Выдержки из Chapter 5 книги Cisco Press "Interconnecting Cisco Network Devices, Part 1 (ICND1): CCNA Exam 640-802 and ICND1 Exam 640-822" для повторения перед экзаменом.
Understanding WAN Technologies
Understanding WAN Technologies
Here are the three major characteristics of WANs:
■ WANs connect devices that are separated by wide geographical areas.
■ WANs use the services of carriers, such as telephone companies, cable companies, satellite systems, and network providers.
■ WANs use serial connections of various types to provide access to bandwidth over large geographical areas.
WAN Devices
Several devices operate at the physical layer in a WAN. The following devices are used for
WAN access:
■ Routers: Routers provide internetworking and WAN access interface ports.
■ Communication servers: Communication servers concentrate dial-in and dial-out user communications
■ Modems or digital service units (DSU)/channel service units(CSU): In analog lines, modems convert the digital signal of the sending device into analog format for transmission over an analog line and then convert this digital signal back to digital form so that it can be received and processed by the receiving device on the network. For digital lines, a DSU and a CSU are required. The two are often combined into a single piece of equipment, called the DSU/CSU. The DSU/CSU can also be built into the interface card in the router.
■ WAN networking devices: Other devices, such as ATM switches, Frame Relay switches, public switched telephone network (PSTN) switches, and core routers, are also used within the cloud to support the access services.
NOTE To support higher densities in a smaller form factor, Cisco introduced a smart serial cable. The serial end of the smart serial cable is a 26-pin connector. It is much smaller than the DB-60 connector that connects to a five-in-one serial port. These transition cables support the same five serial standards, are available in either DTE or DCE configuration, and are used with two-port serial connections and two-port asynchronous and synchronous WICs.
The WAN data link layer protocols are as follows:
■ HDLC
■ PPP
■ Frame Relay (Link Access Procedure for Frame Relay [LAPF])
■ ATM
Two major categories of communication links for WANs exist: dedicated and switched. Within each category, individual types of communication link options exist, as follows:
■ Dedicated communication links: When permanent dedicated connections are required, point-to-point lines are used with various capacities that are limited only by the underlying physical facilities and the willingness of users to pay for these dedicated lines. A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider network to a remote destination. Point-to-point lines are usually leased from a carrier and are also called leased lines.
■ Circuit-switched communication links: Circuit switching dynamically establishes a dedicated virtual connection for voice or data between a sender and a receiver. Before communication can start, you need to establish the connection through the network of the service provider.
■ Packet-switched communication links: Many WAN users do not make efficient use of the fixed bandwidth that is available with dedicated, switched, or permanent circuits because the data flow fluctuates. Communications providers have data networks available to more appropriately service these users. In packet-switched networks, the data is transmitted in labeled cells, frames, or packets.
Summary of Understanding WAN Technologies
The following list summarizes the key points that were discussed in this lesson.
■ A WAN has three major characteristics: the connection of devices that are separated by wide geographical distances; the use of the services of carriers, such as telephone companies, cable companies, satellite systems, and network providers; and the use of serial connections of various types to access bandwidth over large geographic areas.
■ Many business and home needs require communication among remote users, including communication between users in remote company locations, data sharing among different organizations, access to corporate information by traveling workers, and access to Internet.
■ LANs connect computers, peripherals, and other devices in a single building or other small geographic area; WANs transmit data across broad geographic distances.
■ A company, organization, or individual must subscribe to an outside WAN service provider to use WAN network services, whereas LANs are owned typically by the company, organization, or individual that uses them.
■ WAN access functions in relation to the OSI reference model; the WAN function focuses primarily on Layer 1 and Layer 2.
■ The major types of devices used for WAN access environments include routers, communication servers, modems (DSU/CSUs).
■ Routers have both LAN and WAN interfaces, and whereas a router segments LANs, it is also used as the WAN connection device.
