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Study Guide 2 for the Cisco Certification

Study Sheet Quick Links

OSI Model
LAN Design
Network Devices
Lan Protocols
   TCP/IP
   IPX/SPX
VLANs
WAN Protocols
   Frame Relay
   ISDN
   ATM
   PPP
Cisco IOS
Security
Routing
   RIP
   OSPF
   IGRP and EIGRP
Other Routing Info

OSI Model: Top
The OSI model is a layered model and a conceptual standard used for defining standards to promote multi-vendor integration as well as maintain constant interfaces and isolate changes of implementation to a single layer. It is NOT application or protocol specific. In order to pass any Cisco exam, you need to know the OSI model inside and out.

The OSI Model consists of 7 layers.

Layer	Description	Device	Protocol
Application	Provides network access for applications, flow control and error recovery. Provides communications services to applications by identifying and establishing the availability of other computers as well as to determine if sufficient resources exist for communication purposes.	Gateway	NCP, SMB, SMTP, FTP, SNMP, Telnet, Appletalk
Presentation	Performs protocol conversion, encryption and data compression	Gateway and redirectors	NCP, AFP, TDI
Session	Allows 2 applications to communicate over a network by opening a session and synchronizing the involved computers. Handles connection establishment, data transfer and connection release	Gateway	NetBios
Transport	Repackages messages into smaller formats, provides error free delivery and error handling functions	Gateway	NetBEUI, TCP, SPX, and NWLink
Network	Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management.	Router and brouter	IP, IPX, NWLink, NetBEUI
**Data Link	Packages raw bits into frames making it transmittable across a network link and includes a cyclical redundancy check (CRC). It consists of the LLC sublayer and the MAC sublayer. The MAC sublayer is important to remember, as it is responsible for appending the MAC address of the next hop to the frame header. On the contrary, LLC sublayer uses Destination Service Access Points and Source Service Access Points to create links for the MAC sublayers.	Switch, bridge and brouter	None
Physical	Physical layer works with the physical media for transmitting and receiving data bits via certain encoding schemes. It also includes specifications for certain mechanical connection features, such as the adaptor connector.	Multiplexor and repeater	None

Here is an easy way to memorize the order of the layers:
All People Seem To Need Data Processing.
The first letter of each word corresponds to the first letter of one of the layers. It is a little corny, but it works.

The table above mentions the term "MAC Address". A MAC address is a 48 bit address for uniquely identifying devices on the network. Something likes 00-00-12-33-FA-BC, we call this way of presenting the address a 12 hexadecimal digits format. The first 6 digits specify the manufacture, while the remainders are for the host itself. ARP Protocol is used to determine the IP to MAC mapping. And of course, MAC address cannot be duplicated in the network or problems will occur.

Data encapsulation takes place in the OSI model. It is the process in which the information in a protocol is wrapped in the data section of another protocol. The process can be broken down into the following steps:

User data -> segments -> packets/datagrams -> frames -> bits.

When discussing the OSI model it is important to keep in mind the differences between "Connection-oriented" and "Connectionless" communications. A connection oriented communication has the following characteristics:

• A session is guaranteed.

• Acknowledgements are issued and received at the transport layer, meaning if the sender does not receive an acknowledgement before the timer expires, the packet is retransmitted.

• Phrases in a connection-oriented service involves Call Setup, Data transfer and Call termination.

• All traffic must travel along the same static path.

• A failure along the static communication path can fail the connection.

• A guaranteed rate of throughput occupies resources without the flexibility of dynamic allocation.

• Reliable = SLOW (this is always the case in networking).

In contrast, a connectionless communication has the following characteristics:

• Often used for voice and video applications.

• NO guarantee nor acknowledgement.

• Dynamic path selection.

• Dynamic bandwidth allocation.

• Unreliable = FAST.

LAN Design: Top
Ethernet
When we talk about a LAN, Ethernet is the most popular physical layer LAN technology today. Its standard is defined by the Institute for Electrical and Electronic Engineers as IEEE Standard 802.3. According to IEEE, information for configuring an Ethernet as well as specifying how elements in an Ethernet network interact with one another is clearly defined in 802.3.

