Ccna: Routing Protocols And Concepts.

Front 1

active state

Back 1

a state in which there is no feasible successor in the topology table and the local router goes into active state and queries its neighbors for routing information

Front 2

adjacency

Back 2

a relationship formed between selected neighboring routers and end nodes for the purpose of exchaning routing information. Adjacency is based on the use of a common media segment

Front 3

administrative distance

Back 3

rating of the trustworthiness of a routing information source. Adminstrative distance is often expressed as a numerical value between 0 and 255. The higher the value, the lower the trustworthiness rating. If a router has multiple routing protocols in its routing table, it will select the route with the lowest administrative distance.

Front 4

ALL SPF Routers

Back 4

a multicast group used in the OSPF routing protocol. the address is 224.0.0.5

Front 5

ARP

Back 5

address resolution protocol. Internet protocol used to map an IP address to a MAC address.

Front 6

Asynchronous Transfer Mode

Back 6

the international standard for cell relay in which multiple service types (voice, video and data) are conveyed in fixed-lenght cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media, such as e3, SONET and T3.

Front 7

automatic summarization

Back 7

consolidation of network and advertised in classful network advertisements. In RIP, this cause a single summary route to be advertised to other routers.

Front 8

autonomous system

Back 8

a collection of networks under a common administration sharing a common routing strategy. Autonomous systems are subdivided by areas. An autonomous system must be assigned a unique 16-bit number by the IANA.

Front 9

Autonomous System Boundary Router

Back 9

The OSPF router located between an OSP autonomous system network and a non-OSPF network. ASBRs run both OSPF and another routing protocol, such as RIP. ASBRs must reside in a nonstub OSPF area.

Front 10

backup designated router

Back 10

a router that becomes the disignated router if the current designated router fails. The BDR is the OSPF router with the second-highest priority at the time of the last DR election.

Front 11

Bellman-Ford Algorithm

Back 11

class of routing algorithms that iterate on the number of hops in a router to find a shortest-path spanning tree. Distance vector routing algorithms call for each router to send its entire routing table in each update, but only to its neighbors. Distance vector routing algorithms can be prone to routing loops but are computationally simpler than link-state routing algorithms.

Front 12

best path

Back 12

the fastest path to a certain destination. The fastest path is based on the routing protocol's metric.

Front 13

BGP

Back 13

border gateway protocol. Interdomain routing protocol that replaces EGP. BGP exchanges reachability information with other BGP systems. It is defined by RFC 1163.

Front 14

boundary router

Back 14

a router that sits on the edge of two discontiguous classful networks. A boundary router can laos be known as a router that sits on the edge of two different networks that have different routing protocols. Sometimes the term boundary router is loosely used when discussing OSPF and Autonomous System Boundary Routers.

Front 15

bounded updates

Back 15

updates that are bounded to those very routers that need the updated information instead of sending updates to all routers.

Front 16

CIDR

Back 16

classless interdomain routing. Technique supported by BGP4 and based on route aggregation. CIDR allows routers to group routs to reduce the quantity of routing information carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity. With CIDR, IP addresses and their subnet masks are written as four octets, separated by periods, followed by a forward slash and a two-digit number that represents the subnet mask.

Front 17

Cisco Discovery Protocol

Back 17

a cisco proprietary protocol that defines a set of messages that cisco devices send. The messages include basic statement about the device sending the message, such as the device's name, OS level, type of device, and other confiuration information. Cisco devices on neighboring data links receive these multicast CDP messages and learn about the neighboring devices.

Front 18

classful IP addressing

Back 18

In the early days of IPv4, IP addresses were divided into five classes, namely, Class A, B, C, D, E/

Front 19

classful routing protocols

Back 19

routing protocols that use classful IP addressing. They do not use subnet mask information in their routing operation. They automatically assume classful masks.

Front 20

classless routing protocols

Back 20

routing protocols that include the subnet mask in their routing operation.

Front 21

clocking

Back 21

the configuration of a clock rate on the DCE interface of a serial connection.

Front 22

clocking signal

Back 22

a signal used to coordinate the transmission on two or more circuits.