■ The data link layer protocols define how data is encapsulated for transmission toward remote sites in a WAN environment and the mechanisms for transferring the resulting frames.
Packet-Switched Communication Links
Packet switching is a switching method in which no dedicated path between source and destination endpoints exists, allowing for the sharing of connection links and common carrier resources for data transmission.
Digital Subscriber Line
DSL technology is an always-on connection technology that uses existing twisted-pair telephone lines to transport high-bandwidth data and provides IP services to subscribers.
DSL Types and Standards
The two basic types of DSL technologies are as follows:
■ Asymmetric DSL (ADSL): Provides higher download bandwidth than upload
bandwidth
■ Symmetric DSL (SDSL): Provides the same capacity of bandwidth in both directions
All forms of DSL services are categorized as asymmetric or symmetric, but several varieties of each type exist. ADSL includes the following forms:
■ ADSL
■ Consumer DSL (CDSL), also called G.Lite or G.992.2
■ Very-high-data-rate DSL (VDSL)
SDSL includes the following forms:
■ SDSL
■ High-data-rate DSL (HDSL)
■ ISDN DSL (IDSL)
■ Symmetric high-bit-rate DSL (G.shdsl)
Introducing NAT and PAT
NAT enables private IP intranets that use nonregistered IP addresses to connect to the Internet. Usually, NAT connects two networks together and translates the private (inside local) addresses in the internal network into public addresses (inside global) before packets are forwarded to another network
Several internal addresses can be translated using NAT into just one or a few external addresses by using PAT. PAT uses unique source port numbers on the inside global IP address to distinguish between translations. Because the port number is encoded in 16 bits, the total number of internal addresses that NAT can translate into one external address is, theoretically, as many as 65,536 addresses. PAT attempts to preserve the original source port. If the source port is already allocated, PAT attempts to find the first available port number. It starts from the beginning of the appropriate port group, 0–511, 512–1023, or 1024–65535. If PAT does not find a port that is available from the appropriate port group and if more than one external IP address is configured, PAT moves to the next IP address and tries to allocate the original source port again.
In NAT terminology, the “inside network” is the set of networks that are subject to translation. The “outside network” refers to all other addresses. Usually these are valid addresses located on the Internet. Cisco defines the following NAT terms:
■ Inside local address: The IP address assigned to a host on the inside network. The inside local address is likely not an IP address assigned by the Internet Assigned Numbers Authority (IANA) or service provider.
■ Inside global address: A legitimate IP address assigned by the NIC or service provider that represents one or more inside local IP addresses to the outside world.
■ Outside local address: The IP address of an outside host as it appears to the inside network. Not necessarily legitimate, the outside local address is allocated from an address space routable on the inside.
■ Outside global address: The IP address assigned to a host on the outside network by the host owner. The outside global address is allocated from a globally routable address or network space.
Verifying the NAT and PAT Configuration
You can verify the NAT and PAT configuration with the command show ip nat translation.
Summary of Enabling the Internet Connection
This topic summarizes the key points that were discussed in this section.
■ Packet-switched networks send data packets over different routes of a shared public network owned by a carrier to reach the same destination. The route that the packets take to reach the destination site, however, varies.
■ Packet-switched networks send data packets over different routes of a shared public network owned by a carrier to reach the same destination. The route that the packets take to reach the destination site, however, varies.
■ DSL comes in several varieties, including ADSL, SDSL, HDSL, IDSL, and CDSL. DSL has both advantages (speed, always on, and so on) and disadvantages (availability).
■ The global Internet grew from a U.S. Department of Defense plan to build a commandand-control network in the 1960s to its present state as the largest WAN on earth, with multiple ways to access it and multiple communication, research, and commercial uses.
■ An interface can get its IP address from a DHCP server.
■ NAT enables private IP internetworks that use nonregistered IP addresses to connect to the Internet.
■ You can translate your own IP addresses into globally unique IP addresses when you are communicating outside of your network.
■ Overloading is a form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address (many-to-one) by using different ports, known also as PAT.
