Cisco Hierarchical Model
OSI Model
LAN Design
Network Devices
Bridging/Switching
VLANs
Lan Protocols
TCP/IP
IPX/SPX
WAN Protocols
Frame Relay
ATM
PPP
Cisco IOS
Security
Routing
RIP
OSPF
IGRP and EIGRP
Other Routing Info
Cisco Hierarchical Model:
For more information about this, please read our separate tutorial titled "The Cisco Hierarchical Model".
OSI Model:
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 as follows:
| 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 transmitable 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. | Multiplexer 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. The ARP Protocol is used to determine the IP to MAC mapping. And of course, MAC addresses cannot be duplicated in the network or problems will occur. For more information about ARP and related protocols, read Guide To ARP, IARP, RARP, and Proxy ARP.
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 information -> 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).
- Often used for voice and video applications.
- NO guarantee nor acknowledgement.
- Dynamic path selection.
- Dynamic bandwidth allocation.
- Unreliable = FAST.
LAN Design:
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, but was originally created by Digital Intel Xerox (DIX). 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, known as a jam signal. Also, with full-duplex transmissions the available bandwidth is 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, and NICs that are capable of handling full duplex. Ethernet’s media access control method is called Carrier sense multiple access with collision dectection (CSMA/CD). Because of Ethernets collision habits it is also known as the “best effort delivery system.” Ethernet cannot carry data over 1518 bytes, anything over that is broken down into “travel size packets.”
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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 3bit 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, read Cisco's Token Ring/IEEE 802.5 tutorial.
Network Devices:
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.
Bridging/Switching:
Store-and-forward - The entire frame is received before any forwarding takes place. The destination and/or the source addresses are read and filters are applied before the frame is forwarded. Latency occurs while the frame is being received; the latency is greater with larger frames because the entire frame takes longer to read. Error detection is high because of the time available to the switch to check for errors while waiting for the entire frame to be received. This method discards frames smaller than 64 bytes (runts) and frames larger than 1518 bytes (giants). Cut-Through - The switch reads the destination address before receiving the entire frame. The frame is then forwarded before the entire frame arrives. This mode decreases the latency of the transmission and has poor error detection. This method has two forms, Fast-forward and fragment-free.
The Spanning-Tree Algorithm, implemented by the Spanning-Tree Protocol, prevents loops by calculating stable spanning-tree network topology. When creating a fault-tolerant network, a loop-free path must exist between all nodes in the network The Spanning-Tree Algorithm is used to calculate a loop-free paths. Spanning-tree frames, called bridge protocol data units (BPDUs), are sent and received by all switches in the network at regular intervals and are used to determine the spanning-tree topology. A switch uses Spanning-Tree Protocol on all Ethernet-and Fast Ethernet-based VLANs. Spanning-tree protocol detects and breaks loops by placing some connections in standby mode, which are activated in the event of an active connection failure. A separate instance Spanning-Tree Protocol runs within each configured VLAN, ensuring topologies, mainly Ethernet topologies that conform to industry standards throughout the network. These modes are as follows:
VLANs: A VLAN is a logical grouping of devices or users. These devices or users can be grouped by function, department application and so on, regardless of their physical segment location. VLAN configuration is done at the switch via switching fabric. 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. Frame tagging at layer 2 does this. Frame tagging is a gaining recognition as the standard for implementing VLANs, and is recognized by IEEE 802.1q. Frame tagging uniquely assigns a VLAN ID to each frame. This identifier is understood and examined by each switch prior to any broadcasts or transmissions to other switches, routers, and end-stations devices. When the frame exits the network backbone, the switch removes the identifier before the frame is transmitted to the target end station. 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. Frame filtering examines particular information about each frame. A filtering table is developed for each switch; this provides a high level of administrative control because it can examine many attributes of each frame. Frame filtering is slowly being erased and replaced by the frame tagging method. 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. VLANs are configured on the switch three ways, port centric, static and dynamically. In port-centric VLANs, all the nodes connected to ports in the same VLAN are assigned the same VLAN ID. Packets do not “leak” into other domains, and are easily administered and provide great security between VLANs. Some say that static configured VLANs are the same as port centric, because static VLANs use the port centric method for assigning them to switch ports. Dynamic VLANs are ports on a switch that can automatically determine their VLAN assignments. Dynamic VLAN functions are based on MAC addresses, logical addressing, or protocol type of the data packets. When a station is initially connected to an unassigned switch port, the appropriate switch checks the MAC entry in the management database and dynamically configures the port with the corresponding VLAN configuration. The major high points of this method are less administration overhead, of course only after the first administration of the database within the VLAN management software. Creating and Maintaining VLANs VLAN Considerations Lan Protocols: The following sections will introduce the core LAN protocols that you will need to know for the exam. TCP/IP: 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:
IP addresses are divided into 3 classes as shown below:
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:
In these cases, the part of the IP address blocked out by 255 is the Net ID. 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.
