update period, 630 seconds.

IGRP Configurations

Configuring IGRP is similar to configuring RIP in that after the router command you must specify onlydirectly connected (system routes) networks. The only difference is in the command to enable the routingprotocol. You must specify an AS number when enabling IGRP. The AS number parameter specifies theautonomous system number that is supported by this IGRP process and allows multiple IGRP processes torun on a single router. The AS number can be between 1 and 65,655.

For example:RTR(config)# router igrp 10

RTR(config-router)# network 200.40.0.0RTR(config-router)# network 200.30.0.0

Monitoring and Verifying IGRP

Command Description

Show ip protocols Shows routing protocol parameters and current timer values

Debug ip igrp transactions Issues log messages with details of the IGRP updates.

Debug ip igrp events Issues log messages for each igrp updates

Ping Sends and receive ICMP echo messages to verify connectivity

trace Sends a series of ICMP echoes with increasing TTL value

Show ip route Shows routing protocol parameters and current timer values

Cisco Hierarchical Inter Network Model

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