Front 23

contiguous

Back 23

consistent or adjacent. In terms of continguous networks, the word contiguous means network blocks that are hierarchical in nature.

Front 24

convergence

Back 24

speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology.

Front 25

cost

Back 25

an arbitrary value, typically based on hop count, media bandwidth, or other measures, that is assigned by a network administrator and used to compare various paths through an internetwork environment. Routing protocols use cost values to calculate the most favorable path to a particular destination: the lower the cost, the better the path

Front 26

count to infinity

Back 26

problem that can occur in routing algorithms that are slow to converge, in which routers continuously increment the hop count to particular networks. Typically some arbritrary hop-count limit is imposed to prevent this problem.

Front 27

data link

Back 27

Layer 2 of the OSI reference model. Provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control. The IEEE divided this layer into two sublayers: the MAC sublayer and the LLC sublayer. Sometimes simply called the link layer. Roughly corresponds to the data-link control layer of the SNA model.

Front 28

database description

Back 28

a packet used in OSPF that contains link-state advertisements LSA headers only and describes the contents of the entire link-state database. Routers exchange DBDs during the exchange phase of adjacency creation.

Front 29

datagram

Back 29

logical grouping of information sent as a network layer unit over a transmission medium without prior establishment of a virtual circuit. IP datagrams are the primary information units in the Internet. The terms cell, frame, message, packet and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

Front 30

designated router

Back 30

OSPF router that generates LSAs for a multiaccess network and has other special responsibilities in running OSPF. Each multiaccess OSPF network that has at least two attached routers has a designated router that is elected by the OSPF Hello protocol. The designated router enables a reduction in the number of adjacencies required on a mutliaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topological database.

Front 31

diffusing update algorithm

Back 31

convergence algorithm used in enhanced IGRP that provides loop-free operation at every instant throughout a route computation. Allows routers involved in a topology change to synchronize at the same time, while not involving routers that are unaffected by the change.

Front 32

discontiguous

Back 32

components that are fragmented. For example, a discontiguous network is composed of a major network that separates another major network.

Front 33

discontiguous address assignment

Back 33

fragmented network assignment that does not follow a consistent pattern.

Front 34

discontiguous network

Back 34

fragmented network addressing. networks that do not have a hierarchical scheme.

Front 35

DROthers

Back 35

routers that are not a DR or BDR. They are the routers in the OSPF network.

Front 36

DSL

Back 36

digital subscriber line. Network technology that delivers high bandwidth over conventional coppoer wiring at limited distances. There are four types of DSL: ADSL, HDSL, SDSL, and VDSL. All are provisioned through modem pairs, with one modem located at a central office and the other at the customer site. Because most DSL technologies do not use the whole bandwidth of the twisted pair, there is room remaining for a voice channel.

Front 37

dynamic routing protocols

Back 37

routing that adjusts automatically to network topology or traffic changes.

Front 38

EIGRP

Back 38

enhanced interior gateway routing protocol. advance version of IGRP developed by cisco. provides superior convergence properties and operating efficiency, and combines the advantages of link-state protocols with those of distance vector protocols.

Front 39

equal-cost load balancing

Back 39

when a a router utilizes multiple paths with the same administrative distance and cost to a destination.

Front 40

equal-cost metric

Back 40

a metric that has the same value on multiple paths to the same destination. When multiple paths have equal-cost metrics, a router can execute equal cost load balancing among those paths.

Front 41

ethernet

Back 41

baseband LAN specification invented by Xerox Corp and developed jointly by Xerox, Intel and DEC. Ethernet networks use CSMA/CD and run over a variety of cable types of 10mbps. Ethernet is similar to the IEEE 802.3 series of standards.

Front 42

Exterior gateway protocol

Back 42

routing protocols used to route between autonomous systems. The current exterior routing protocol of the internet is BGP.

Front 43

FDDI

Back 43

Fiber distributed data interface. LAN standard, defined by ANSI X3T9.5. specifying a 100 Mbps toke-passing network using fiber-optic cable, with transmission distances of up to 2km. FDDI uses dual-ring architecutre to provide redundancy.

