Interior Gateway Protocol (IGP) An IGP (Interior Gateway Protocol) is a protocol for exchanging routing information between gateways (hosts with routers) within an autonomous network (for example, a system of corporate local area networks). The routing information can then be used by the Internet Protocol (IP) or other network protocols to specify how to route transmissions.  There are two commonly used IGPs: the Routing Information Protocol (RIP) and the Open Shortest Path First (OSPF) protocol.  Also see the Exterior Gateway Protocol (EGP).  Autonomous System On the Internet, an autonomous system (AS) is the unit of router policy, either a single network or a group of networks that is controlled by a common network administrator (or group of administrators) on behalf of a single administrative entity (such as a university, a business enterprise, or a business division). An autonomous system is also sometimes referred to as a routing domain. An autonomous system is assigned a globally unique number, sometimes called an Autonomous System Number (ASN).  Networks within an autonomous system communicate routing information to each other using an Interior Gateway Protocol (IGP). An autonomous system shares routing information with other autonomous systems using the Border Gateway Protocol (BGP). Previously, the Exterior Gateway Protocol (EGP) was used. In the future, the BGP is expected to be replaced with the OSI Inter-Domain Routing Protocol (IDRP). The Internet's protocol guideline for autonomous systems, after offering a definition similar to the one above, provides a more technical definition as follows:  An AS is a connected group of one or more Internet Protocol prefixes run by one or more network operators which has a SINGLE and CLEARLY DEFINED routing policy.   Routing Information Protocol (RIP) RIP (Routing Information Protocol) is a widely-used protocol for managing router information within a self-contained network such as a corporate local area network () or an interconnected group of such LANs. RIP is classified by the Internet Engineering Task Force (IETF) as one of several internal gateway protocols (Interior Gateway Protocol).  Using RIP, a gatewayhost (with a router) sends its entire routing table (which lists all the other hosts it knows about) to its closest neighbor host every 30 seconds. The neighbor host in turn will pass the information on to its next neighbor and so on until all hosts within the network have the same knowledge of routing paths, a state known as network convergence. RIP uses a hop count as a way to determine network distance. (Other protocols use more sophisticated algorithms that include timing as well.) Each host with a router in the network uses the routing table information to determine the next host to route a packet to for a specified destination.  RIP is considered an effective solution for small homogeneous networks. For larger, more complicated networks, RIP's transmission of the entire routing table every 30 seconds may put a heavy amount of extra traffic in the network.  The major alternative to RIP is the Open Shortest Path First Protocol (OSPF).  Open Shortest Path First (OSPF) OSPF (Open Shortest Path First) is a routerprotocol used within larger autonomous system networks in preference to the Routing Information Protocol (RIP), an older routing protocol that is installed in many of today's corporate networks. Like RIP, OSPF is designated by the Internet Engineering Task Force (IETF) as one of several Interior Gateway Protocols (IGPs).  Using OSPF, a host that obtains a change to a routing table or detects a change in the network immediately multicasts the information to all other hosts in the network so that all will have the same routing table information. Unlike the RIP in which the entire routing table is sent, the host using OSPF sends only the part that has changed. With RIP, the routing table is sent to a neighbor host every 30 seconds. OSPF multicasts the updated information only when a change has taken place.  Rather than simply counting the number of hops, OSPF bases its path descriptions on "link states" that take into account additional network information. OSPF also lets the user assign cost metrics to a given host router so that some paths are given preference. OSPF supports a variable network subnet mask so that a network can be subdivided. RIP is supported within OSPF for router-to-end station communication. Since many networks using RIP are already in use, router manufacturers tend to include RIP support within a router designed primarily for OSPF.  Exterior Gateway Protocol (EGP) Exterior Gateway Protocol (EGP) is a protocol for exchanging routing information between two neighbor gatewayhosts (each with its own router) in a network of autonomous systems. EGP is commonly used between hosts on the Internet to exchange routing table information. The routing table contains a list of known routers, the addresses they can reach, and a cost metric associated with the path to each router so that the best available route is chosen. Each router polls its neighbor at intervals between 120 to 480 seconds and the neighbor responds by sending its complete routing table. EGP-2 is the latest version of EGP.  A more recent exterior gateway protocol, the Border Gateway Protocol (BGP), provides additional capabilities.  