For half-duplex Ethernet 10BaseT topologies, data transmissions occur in one direction at a time, leading to frequent collisions and data retransmission. In contrast, full-duplex devices use separate circuits for transmitting and receiving data and as a result, collisions are largely avoided. A collision is when two nodes are trying to send data at the same time. On an ethernet network, the node will stop sending when it detects a collision, and will wait for a random amount of time before attempting to resend. Also, with full-duplex transmissions the available bandwidth are effectively doubled, as we are using both directions simultaneously. You MUST remember: to enjoy full-duplex transmission, we need a switch port, not a hub. Click here for a website with tons of information related to ethernet.

Fast Ethernet
For networks that need higher transmission speeds, there is the Fast Ethernet standard called IEEE 802.3u that raises the Ethernet speed limit to 100 Mbps! Of course, we need new cabling to support this high speed. In 10BaseT network we use Cat3 cable, but in 100BaseT network we need Cat 5 cables. The three types of Fast Ethernet standards are 100BASE-TX for use with level 5 UTP cable, 100BASE-FX for use with fiber-optic cable, and 100BASE-T4 which utilizes an extra two wires for use with level 3 UTP cable.

Gigabit Ethernet
Gigabit Ethernet is an emerging technology that will provide transmission speeds of 1000mbps. It is defined by the IEEE standard The 1000BASE-X (IEEE 802.3z). Just like all other 802.3 transmission types, it uses Ethernet frame format, full-duplex and media access control technology.

Token Ring
Token Ring is an older standard that isn't very widely used anymore as most have migrated to some form of Ethernet or other advanced technology. Ring topologies can have transmission rates of either 4 or 16mbps. Token passing is the access method used by token ring networks, whereby, a packet called a token is passed around the network. A computer that wishes to transmit must wait until it can take control of the token, allowing only one computer to transmit at a time. This method of communication aims to prevent collisions. Token Ring networks use multistation access units (MSAUs) instead of hubs on an Ethernet network. For extensive information on Token Ring, visit Cisco's website.

Network Devices: Top
In a typical LAN, there are various types of network devices available as outlined below.

• Hub Repeat signals received on each port by broadcasting to all the other connected ports.

• Repeaters Used to connect two or more Ethernet segments of any media type, and to provide signal amplification for a segment to be extended. In a network that uses repeater, all members are contending for transmission of data onto a single network. We like to call this single network a collision domain. Effectively, every user can only enjoy a percentage of the available bandwidth. Ethernet is subject to the "5-4-3" rule regarding repeater placement, meaning we can only have five segments connected using four repeaters with only three segments capable of accommodating hosts.

• Bridge A layer 2 device used to connect different networks types or networks of the same type. It maps the Ethernet addresses of the nodes residing on each segment and allows only the necessary traffic to pass through the bridge. Packet destined to the same segment is dropped. This "store-and-forward" mechanism inspects the whole Ethernet packet before making a decision. Unfortunately, it cannot filter out broadcast traffic. Also, it introduces a 20 to 30 percent latency when processing the frame. Only 2 networks can be linked with a bridge.

• Switch Can link up four, six, eight or even more networks. Cut-through switches run faster because when a packet comes in, it forwards it right after looking at the destination address only. A store-and-forward switch inspects the entire packet before forwarding. Most switches cannot stop broadcast traffic. Switches are layer 2 devices.

• Routers Can filter out network traffic also. However, they filter based on the protocol addresses defined in OSI layer 3 (the network layer), not based on the Ethernet packet addresses. Note that protocols must be routable in order to pass through the routers. A router can determine the most efficient path for a packet to take and send packets around failed segments.

• Brouter Has the best features of both routers and bridges in that it can be configured to pass the unroutable protocols by imitating a bridge, while not passing broadcast storms by acting as a router for other protocols.

• Gateway Often used as a connection to a mainframe or the internet. Gateways enable communications between different protocols, data types and environments. This is achieved via protocol conversion, whereby the gateway strips the protocol stack off of the packet and adds the appropriate stack for the other side. Gateways operate at all layers of the OSI model without making any forwarding decisions.