■ After NAT is configured, the clear and show commands can be used to verify that it operates as expected.
Routing Overview
To be able to route anything, a router, or any entity that performs routing, must do the following:
■ Identify the destination address: Determine the destination (or address) of the item that needs to be routed
■ Identify sources of routing information: Determine from which sources (other routers) the router can learn the paths to given destinations.
■ Identify routes: Determine the initial possible routes, or paths, to the intended destination.
■ Select routes: Select the best path to the intended destination.
■ Maintain and verify routing information: Determine if the known paths to the destination are the most current.
Static and Dynamic Route
Routers can forward packets over static routes or dynamic routes based on the router configuration. The two ways to tell the router where to forward packets to destination networks that are not directly connected are as follows:
■ Static route: The router learns routes when an administrator manually configures the static route. The administrator must manually update this static route entry whenever an internetwork topology change requires an update. Static routes are user-defined routes that specify the path that packets take when moving between a source and a destination. These administrator-defined routes allow very precise control over the routing behavior of the IP internetwork.
■ Dynamic route: The router dynamically learns routes after an administrator configures a routing protocol that helps determine routes. Unlike the situation with static routes, after the network administrator enables dynamic routing, the routing process automatically updates route knowledge whenever new topology information is received. The router learns and maintains routes to the remote destinations by exchanging routing updates with other routers in the internetwork.
Default Route Forwarding
You should use a default route in situations in which the route from a source to a destination is not known or when it is not feasible for the router to maintain many routes in its routing table.
Static Route Configuration Verification
To verify that you have properly configured static routing, enter the show ip route.
Summary of Enabling Static Routing
The following summarizes the key points that were discussed in this section:
■ Routing is the process by which items get from one location to another. In networking, a router is the device used to route traffic. Routers can forward packets over static routes or dynamic routes based on the router configuration.
■ Static routers use a route that a network administrator enters into the router manually. Dynamic routes use a router that a network routing protocol adjusts automatically for topology or traffic changes.
■ Unidirectional static routes must be configured to and from a stub network to allow communications to occur.
■ The ip route command can be used to configure default route forwarding.
■ The show ip route command verifies that static routing is properly configured. Static routes are signified in the command output by “S.”
Circuit-Switched Communication Links
Switched circuits allow connections to be initiated when transmission is needed and terminated when the transmission is complete.
Traditional telephony uses a copper cable, called the local loop, to connect the telephone handset in the subscriber premises to the telephone network.
Using PSTN has a number of advantages, including the following:
■ Simplicity: Other than a modem, no additional equipment is required, and analog modems are easy to configure.
■ Availability: Because a public telephone network is available virtually everywhere, it is easy to locate a telephone service provider, and the maintenance of the telephone system is very high quality, with few instances in which lines are not available.
■ Cost: The cost associated with the implementation of a PSTN connection link for a WAN is relatively low, consisting primarily of line charges and modems.
Using PSTN also has some disadvantages, including the following:
■ Low data rates: Because the telephone system was designed to transmit voice data, the transmission rate for large data files is noticeably slow.
■ Relatively long connection setup time: Because the connection to the PSTN requires a dialup activity, the time required to connect through the WAN is very slow compared to other connection types.
Point-to-Point Communication Links
A point-to-point (or serial) communication link provides a single, established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network.
Bandwidth
Bandwidth refers to the rate at which data is transferred over the communication link. The underlying carrier technology depends on the bandwidth available.
Clock rates in bits per second are as follows: 1200, 2400, 4800, 9600, 19200, 38400, 56000, 64000, 72000, 125000, 148000, 500000, 800000, 1000000, 1300000, 2000000, and 4000000.
The bandwidth command overrides the default bandwidth that is displayed in the show interfaces command and is used by some routing protocols.
The show controller command displays information about the physical interface itself.