Front 44

feasibility condition

Back 44

If the receiving router has a feasible distance to a particular network and it receives an update from a neighbor with a lower advertised distance to that network, there is a feasibility condition. Used in EIGRP

Front 45

feasible distance

Back 45

the metric of a network advertised by the connected neighbor plus the cost of reaching that neighbor. The path with the lowest metric is added to the routing table and is called FD or feasible distance. Used in EIGRP routing

Front 46

feasible successor

Back 46

a next-hop router that leads to a certain destination network. The feasible successor can be thought of as a backup next hop if the primary next hop goes down. Used in EIGRP routing.

Front 47

flapping link

Back 47

Routing problem where an advertised route between two nodes alternates back and forth between two paths because of a network problem that causes internmittent interface failures.

Front 48

frame realy

Back 48

a packet-switched data link layer protocol that handles multiple virtual circuits used between connected devices. Frame realy is more efficient than x.25 the protocol for which it generally is considered a replacement.

Front 49

gateway

Back 49

a device on a network that serves as an access point to other networks. A default gateway is used by a host when an IP packet's destination address belongs to someplace outside the local subnet. A router is a good example of a default gateway.

Front 50

high-order bit

Back 50

a bit of a binary number that carries the most weight, the one written farthest to the left. High-order bits are 1s in the network mast.

Front 51

hold time

Back 51

the maximum time a router waits to receive the next Hello packet or routing update. When the hold-time counter expires, that route will become unreachable.

Front 52

hold-down timers

Back 52

timers that a route is placed in so that routers neither advertise the route nor accept advertisements about the route for a sepcific length of time. Holddown is used to flush bad info about a route from all routers in a network. A route typically is placed in holddown when a link in that route fails.

Front 53

host

Back 53

computer system on a network. Similar to node, except that host usually implies a computer system, whereas node generally applies to any networked system, including access servers and routers.

Front 54

hub and spoke

Back 54

a wan topology in which various branch offices are connected through a centralized hub or headquarters.

Front 55

ICMP

Back 55

internet control message protocol. Network layer internet protocol that reports errors and provides other information relevant to IP packet processing. Documented in RFC 792.

Front 56

IGRP

Back 56

Interior gateway routing protocol. IGP developed by cisco to address the issues associated with routing in large, heterogeneous networks.

Front 57

Interior gateway protocol

Back 57

Internet protocol used to exchange routing information within an autonomous system. Examples of common internet IGPs include IGRP, OSPF and RIP.

Front 58

IP

Back 58

Network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides features for addressing, type of service specification, fragmentation and reassembly and security RFC 791

Front 59

IPv6

Back 59

a network layer protocol for packet-switched internetworks. This is the successor of IPv4 for general use on the Internet.

Front 60

IPX

Back 60

Internetwork Packet Exchange. NetWare network layer protocol used for transferring data from servers to workstations. IPX is similar to IP and XNS.

Front 61

ISDN

Back 61

Integrated Services Digital Network. Communication protocol offered by telephone companies that permits telephone networks to carry data, voice and other source traffic.

Front 62

IS-IS

Back 62

the intermediate system to intermediate system protocol is based on a routing method known as DECnet Phase V routing, in which routers known as intermediate systems exchange data about routing using a single metric to determine the network topology. IS-IS was developed by the International Organization for Stadardization as part of its Open systems interconnection model.

Front 63

LAN

Back 63

High-speed, low error data network covering a relatively small geographic area. LANs connect workstations, peripherals, terminals and other devices in a single building or other geographically limited areas. LAN standards specify cabling and signaling at the physical and data link layers of the OSI model.

Front 64

level 1 parent route

Back 64

a first-level route in the routing table that has subnets catalogued under it. A first-level parent route does not contain a next-hop IP address or exit interface information, but serves as a type of header information for the child routes.

Front 65

Level 1 route

Back 65

a route with a subnet mask equal to or less than the calssful mask of the network address.

Front 66

level 2 child route

Back 66

the subnets that belong to the parent route.

Front 67

level 2 route

Back 67

a subnet is the level 2 route of the parent route.