Also see Interior Gateway Protocol (IGP). Border Gate Protocol (BGP) BGP (Border Gateway Protocol) is a protocol for exchanging routing information between gatewayhosts (each with its own router) in a network of autonomous systems. BGP is often the protocol used between gateway hosts on the Internet. The routing table contains a list of known routers, the addresses they can reach, and a cost metric associated with the path to each router so that the best available route is chosen.  Hosts using BGP communicate using the Transmission Control Protocol (TCP) and send updated router table information only when one host has detected a change. Only the affected part of the routing table is sent. BGP-4, the latest version, lets adminstrators configure cost metrics based on policy statements. (BGP-4 is sometimes called BGP4, without the hyphen.)  BGP communicates with autonomous (local) networks using Internal BGP (IBGP) since it doesn't work well with IGP. The routers inside the autonomous network thus maintain two routing tables: one for the interior gateway protocol and one for IBGP.  BGP-4 makes it easy to use Classless Inter-Domain Routing (CIDR), which is a way to have more addresses within the network than with the current IP address assignment scheme.  BGP is a more recent protocol than the Exterior Gateway Protocol (EGP).  Also see the Interior Gateway Protocol (IGP) and the Open Shortest Path First (OSPF) interior gateway protocol.  Protocol In information technology, a protocol (pronounced PROH-tuh-cahl, from the Greek protocollon, which was a leaf of paper glued to a manuscript volume, describing its contents) is the special set of rules that end points in a telecommunication connection use when they communicate. Protocols exist at several levels in a telecommunication connection. There are hardware telephone protocols. There are protocols between each of several functional layers and each corresponding layer at the other end of a communication. Both end points must recognize and observe a protocol. Protocols are often described in an industry or international standard.  On the Internet, there are the TCP/IP protocols, consisting of:  Transmission Control Protocol (TCP), which uses a set of rules to exchange messages with other Internet points at the information packet level  Internet Protocol (IP), which uses a set of rules to send and receive messages at the Internet address level  Additional protocols that are usually packaged with a TCP/IP suite, including the Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP), each with defined sets of rules to use with corresponding programs elsewhere on the Internet  There are many other Internet protocols, such as the Border Gateway Protocol (BGP) and the Dynamic Host Configuration Protocol (DHCP).  Router On the Internet, a router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination. The router is connected to at least two networks and decides which way to send each information packet based on its current understanding of the state of the networks it is connected to. A router is located at any gateway (where one network meets another), including each Internet point-of-presence. A router is often included as part of a network switch.  A router may create or maintain a table of the available routes and their conditions and use this information along with distance and cost algorithms to determine the best route for a given packet. Typically, a packet may travel through a number of network points with routers before arriving at its destination. Routing is a function associated with the Network layer (layer 3) in the standard model of network programming, the Open Systems Interconnection (OSI) model. A layer-3 switch is a switch that can perform routing functions.  An edge router is a router that interfaces with an asynchronous transfer mode (ATM) network. A brouter is a network bridge combined with a router.  Failover Failover is a backup operational mode in which the functions of a system component (such as a processor, server, network, or database, for example) are assumed by secondary system components when the primary component becomes unavailable through either failure or scheduled down time. Used to make systems more fault-tolerant, failover is typically an integral part of mission-critical systems that must be constantly available. The procedure involves automatically offloading tasks to a standby system component so that the procedure is as seamless as possible to the end user. Failover can apply to any aspect of a system: within an personal computer, for example, failover might be a mechanism to protect against a failed processor; within a network, failover can apply to any network component or system of components, such as a connection path, storage device, or Web server. Originally, stored data was connected to servers in very basic configurations: either point-to-point or cross-coupled. In such an environment, the failure (or even maintenance) of a single server frequently made data access impossible for a large number of users until the server was back online. More recent developments, such as the storage area network (SAN), make any-to-any connectivity possible among servers and data storage systems. In general, storage networks use many paths - each consisting of complete sets of all the components involved - between the server and the system. A failed path can result from the failure of any individual component of a path. Multiple connection paths, each with redundant components, are used to help ensure