Point-to-Point Communication Considerations
Point-to-point links have been the traditional connection of choice. The advantages to this type of WAN access include the following:
■ Simplicity: Point-to-point communication links require minimal expertise to install and maintain.
■ Quality: Point-to-point communication links usually offer a high quality of service, provided that they have adequate bandwidth. The dedicated capacity gives no latency or jitter between the endpoints.
■ Availability: Constant availability is essential for some applications, such as electronic commerce, and point-to-point communication links provide permanent, dedicated capacity that is always available.
This type of WAN access also has some disadvantages, including the following:
■ Cost: Point-to-point links are generally the most expensive type of WAN access, and this cost can become significant when they connect many sites. In addition, each endpoint requires an interface on the router, which increases equipment costs.
■ Limited flexibility: WAN traffic is often variable, and leased lines have a fixed capacity, resulting in the bandwidth of the line seldom being exactly what is needed. Any changes to the leased line generally require a site visit by the ISP or carrier personnel to adjust capacity.
High-Level Data Link Control Protocol
The High-Level Data Link Control (HDLC) protocol is one of two major data-link protocols commonly used with point-to-point WAN connections.
HDLC specifies an encapsulation method for data on synchronous serial data links using frame character and checksum. HDLC supports both point-to-point and multipoint configurations and includes a means for authentication. However, HDLC might not be compatible between devices from different vendors because of the way each vendor might have chosen to implement it.
A Cisco implementation of HDLC exists; it is the default encapsulation for serial lines. Cisco HDLC is streamlined. It has no windowing or flow control, and only point-to-point connections are allowed. The Cisco HDLC implementation includes proprietary extensions in the data field, as shown in Figure 5-31; the extensions allowed multiprotocol support at a time before PPP was specified. Because of the modification, the Cisco HDLC implementation does not interoperate with other HDLC implementations.
Use the encapsulation hdlc interface configuration command to specify HDLC encapsulation on the interface
Point-to-Point Protocol
PPP originally emerged as an encapsulation protocol for transporting IP traffic over pointto-point links. PPP also established a standard for the assignment and management of IP addresses, asynchronous (start and stop bit) and bit-oriented synchronous encapsulation, network protocol multiplexing, link configuration, link quality testing, error detection, and option negotiation for such capabilities as network layer address negotiation and datacompression negotiation. PPP provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits. An example of an asynchronous connection is a dialup connection. An example of a synchronous connection is a leased line.
PPP provides a standard method for transporting multiprotocol datagrams (packets) over point-to-point links. PPP comprises these three main components:
■ A method for encapsulating multiprotocol datagrams
■ A link control protocol (LCP) for establishing, configuring, and testing the data-link connection
■ A family of Network Control Programs (NCP) for establishing and configuring different network layer protocols
PPP provides that an LCP be sufficiently versatile and portable to a wide variety of environments. The LCP is used to automatically determine the encapsulation format option, handle varying limits on sizes of packets, and detect a loopback link and terminate the link.
Other optional facilities provided are authentication of the identity of its peer on the link and determination of when a link is functioning properly or failing.
The authentication phase of a PPP session is optional. After the link has been established and the authentication protocol chosen, the peer can be authenticated. If the authentication option is used, authentication takes place before the network layer protocol configuration phase begins.
You can configure PPP on the following types of physical interfaces:
■ Asynchronous serial
■ Synchronous serial
■ Basic Rate Interface (BRI)
■ High-Speed Serial Interface (HSSI)
To enable PPP encapsulation, enter interface configuration mode. Use the encapsulation ppp interface configuration command to specify PPP encapsulation on the interface.
Use the show interface command to verify proper configuration.
Frame Relay
Frame Relay is a packet-switching protocol that grew in its popularity by being much more cost-effective and thereby replaced older technologies such as X.25 and leased lines.