The goal of LAN segmentation is to effectively reduce traffic and collisions by segmenting the network. In a LAN segmentation plan, we do not consider the use of gateways and hubs at all and the focus turns to device such as switches and routers.

Lan Protocols: Top
The following sections will introduce the core LAN protocols that you will need to know for the exam.

TCP/IP: Top
Every IP address can be broken down into 2 parts, the Network ID (netid) and the Host ID (hostid). All hosts on the same network must have the same netid. Each of these hosts must have a hostid that is unique in relation to the netid. IP addresses are divided into 4 octets with each having a maximum value of 255. We view IP addresses in decimal notation such as 124.35.62.181, but it is actually utilized as binary data so one must be able to convert addresses back and forth.

The following table explains how to convert binary into decimal and visa versa:

Decimal	Binary	When converting binary data to decimal, a "0" is equal to 0. "1" is equal to the number that corresponds to the field it is in. For example, the number 213 would be 11010101 in binary notation. This is calculated as follows: 128+64+0+16+0+4+0+1=213. Remember that this only represents 1 octet of 8 bits, while a full IP address is 32 bits made up of 4 octets. This being true, the IP address 213.128.68.130 would look like 11010101 10000000 01000100 10000010.
128	10000000
64	01000000
32	00100000
16	00010000
8	00001000
4	00000100
2	00000010
1	00000001

IP addresses are divided into 3 classes as shown below:

Class	Range
A	1-126	IP addresses can be class A, B or C. Class A addresses are for networks with a large number of hosts. The first octet is the netid and the 3 remaining octets are the hostid. Class B addresses are used in medium to large networks with the first 2 octets making up the netid and the remaining 2 are the hostid. A class C is for smaller networks with the first 3 octets making up the netid and the last octet comprising the hostid.
B	128-191
C	192-223

A subnet mask blocks out a portion of an IP address and is used to differentiate between the hostid and netid. The default subnet masks are as follows:

Class	Default Subnet	# of Subnets	# of Hosts Per Subnet
Class A	255.0.0.0	126	16,777,214
Class B	255.255.0.0	16,384	65,534
Class C	255.255.255.0	2,097,152	254

In these cases, the part of the IP address blocked out by 255 is the netid.

In the table above, the it shows the default subnet masks. What subnet mask do you use when you want more that 1 subnet? Lets say, for example, that you want 8 subnets and will be using a class C address. The first thing you want to do is convert the number of subnets into binary, so our example would be 00001000. Moving from left to right, drop all zeros until you get to the first "1". For us that would leave 1000. It takes 4 bits to make 8 in binary so we add a "1" to the first 4 high order bits of the 4th octet of the subnet mask (since it is class C) as follows:
11111111.11111111.11111111.11110000 = 255.255.255.240. There is our subnet mask.

Lets try another one...Lets say that you own a chain of stores that sell spatulas in New York and you have stores in 20 different neighborhoods and you want to have a separate subnet on your network for each neighborhood. It will be a class B network. First, we convert 20 to binary - 00010100. We drop all zeros before the first "1" and that leaves 10100. It takes 5 bits to make 20 in binary so we add a "1" to the first 5 high order bits which gives: 11111111.11111111.11111000.00000000 = 255.255.248.0. The following table shows a comparison between the different subnet masks.

Mask	# of Subnets	Class A Hosts	Class B Hosts	Class C Hosts
192	2	4,194,302	16,382	62
224	6	2,097,150	8,190	30
240	14	1,048,574	4,094	14
248	30	524,286	2,046	6
252	62	262,142	1,022	2
254	126	131,070	510	Invalid
255	254	65,534	254	Invalid

Note: 127.x.x.x is reserved for loopback testing on the local system and is not used on live systems.

TCP/IP Ports - Ports are what an application uses when communicating between a client and server computer. Some common TCP/IP ports are:

• 20 FTP-DATA

• 21 FTP

• 23 TELNET

• 25 SMTP

• 69 TFTP

• 70 GOPHER

• 80 HTTP

• 110 POP3

• 137 NetBIOS name service

• 138 NetBIOS datagram service

• 139 NetBIOS

• 161 SNMP

You need to understand Buffering, Source quench messages and Windowing. Buffering allows devices to temporarily store bursts of excess data in memory. However, if data keep arriving at high speed, buffers can go overflow. In this case, we use source quench messages to request the sender to slow down.