Front 68

link state

Back 68

refers to the status of a link, including the interface IP address/subnet mask, type of network, cost of the link, and any neighbor routers on that link.

Front 69

link-state acknowledgment

Back 69

acknowledges receipt of LSA packets. Link-state acknowledgment packets are Type 5 OSPF packets.

Front 70

link-state advertisement

Back 70

Broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables.

Front 71

link-state database

Back 71

a table used is OSPF that is a representation of the topology of the autonomous system. It is the method by which routers "see" the state of the links in the autonomous sytem.

Front 72

link-state request

Back 72

link state request packets are Type 3 OSPF packets. The link-state request packet is used to request the pieces of the neighbors database that are more up to date.

Front 73

link state router

Back 73

a router that uses a link-state routing protocol.

Front 74

link state routing protocol

Back 74

a routing protocol in which routers exchange information with one another about the reachability of other networks and the cost or metric to reach the other networks. Link-state routers use Dijkstra's algorithm to calculate shortes paths to a destination, and normally update other routers with whom they are connected only when their own routing tables exchange.

Front 75

link state update

Back 75

Link-state update packets are Type 4 OSPF packets. A link statue update packet carries a collection of link-state advertisements one hop farther from its origin.

Front 76

load balancing

Back 76

In routing, the capability of a router to distribute traffic over all its network ports that are the same distance from the destination address. Good load-balancing algorithms use both line speed and reliability information. Load balancing increases the use of network segments, thus increasing effective network bandwidth.

Front 77

longest match

Back 77

a route in the routing table that has a closer match of leftmost bits with the destination IP address in the IP packet.

Front 78

loopback address

Back 78

127.0.0.1 is an IP address available on all devices to test whether the NIC on that device is functioning. If you send a packet to 127.0.0.1 it loops back on itself, thereby sending the data to the NIC on that device. If you get a positive response to a ping command, you known that your nic is up and running.

Front 79

loopback interface

Back 79

a virtual interface used for management purposes. Unlike a proper loopback interface, this loopback device is not used to talk with itself.

Front 80

MAC address

Back 80

standardized data link layer address that is required for every port or device that connects to a LAN. Other devices in the network use these addreses to locate specific ports in the network and to create and update routing tables and data structures. MAC addresses are 6 bytes long and are conntrolled by the IEEE.

Front 81

metric

Back 81

method by which a routing algorithm determines that one route is better than another. This information is stored in routing tables. Metrics include bandwidth, communication cost, delay, hop count, load, MTU, path cost, and reliability

Front 82

multiaccess network

Back 82

network that allows multiple devices to connect and communicate simultaneously.

Front 83

NAT

Back 83

network address translation. Mechanism for reducing the need for globally unique IP addresses. NAT allows an organization with addresses that are not globally unique to connect to the Internet by translating those addresses into globally routable address space.

Front 84

neighbor

Back 84

in OSPF, two routers that have interfaces to a common network. On multiaccess networks, neighbors are discovered dynamically by the OSPF Hello protocol

Front 85

network prefix

Back 85

Number of bits that defines the subnet mask. For example, the subnet mask 255.255.0.0 is /16

Front 86

next hop

Back 86

the next point of routing. When routers are not directly connected to the destination network, they will have a neighboring router that provides the next step in routing the data to its destination.

Front 87

NIC

Back 87

network interface card. computer hardware that is designed to allow computers to communicate over a computer network.

Front 88

nonbroadcast multiaccess network NBMA

Back 88

term describing a mutliaccess network that either does not support broadcasting or in which broadcasting is not feasible.

Front 89

null interface

Back 89

provides an alternative method of filtering traffic. You can avoid the overhead involved with using access lists by directing undesired network traffic to the null interface. This interface is always up and can never foward or receive traffic. This of it as a black hole.

Front 90

null0 summary routes

Back 90

another mechanism to prevent routing loops. EIGRP always creates a route to the Null0 interface when it summarizes a group of routes. This is because whenever a routing protocol summarizes, the router might receive traffic for any IP address within that summary. Because not all IP addresses are always in use, there is a risk of looping packets in case default routes are used on the router that receives the traffic for the summary route.