Frame Relay provides both permanent virtual circuit (PVC) and switched virtual circuit (SVC) service using shared medium-bandwidth connectivity that carries both voice and data traffic. Available data rates are commonly up to 4 Mbps
Frame Relay is ideal for connecting enterprise LANs, because a router on the LAN needs only a single WAN interface, even when multiple virtual circuits (VC) are used. The dicated line to the Frame Relay network edge allows cost-effective connections between widely scattered LANs.
Frame Relay operates over virtual circuits, which are logical connections created to enable communication between two remote devices across a network. VCs provide a bidirectional communications path from one DTE device to another. A data-link connection identifier (DLCI) within the Frame Relay address header uniquely identifies a virtual circuit. The DLCI is specific only to the router where it is configured. A VC can pass through any number of intermediate DCE devices located within the network. Numerous VCs can be multiplexed into a single physical circuit for access to and transmission across the network.
Summary of Configuring Serial Encapsulation
This topic summarizes the key points that were discussed in this lesson.
■ A point-to-point (or serial) line can connect two geographically distant sites. These lines are usually leased from a carrier and are, therefore, often called leased lines.
■ Bandwidth refers to the rate at which data is transferred over the communication link.In North America, point-to-point leased line bandwidth is typically specified as a DS number (DS0, DS1, and so forth) that technically refers to the rate and format of the signal.
■ The HDLC protocol is one of two major data link layer protocols commonly used with point-to-point WAN connections. HDLC supports both point-to-point and multipoint configurations.
■ The encapsulation hdlc interface configuration command can be used to specify Cisco HDLC encapsulation on the interface.
■ PPP lower-level functions use synchronous and asynchronous physical media. PPP higher-level functions carry packets from several network layer protocols using NCPs.
■ The encapsulation ppp interface configuration command can be used to specify PPP encapsulation on the interface.
■ The show interface command can be used to verify proper configuration of PPP or HDLC encapsulation.
■ Frame Relay data rates are commonly up to 4 Mbps, with some providers offering even higher rates. Frame Relay is a simpler protocol that works at the data link layer rather than at the network layer.
■ ATM is a type of cell-switched connection technology that is capable of transferring voice, video, and data through private and public networks. ATM is used primarily in service provider networks and enterprise LAN backbones.
■ ATM and Frame Relay VCs can be either PVC or SVC.
Enabling RIP
While static routes provide a method for giving the router information about where networks are located so that they can route packets, they are not scalable. For that information you need to use a dynamic routing protocol. While a lot of different routing protocols exist, the Routing Information Protocol (RIP), which is a distance vector routing protocol, is one of the most enduring of all routing protocols.
Further examples of the information that routing protocols describe are as follows:
■ How updates are conveyed
■ What knowledge is conveyed
■ When to convey knowledge
■ How to locate recipients of the updates
■ Interior Gateway Protocols (IGP): These routing protocols are used to exchange routing information within an autonomous system. Routing Information Protocol version 1 (RIPv1), RIPv2, EIGRP, and Open Shortest Path First (OSPF) are examples of IGPs.
■ Exterior Gateway Protocols (EGP): These routing protocols are used to connect autonomous systems. An autonomous system is a collection of networks under a common administration and sharing a common routing strategy. Border Gateway Protocol (BGP) is an example of an EGP.
In an autonomous system, most IGP routing algorithms can be classified as conforming to one of the following algorithms:
■ Distance vector: The distance vector routing approach determines the direction (vector) and distance (hops) to any link in the internetwork.
■ Balanced hybrid: The balanced hybrid approach combines aspects of link-state and distance vector algorithms.
■ Link state: The link-state approach, also known as the shortest path first (SPF) algorithm, creates an abstraction of the exact topology of the entire internetwork, or at least of the partition in which the router is situated.
Classful Routing Versus Classless Routing Protocols
When a classful routing protocol is used, all subnetworks of the same major network (Class A, B, or C) must use the same subnet mask. Routers that are running a classful routing protocol perform automatic route summarization across network boundaries.