Windowing is for flow-control purpose. It requires the sending device to send a few packets to the destination device and wait for the acknowledgment. Once received, it sends the same amount of packets again. If there is a problem on the receiving end, obviously no acknowledgement will ever come back. The sending source will then retransmits at a slower speed. This is like trial and error, and it works. Note that the window size should never be set to 0 - a zero window size means to stop transmission completely.


IPX/SPX: Top
IPX will also be an important issue to consider in network management given the fact there many companies still use Netware servers. There are two parts to every IPX Network address - the Network ID and the Host ID. The first 8 hex digits represent the network ID, while the remaining hex digits represent the host ID, which is most likely the same as the MAC address, meaning we do not need to manually assign node addresses. Note that valid hexadecimal digits range from 0 through 9, and hexadecimal letters range from A through F. FFFFFFFF in hexadecimal notation = 4292967295 in decimal.

Sequenced Packet Exchange (SPX) belongs to the Transport layer, and is connection-oriented. It creates virtual circuits between hosts, and that each host is given a connection ID in the SPX header for identifying the connection. Service Advertisement Protocol (SAP) is used by NetWare servers to advertise network services via broadcast at an interval of every 60 minutes by default.

VLANs: Top
A VLAN can be used to reduce collisions by separating broadcast domains within the switch. In other words, VLANs create separate broadcast domains in a switched network. This is done by frame tagging at layer 2. This effectively creates an environment with fewer collisions. The key to this is that ports in a VLAN share broadcasts, while ports not in that VLAN cannot share the broadcasts. Thus users in the same physical location can be members of different VLANs. We can plug existing hubs into a switch port and assign them a VLAN of their own to segregates users on the hubs.

VLANs can be complicated to set up. VLANs use layer 2 addressing, meaning that routers are required between separate VLANs. The advantage of deploying layer 2 addresses is that layer 2 addressing is faster to process. It is also quite common for administrators to set up multiple VLANs with multiple access lists to control access. Layer 3 routing provides the ability for multiple VLANs to communicate with each other, which means that users in different locations can reside on the same VLAN. This is a flexible approach to network design. There is extensive information published on Cisco's website in the following 2 links:
VLAN Switching
VLAN Considerations

WAN Protocols: Top
In general, there are three broad types of WAN access technology. With Leased Lines, we have point-to-point dedicated connection that uses pre-established WAN path provided by the ISP. With Circuit Switching such as ISDN, a dedicated circuit path exist only for the duration of the call. Compare to traditional phone service, ISDN is more reliable and is faster. With Packet Switching, all network devices share a single point-to-point link to transport packets across the carrier network - this is known as virtual circuits.

When we talk about Customer premises equipment (CPE), we are referring to devices physically located at the subscriber's location. Demarcation is the place where the CPE ends and the local loop begins. A Central Office (CO) has switching facility that provides point of presence for its service. Data Terminal Equipment (DTE) are devices where the switching application resides, and Date Circuit-terminating Equipment (DCE) are devices that convert user data from the DTE into the appropriate WAN protocol. A router is a DTE, while a DSU/CSU device or modem are often being referred to as DCEs.

Frame Relay: Top
Frame Relay has the following characteristics:

• successor to X.25

• has less overhead than X.25 because it relies on upper layer protocols to perform error checking.

• Speed in between the range of 56 Kbps to 2.078 Mbps.

• uses Data Link Connection Identifiers (DLCI) to identify virtual circuits, with DLCI number between 16 and 1007.

• uses Local Management Interfaces (LMI) to provide info on the DLCI values as well as the status of virtual circuits. Cisco routers support Cisco (Default), ANSI and Q933a.

• to set up frame relay, we need to set the encapsulation to frame-relay in either the Cisco (Default) mode or the IETF mode, although Cisco encapsulation is required to connect two Cisco devices.

• LMI type is configurable, but by default it is being auto-sensed.