that the connection is still viable even if one (or more) paths fail. The capacity for automatic failover means that normal functions can be maintained despite the inevitable interruptions caused by problems with equipment. Classless Interdomain Routing (CIDR) CIDR (Classless Inter-Domain Routing, sometimes known as supernetting) is a way to allocate and specify the Internet addresses used in inter-domain routing more flexibly than with the original system of Internet Protocol (IP) address classes. As a result, the number of available Internet addresses has been greatly increased. CIDR is now the routing system used by virtually all gateway hosts on the Internet's backbone network. The Internet's regulating authorities now expect every Internet service provider (ISP) to use it for routing. The original Internet Protocol defines IP addresses in four major classes of address structure, Classes A through D. Each of these classes allocates one portion of the 32-bit Internet address format to a network address and the remaining portion to the specific host machines within the network specified by the address. One of the most commonly used classes is (or was) Class B, which allocates space for up to 65,533 host addresses. A company who needed more than 254 host machines but far fewer than the 65,533 host addresses possible would essentially be "wasting" most of the block of addresses allocated. For this reason, the Internet was, until the arrival of CIDR, running out of address space much more quickly than necessary. CIDR effectively solved the problem by providing a new and more flexible way to specify network addresses in routers. (With a new version of the Internet Protocol - IPv6 - a 128-bit address is possible, greatly expanding the number of possible addresses on the Internet. However, it will be some time before IPv6 is in widespread use.) Using CIDR, each IP address has a network prefix that identifies either an aggregation of network gateways or an individual gateway. The length of the network prefix is also specified as part of the IP address and varies depending on the number of bits that are needed (rather than any arbitrary class assignment structure). A destination IP address or route that describes many possible destinations has a shorter prefix and is said to be less specific. A longer prefix describes a destination gateway more specifically. Routers are required to use the most specific or longest network prefix in the routing table when forwarding packets. A CIDR network address looks like this: 192.30.250.00/18 The "192.30.250.00" is the network address itself and the "18" says that the first 18 bits are the network part of the address, leaving the last 14 bits for specific host addresses. CIDR lets one routing table entry represent an aggregation of networks that exist in the forward path that don't need to be specified on that particular gateway, much as the public telephone system uses area codes to channel calls toward a certain part of the network. This aggregation of networks in a single address is sometimes referred to as a supernet. CIDR is supported by the Border Gateway Protocol, the prevailing exterior (interdomain) gateway protocol. (The older exterior or interdomain gateway protocols, Exterior Gateway Protocol and Routing Information Protocol, do not support CIDR.) CIDR is also supported by the OSPF interior or intradomain gateway protocol.   Multihomed Multihomed describes a computer host that has multiple IP addresses to connected networks. A multihomed host is physically connected to multiple data links that can be on the same or different networks. For example, a computer with a Windows NT 4.0 Server and multiple IP addresses can be referred to as "multihomed" and may serve as an IP router. Using the Stream Control Transmission Protocol (SCTP), multihoming allows a single SCTP endpoint to support multiple IP addresses, which means that a session is more likely to survive a network failure. In a single-homed session, a network failure can isolate the end system or make transport temporarily unavailable. Multihoming means that redundant local area networks (LANs) can be used to support local access. Various approaches, such as using addresses with different prefixes to force routing through different carriers, or even using redundant core networks, can be taken to reduce the effects of failures. Multihoming is commonly used in Web management for load balancing, redundancy, and disaster recovery.   Route Dampening When there is a route change, how does a BGP router handle that? At what time scale can a BGP router discover a route change?   