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: 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 transmittion completely. 3COM’s IP addressing tutorial is just superior. It covers basic IP addressing options as well as subnetting and VLSM/CIDR. IPX/SPX: 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. WAN Protocols: 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: Frame Relay has the following characteristics:
ATM: ATM stands for Asynchronous Transfer Mode and is a high-speed, packet-switching technique that uses short fixed length packets called cells which are about 53 bits in length. 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: 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:
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:
Cisco IOS: 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 dialin 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. 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:
You will find most of the IOS commands at the following 2 links: Router and Switch Commands http://www.cisco.com/warp/cpropub/45/tutorial.htm Security: 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:
When dealing with Access Control Lists or preparing for your CCNA exam, you have to deal with a 32-bit wild card address in dotted-decimal form, known as your inverse mask. By Cisco’s definition it is called inverse, but you can think of it as the “reverse” of your subnet mask in most cases. When dealing with your wild card mask, you have two values that you are working with. Like subnetting you have a 0 as "off" and a 1 as the "on" value. Wild cards deal with the 0 value as “match” and the 1 value as "ignore". What do I mean by ignore or match? If you have studied ACLs you should know that your goal is to set criteria to deny or permit and that is where your Inverse mask comes into play. It tells the router which values to seek out when trying to deny or permit in your definition. If you have dealt with subnetting you know that most of your address ended with an even number. With your inverse mask you will end up with an odd number. There are several different ways to come up with your inverse mask; the easiest is to subtract your subnet mask from the all routers broadcast address of 255.255.255.255. Example: You have a subnet mask of 255.255.255.0. To get your wild card mask all you have to do is: 255.255.255.255. -255.255.255.0 0.0.0.255 Then you can apply it to the definition, whether using a standard or extended ACL. Standard example: Router(config)# access-list 3 deny 170.10.1.0 0.0.0.255 How you would read this list. With this wild card you told the router to “match” the first three octets and you don’t care what’s going on in the last octet. Extended example: Router(config)# access-list 103 permit 178.10.2.0 0.0.0.255 170.10.1.0 0.0.0.255 eq 80 How you would read this list? With this wild card you have told the router to match the first three octets and you don’t care what’s going on in the last octet. Think of it this way. If you had broken the decimal form down to binary, the wild card mask would look like this. 00000000.00000000.00000000.11111111 As you know the “1” means ignore and “0” means match. So in that last octet it could have been any value on that subnet line ranging from 0-255. For more information on IP Access Lists, read Configuring IP Access Lists Routing: There are 2 main types of routing, which are static and dynamic, the third type of routing is called Hybrid. 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: 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: 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. 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. For commands and methods to configure OSPF read OSPF Commands IGRP and EIGRP: 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. (Note: RIPv2, OSPF and EIGRP include the subnet mask in routing updates which allows for VLSM (Variable Length Subnet Mask), hence VLSM is not supported by RIP-1 or IGRP.) For more information about IGRP, read Configuring IGRP For a detailed guideline on configuring EIGRP, read Configuring IP Enhanced IGRP Other Routing Info: 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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