• generally transfer data with permanent virtual circuits (PVCs), although we can use switched virtual circuits (SVCs) as well.

• SVC is for transferring data intermittently.

• PVC does not have overhead of establishing and terminating a circuit each time communication is needed.

• Committed Information Rate (CIR) is the guaranteed minimum transfer rate of a connection

ISDN: Top
ISDN has the following characteristics:

• Works at the Physical, Data Link, and Network Layers.

• Often used in backup DDR Dial on Demand Routing.

• Makes use of existing telephone.

• Supports simultaneous data and voice.

• Max speed at 125 Kbps with PPP Multilink.

• Call setup and data transfer is faster than typical modems.

• BRI has 2 x 64 1Kbps B Channels for data and one 16 Kbps D Channel for control.

• PRI has 23 x B Channels and one D Channel in the US, or 30 x B Channel and one D Channel in Europe.

• E protocol specifies ISDN on existing telephone network.

• I protocol specifies Concepts, terminology, and Services.

• Q protocol specifies switching and signaling.

• ISDN Reference Points include R (between non ISDN equipment and TA), S (between user terminals and NT2), T (between NTI and NT2 devices) and U (between NTI devices and Line Termination Equipment in North America).

• router always connected by the U interface into NT1.

• BRI interface is considered Terminal Equipment type 1 TE1.

• TE1 is built into the ISDN standards.

• Needs to have Terminal Adapter TA to use TE2

Cisco has a web page with links about the configuration and troubleshooting of ISDN here

ATM: Top
ATM stands for Asynchronous Transfer Mode and is a high-speed, packet-switching technique that uses short fixed length packets called cells. ATM can transmit voice, video, and data over a variable-speed LAN and WAN connections at speeds ranging from 1.544Mbps to as high as 622Mbps. I recently read that the new standard may be 2Gbps. ATM's speed is derived from the use of short fixed length cells which reduce delays and the variance of delay for delay-sensitive services such as voice and video. ATM is capable of supporting a wide range of traffic types such as voice, video, image and data.

PPP: Top
As an improvement to Serial Line Internet Protocol (SLIP), Point-to-Point Protocol (PPP) was mainly for the transfer of data over slower serial interfaces. It is better than SLIP because it provides multiprotocol support, error correction as well as password protection. It is a Data Link Layer protocol used to encapsulate higher protocols to pass over synchronous or asynchronous communication lines. PPP is capable of operating across any DTE/DCE device, most commonly modems, as long as they support duplex circuits. There are 3 components to PPP:

• HDLC (High-level Data Link Control) - Encapsulates the data during transmission and is a link layer protocol which is also the default Cisco encapsulation protocol for synchronous serial links. HDLC is supposed to be an open standard, but Cisco's version is proprietary, meaning it can only function with Cisco routers.

• LCP (Link Control Protocol) - Establishes, tests and configures the data link connection.

• NCPs (Network Control Protocols) - Used to configure the different communication protocols, allowing them on the same line simultaneously. Microsoft uses 3 NCPs for the 3 protocols at the Network Layer (IP, IPX and NetBEUI).

PPP communication occurs in the following manner: PPP sends LCP frames to test and configure the data link. Next, authentication protocols are negotiated to determine what sort of validation is used for security. Below are 2 common authentication protocols:

• PAP is similar to a network login but passwords are sent as clear text. It is normally only used on FTP sites.

• CHAP uses encryption and is a more secure way of sending passwords.

Then NCP frames are used to setup the network layer protocols to be used. Finally, HDLC is used to encapsulate the data stream as it passes through the PPP connection.

Point-to-Point Tunneling Protocol (PPTP) provides for the secure transfer of data from a remote client to a private server by creating a multi-protocol Virtual Private Network (VPN) by encapsulating PPP packets into IP datagrams. There are 3 steps to setup a secure communication channel:

1. PPP connection and communication to the remote network are established.

2. PPTP creates a control connection between the client and remote PPTP server

3. PPTP creates the IP datagrams for PPP to send.

The packets are encrypted by PPP and sent through the tunnel to the PPTP server which decrypts the packets, disassembles the IP datagrams and routes them to the host. Setting Up PPTP requires a PPTP Client, PPTP Server and a Network Access Server (NAS).