Front 91

NVRAM

Back 91

nonvolatile random-access memory. Random-access memory that retains its contents when the power is shut off.

Front 92

OSPF

Back 92

software that performs basic tasks such as controlling and allocating memroy, prioritizing system requests, controlling input and output devices, facilitating networking, and managing file systems.

Front 93

OSPF area

Back 93

a logical set of network segments and their attached devices. Areas are usually connected to other areas through routers, making up a single autonomous system.

Front 94

packet

Back 94

logical grouping of information that includes a header containing control information and user data. Packets most often are used to refer to network layer units of data. The terms datagram, frame, message, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circle.

Front 95

passive state

Back 95

a state when the router has identified the successor for a certain destination and it becomes stabl. A term used in conjunction with EIGRP.

Front 96

packet

Back 96

logical grouping of information that includes a header containg control information and user data. Packets most often are used to refer to network layer units of data. The terms datagram, frame, message and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

Front 97

passive state

Back 97

a state when the router has identified the successors for a certain destination and ti becomes stable. A term used in conjunction with EIGRP

Front 98

path vector protocol

Back 98

a routing protocol that marks and shows the path that update information takes as it diffuses through the network. BGP is a user of this kind of protocol because it verifies what autonomous systems the update has passed through to very loops.

Front 99

point-to-point protocol

Back 99

successor to SLIP that provides router to router and host to network connections over synchronous and asynchronous circuits. Whereas SLIP was designed to work with IP, PPP was designed to work with several network layer protocols, such as IP, IPX and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP.

Front 100

poison revers

Back 100

a variation of the split horizon technique that specifies that all routes should be included in an update out a particular interface, but that the metric should be set to infinity for those routes acquired over that interface.

Front 101

prefix agregation

Back 101

also known as network summarization. Multiple IP addresses and IP prefixes can be summarized into a single IP previs and be announced to other routers as only the resulting less-specific prefix instead of the most specific IP addresses and prefixes that it covers. For example, the 172.16.1.0/24, 172.16.2.0/24 and 172.16.3.0/24 networks can all be summarized as a single 172.16.0.0/22 network

Front 102

private address

Back 102

an address that is used for internal networks. This address follows RFC 1918 addressing. Not routable on the Internet.

Front 103

privileged EXEC mode

Back 103

the administration mode for the router or switch. This node allows you to view router settings that are considered only accessible to the administrator. This mode also allows you to enter global configuration mode. To enter privileged EXEC mode, you must use the enable command.

Front 104

protocol-dependent module

Back 104

a component that depends on a certain routed protocol. For example, protocol dependent modules in EIGRP allow it to work with various routed protocols. PDMS allow EIGRP to keep a topology table for each routed protocol such as IP, IPX, RIP, AppleTalk Routing Table maintenance Protocol (RTMP), and IGRP.

Front 105

quad zero route

Back 105

default route, 0.0.0.0 representing either the network address or the subnet mast

Front 106

recursive route lookup

Back 106

route lookup that occurs to resolve the next hop IP address of a previous route.

Front 107

redistribution

Back 107

allowing routing information discovered through one routing protocol to be distributed in the update messages of another routing protocol.

Front 108

reference bandwidth

Back 108

the bandwidth referenced by the SPF algorithm when calculating the shorted path. In OSPF, the reference bandwidth is 10^8 divided by the actual interface bandwidth.

Front 109

reported distance

Back 109

the total metric along a path to a destination network as adveritsed by an upstream neighbor in EIGRP

Front 110

RIP

Back 110

routing information protocol. IGP supplied with UNIX BSD systems. The most common IGP in the Internet. RIP uses hop count as a routing metric.

Front 111

route poisoning

Back 111

routing updates that explicitly indicate that a network or subnet is unreachable, rather than implying that a network is unreachable by not including it in updates. Poison reverse updates are sent to defeat large routing loops. The cisco IGRP implement uses poison reverse updates.

Front 112

route summarization

Back 112

consolidation of advertised addresses in OSPF and IS-IS. In OSPF, this causes a single summary route to be advertised to other areas by an area border route.

Front 113

router

Back 113

network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information. Occasionally called a gateway.