BGP employs a process called route dampening, which takes care of routes that change frequently. It assigns those routes a penalty and after a certain limit those routes are rendered useless. Now while in suspended state routes are again monitored and if they are stable for a time period (called half-life) there penalty is reduced. Each half-life penalty is reduced to a certain limit. When this penalty is reduced below a certain limit (reuse limit) they are put back again in routing table. All these parameters are configurable. As in route dampening you can configure half-life. If it's the initial flapped route, as soon as NLRI is not there or the TCP connection breaks BGP will come to know that route has changed. Channel Service Unit/Data Service Unit (CSU/DSU) A CSU/DSU (Channel Service Unit/Data Service Unit) is a hardware device about the size of an external modem that converts a digital data frame from the communications technology used on a local area network (LAN) into a frame appropriate to a wide-area network (WAN) and vice versa. For example, if you have a Web business from your own home and have leased a digital line (perhaps a T-1 or fractional T-1 line) to a phone company or a gateway at an Internet service provider, you have a CSU/DSU at your end and the phone company or gateway host has a CSU/DSU at its end. The Channel Service Unit (CSU) receives and transmits signals from and to the WAN line and provides a barrier for electrical interference from either side of the unit. The CSU can also echo loopback signals from the phone company for testing purposes. The Data Service Unit (DSU) manages line control, and converts input and output between RS-232C, RS-449, or V.xx frames from the LAN and the time-division multiplexed (TDM) DSX frames on the T-1 line. The DSU manages timing errors and signal regeneration. The DSU provides a modem-like interface between the computer as Data Terminal Equipment (DTE) and the CSU. CSU/DSUs are made as separate products or are sometimes part of a T-1 WAN card. A CSU/DSU's Data Terminal Equipment interface is usually compatible with the V.xx and RS-232C or similar serial interface. Manufacturers of separate unit or integrated CSU/DSUs include Adtran, Cisco, and Memotec. The CSU originated at AT&T as an interface to their nonswitched digital data system. The DSU provides an interface to the data terminal equipment (DTE) using a standard (EIA/CCITT) interface. It also provides testing capabilities.   Egress Egress (pronounced EE-grehs, from Latin egressus, or going out) is the act of going out of something. For example, in telecommunications, an egress router is a router through which a data packet leaves one network for another network.   Ingress Ingress (pronounced IHN-grehs, from Latin ingressus or stepping into) is the act of entering something. For example, in telecommunications, an ingress router is a router through which a data packet enters a network from another network. Peering Peering is the arrangement of traffic exchange between Internet service providers (ISPs). Larger ISPs with their own backbone networks agree to allow traffic from other large ISPs in exchange for traffic on their backbones. They also exchange traffic with smaller ISPs so that they can reach regional end points. Essentially, this is how a number of individual network owners put the Internet together. To do this, network owners and access providers, the ISPs, work out agreements that describe the terms and conditions to which both are subject. Bilateral peering is an agreement between two parties. Multilateral peering is an agreement between more than two parties. Peering requires the exchange and updating of router information between the peered ISPs, typically using the Border Gateway Protocol (BGP). Peering parties interconnect at network focal points such as the network access points (NAP) in the United States and at regional switching points. Initially, peering arrangements did not include an exchange of money. More recently, however, some larger ISPs have charged smaller ISPs for peering. Each major ISP generally develops a peering policy that states the terms and conditions under which it will peer with other networks for various types of traffic. Private peering is peering between parties that are bypassing part of the public backbone network through which most Internet traffic passes. In a regional area, some ISPs exchange local peering arrangements instead of or in addition to peering with a backbone ISP. In some cases, peering charges include transit charges, or the actual line access charge to the larger network. Properly speaking, peering is simply the agreement to interconnect and exchange routing information.   Network Access Point (NAP) In the United States, a network access point (NAP) is one of several major Internet interconnection points that serve to tie all the Internet access providers together so that, for example, an AT&T user in Portland, Oregon can reach the Web site of a Bell South customer in Miami, Florida. Originally, four NAPs - in New York, Washington, D.C., Chicago, and San Francisco - were created and supported by the National Science Foundation as part of the transition from the original U.S. government-financed Internet to a commercially operated Internet. Since that time, several new NAPs have arrived, including WorldCom's "MAE West" site in San Jose, California and ICS Network Systems' "Big East." The NAPs provide major switching facilities that serve the public in general. Using companies apply to use the NAP facilities and make their own intercompany peering arrangements. Much Internet traffic is handled without involving NAPs, using peering arrangements and interconnections within geographic regions. The vBNS network, a separate network supported by the National Science Foundation for research purposes, also makes use of the NAPs. Internet Engineering Task Force (IETF) The IETF (Internet Engineering Task Force) is the body that defines standard Internet operating protocols such as TCP/IP. The IETF is supervised by the Internet Society Internet Architecture Board (IAB). IETF members are drawn from the Internet Society's individual and organization membership. 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