There is a very helpful web site with detailed tutorials on ISDN, Frame Relay, X.25, ATM and other serial WAN technologies located here.

Cisco IOS: Top
Cisco routers use the Internetworking Operating System (IOS) which stores the configuration information in Non-Volatile RAM (NVRAM) and the IOS itself is stored in flash. The IOS can be accessed via Telnet, console connection (such as HyperTerminal) or dial in connection. You can also configure the router as a web server and then access a web-based configuration panel via http.

There are a variety of sources for booting include Flash memory, TFTP and ROM. It is always recommended that new image of IOS be loaded on a TFTP server first, and then copy the image from the TFTP server to the flash memory as a backup mechanism. The copy command such as "copy tftp flash" allows us to copy the IOS image from TFTP server to the Flash memory. And of course, we can always do the reverse. Now, we need to inform the router to boot from the correct source. The following commands are examples of what we should type in depending on the situation. Typically, it is a good idea to specify multiple boot options as a fall back mechanism.

• boot system flash {filename}

• boot system tftp {filename} {tftp server IP address}

• boot system rom

After the boot up process we can prepare to login. The User EXEC is the first mode we encounter. It gives us a prompt of "Router>". To exit this mode means to log out completely, this can be done with the logout command. If we want to proceed to the Privileged EXEC, we need to use the enable EXEC command. Once entered, the prompt will be changed to "Router#". To go back to user EXEC mode, we need to use the disable command. Note that all the configuration works requires the administrator to be in the Privileged mode first. Put it this way, Privileged EXEC mode includes support for all commands in user mode plus those that provide access to global and system settings.

The setup command facility is for making major changes to the existing configurations, such as adding a protocol suite, modifying a major addressing scheme changes, or configuring a newly installed interface.

If you aren't big on reading manuals, finding out the way to access help information is a MUST. To display a list of commands available for each command mode, we can type in a ? mark. IOS also provides context-sensitive help feature to make life easier. In order to pass this exam, you will need to be able to find your away around the IOS. We will list some the information here, but there is too much to list all of it. You will definitely need access to a router or get the software listed at the beginning of this study guide so that you can practice.

Useful editing commands include:

Command	Purpose
Crtl-P	Recall commands in the history buffer starting with the most recent command.
Crtl-N	Return to more recent commands in the history buffer after recalling commands with Crtl-P or the up arrow key.
Crtl-B	Move the cursor back one character
Crtl-F	Move the cursor forward one character
Crtl-A	Move the cursor to the beginning of the command line
Crtl-E	Move the cursor to the end of the command line
Esc B	Move the cursor back one word
Esc F	Move the cursor forward one word
Crtl-R or Crtl-L	Redisplay the current command line

You will find most of the IOS commands at the following link:
http://www.cisco.com/warp/cpropub/45/tutorial.htm

Security: Top
Access Lists allow us to implement some level of security on the network by inspecting and filtering traffic as it enters or exits an interface. Each router can have many access lists of the same or different types. However, only one can be applied in each direction of an interface at a time (keep in mind that inbound and outbound traffic is determined from the router's perspective). The two major types of access lists that deserve special attention are the IP Access Lists and the IPX Access Lists.

Standard IP access lists can be configured to permit or deny passage through a router based on the source host's IP address. Extended IP access list uses destination address, IP protocol and port number to extend the filtering capabilities. Access can be configured to be judged based on a specific destination address or range of addresses, on an IP protocol such as TCP or UDP, or on port information such as http, ftp, telnet or snmp. We use access list number to differentiate the type of access list. In standard IP access lists we have numbers from 1 through 99, and in extended IP access lists we have numbers from 100 through 199:

1-99	Standard IP
100-199	Extended IP
200-299	Protocol type-code
300-399	DECnet
600-699	Appletalk
700-799	Standard 48-bit MAC Address
800-899	Standard IPX
900-999	Extended IPX
1000-1099	IPX SAP
1100-1199	Extended 48-bit MAC Address
1200-1299	IPX Summary Address

Routing: Top
There are 2 types of routing which are static and dynamic. Static routing involves the cumbersome process of manually configuring and maintaining route tables by an administrator. Dynamic routing enables routers to "talk" to each other and automatically update their routing tables. This process occurs through the use of broadcasts. Next is an explanation of the various routing protocols.