Front 114

routing table

Back 114

a table stored in the memory of a router or some other internetworking device that keeps track of routes to particular network destinations. A router uses this list of networks to determine where to send ata.

Front 115

serial

Back 115

method of data transmission in which the bits of data characters are transmitted sequentially over a single channel.

Front 116

shortest path first SPF algorithm

Back 116

routing algorithm that iterates on the length of path to determine a shortest-path spanning tree. Commonly used in link-state routing algorithms. Sometimes called Dijkstra's algorithm

Front 117

SPF schedule delay

Back 117

after executing the show ip ospf command, you will see the parameter SPF schedule delay X secs. This is the delay time of SPF calculations.

Front 118

split horizon

Back 118

Routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split horizon update are useful in preventing routing loops.

Front 119

static routing

Back 119

routing that depends on manually entered routes in the routing table

Front 120

stub network

Back 120

a network that only has a single exit point to the Internet or other networks.

Front 121

stub router

Back 121

a router that is connected to only one other router.

Front 122

successor

Back 122

the path to a destination. The successor is chosen using DUAL from all the known paths or feasible successors to the end destination. Used in EIGRP

Front 123

summary route

Back 123

a summary route is the result of route summarization. Route summarization reduces the number of routes that a route must maintain. It is a method of representing a series of network addresses as a single summary address. A summary route would be a single route representing multiple specific routes. For example the networks 172.16.1.0/24, 172.16.2.0/24 and 172.16.3.0/24 can be summarized into a single 172.16.0.0/22 network. The 172.16.0.0/22 network can then be advertised as a single summary route for all three networks.

Front 124

supernet

Back 124

aggregation of IP network addresses advertised as a single classless network address. For example, given four class c ip networks - 192.0.8.0, 192.0.9.0, 192.0.10.0 and 192.0.11.0 each having the intrinsic network mask of 255.255.255.0 one can advertise the address 192.0.8.0 with a subnet mask of 255.255.252.0

Front 125

TLV

Back 125

type/length/value the data portion of the EIGRP packet. All TLVs begin with a 16bit Type field and a 16-bit length field. Diferent TLC values exist according to the routed protocol. There is, however a general TLC that describes generic EIGRP parameters such as Sequence and EIGRP software version.

Front 126

Toke ring

Back 126

token-passing LAN developed and supported by IBM. Token Ring runs at 4 - 16 MBPS over a ring topology

Front 127

topology datatbase

Back 127

also know as the topology table, the topology database holds the information about the successor, feasible distance and any feasible successors with their reported distances. Used in EIGRP routing.

Front 128

triggered update

Back 128

a routing update that is triggered by an event in the network

Front 129

TTL

Back 129

time to live, Field in an IP header that indicates how long a packet is considered valid.

Front 130

ultimate rout

Back 130

also know as a level 1 route, an ultimate route is a route in the routing table that includes a next-hop address and an outgoing interface.

Front 131

unequal-cost load balancing

Back 131

load balancing that uses multiple paths to the same destination that have different costs or metrics. EIGRP uses unequal-cost load balancing with variance command.

Front 132

unified communications

Back 132

a communications system for voice, video and data. The system integrates wired, wireless and mobile devices to create a secure solution for enterprise networks.

Front 133

vector

Back 133

a quantity characterized by a magnitude (for example, hops in a path) and a direction.

Front 134

VLSM

Back 134

variable-length subnet mask. Capability to specify a different subnet mask for the same network number on different subnets. VLSM can help optimize available address space.

Front 135

WAN

Back 135

data communications network that serves users across a broad geographic area and often uses transmission devices provided by common carriers. Frame relay, SMDS, and x.25 are examples of WANs

Front 136

wildcard mask

Back 136

a 32-bit quantity used in conjunction with an IP address to determine which bits in an IP address should be ignored when comparing that address with another IP address. A wildcard mask is specified when setting up access lists.

Front 137

XNS

Back 137

xerox network systems. a protocol stack developed by xerox that contains network protocols that closely resemble IP and TCP. XNS was one of the first protocol stacks used in the first local-area network implementations.