RIP: Top
Routing Information Protocol (RIP) is a distance vector dynamic routing protocol. RIP measures the distance from source to destination by counting the number of hops (routers or gateways) that the packets must travel over. RIP sets a maximum of 15 hops and considers any larger number of hops unreachable. RIP's real advantage is that if there are multiple possible paths to a particular destination and the appropriate entries exist in the routing table, it will choose the shortest route. Routers can talk to each other, however, in the real routing world, there are so many different routing technologies available, that it is not as simple as just enabling Routing Information Protocol (RIP). For information on RIP configuration, read Configuring RIP.

OSPF: Top
Open Shortest Path First (OSPF) is a link-state routing protocol that converges faster than a distance vector protocol such as RIP. What is convergence? This is the time required for all routers to complete building the routing tables. RIP uses ticks and hop counts as measurement, while OSPF also uses metrics that takes bandwidth and network congestion into making routing decisions. RIP transmits updates every 30 seconds, while OSPF transmits updates only when there is a topology change. OSPF builds a complete topology of the whole network, while RIP uses second handed information from the neighboring routers. To summarize, RIP is easier to configure, and is suitable for smaller networks. In contrast, OSPF requires high processing power, and is suitable if scalability is the main concern. For commands and methods to configure OSPF read Configuring OSPF on Cisco Routers.

We can tune the network by adjusting various timers. Areas that are tunable include: the rate at which routing updates are sent, the interval of time after which a route is declared invalid, the interval during which routing information regarding better paths is suppressed, the amount of time that must pass before a route is removed from the routing table, and the amount of time for which routing updates will be postponed. Of course, different setting is needed in different situation. In any case, we can use the "show ip route" command to display the contents of routing table as well as how the route was discovered.

IGRP and EIGRP: Top
RIP and OSPF are considered "open", while IGRP and EIGRP are Cisco proprietary. Interior Gateway Routing Protocol (IGRP) is a distance vector routing protocol for the interior networks, while Enhanced Interior Gateway Routing Protocol (EIGRP) is a hybrid that combines distance vector and link-state technologies. Do not confuse these with NLSP. Link Services Protocol (NLSP) is a proprietary link-state routing protocol used on Novell NetWare 4.X to replace SAP and RIP. For IGRP, the metric is a function of bandwidth, reliability, delay and load. One of the characteristics of IGRP is the deployment of hold down timers. A hold-down timer has a value of 280 seconds. It is used to prevent routing loops while router tables converge by preventing routers from broadcasting another route to a router which is off-line before all routing tables converge. For EIGRP, separate routing tables are maintained for IP, IPX and AppleTalk protocols. However, routing update information is still forwarded with a single protocol.

For more information about IGRP, read Configuring IGRP
For a detailed guideline on configuring EIGRP, read Configuring IP Enhanced IGRP

Other Routing Info: Top
In the routing world, we have the concept of autonomous system AS, which represents a group of networks and routers under a common management and share a common routing protocol. ASs are connected by the backbone to other ASs. For a device to be part of an AS, it must be assigned an AS number that belongs to the corresponding AS.

Route poisoning intentionally configure a router not to receive update messages from a neighboring router, and sets the metric of an unreachable network to 16. This way, other routers can no longer update the originating router's routing tables with faulty information.

Hold-downs prevent routing loops by disallowing other routers to update their routing tables too quickly after a route goes down. Instead, route can be updated only when the hold-down timer expires, if another router advertises a better metric, or if the router that originally advertised the unreachable network advertises that the network has become reachable again. Note that hold down timers need to work together with route poisoning in order to be effective.

Split horizon simply prevents a packet from going out the same router interface that it entered. Poison Reverse overrides split horizon by informing the sending router that the destination is inaccessible, while Triggered Updates send out updates whenever a change in the routing table occurs without waiting for the preset time to expire.
 

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