Upon receiving a routing update packet, a router that is running a classful routing protocol takes one of the following actions to determine the network portion of the route:
■ If the routing update information contains the same major network number as is configured on the receiving interface, the router applies the subnet mask that is configured on the receiving interface.
■ If the routing update information contains a major network that is different from that configured on the receiving interface, the router applies the default classful mask (by address class) as follows:
— For Class A addresses, the default classful mask is 255.0.0.0.
— For Class B addresses, the default classful mask is 255.255.0.0.
— For Class C addresses, the default classful mask is 255.255.255.0.
Distance Vector Route
In addition to supporting both classful and classless routing, RIP can be characterized as a distance vector routing protocol. The periodic routing updates that most distance vector routing protocols generate are addressed only to directly connected routing devices. The addressing scheme that is most commonly used is a logical broadcast. Routers that are running a distance vector routing protocol send periodic updates even if no changes exist in the network.
In a pure distance vector environment, the periodic routing update includes a complete routing table. Upon receiving a full routing table from its neighbor, a router can verify all known routes and make changes to the local routing table based on updated information. This process is also known as “routing by rumor” because the router’s understanding of the network is based on the neighboring router’s perspective of the network topology.
RIP Features
The key characteristics of RIP include the following:
■ RIP is a distance vector routing protocol.
■ Hop count is used as the metric for path selection.
■ The maximum allowable hop count is 15.
■ Routing updates are broadcast every 30 seconds by default.
■ RIP is capable of load balancing over as many as 16 equal-cost paths. (Four paths is the default.)
Because of the characteristics of RIP, it always chooses the route with the least number of hops. This is not always, however, the best route.
Dynamic Routing Configuration Tasks
To enable a dynamic routing protocol, you must complete the following steps:
Step 1 Select a routing protocol: RIP, EIGRP, or OSPF.
Step 2 Assign IP network numbers without specifying subnet values (except for OSPF). You must also assign network or subnet addresses and the appropriate subnet mask to the interfaces.
RIP Configuration
The router rip command selects RIP as the routing protocol.
The network command assigns a major network number that the router is directly connected to. The RIP routing process associates interface addresses with the advertised network number and begins RIP packet processing on the specified interfaces.
RIP Configuration Verification
The show ip protocols command displays values about routing protocols and the routing protocol timer information that is associated with the router.
RIP Configuration Troubleshooting
Use the debug ip rip command to display RIP routing updates as they are sent and received.
Summary of Enabling RIP
The following summarizes the key points that were discussed in this section:
■ Routing is the process by which items get from one location to another.
■ Dynamic routing protocols determine how updates are conveyed, what knowledge is conveyed, when to convey knowledge, and how to locate recipients of the updates.
■ A routing protocol that has a lower administrative value is more trustworthy than a protocol that has a higher administrative value.
■ Three classes of routing protocols exist: distance vector, link-state, and balanced hybrid.
■ RIP is a distance vector routing protocol that uses hop count as the matrix for route selection and broadcasts updates every 30 seconds.
■ RIPv1 uses classful routing protocol; RIPv2 uses classless routing protocol. RIPv2 supports VLSM, manual route summarization, and authentication; RIPv1 does not.
■ To enable a dynamic routing protocol, first a routing protocol is selected, and then IP network numbers are assigned without values being specified (except OSPF).
■ The router command starts the routing process. The network command allows the routing process to determine which interfaces participate in sending and receiving the routing updates.
Chapter Summary
The following summarizes the key points that were discussed in this chapter:
■ A WAN allows the transmission of data across broad geographic distances. A number of technologies are involved in the functions of WANs, including hardware devices, such as routers, communication servers, and modems, and software functions.
■ A common type of WAN connection is the point-to-point connection, which is also referred to as a serial or leased-line connection because the lines are leased from a carrier (usually a telephone company) and are dedicated for use by the company leasing the lines.
■ Circuit switching allows multiple sites to connect to the switched network of a carrier and communicate with each other. This technology provides a more cost-effective means of WAN connection and includes its own set of technologies, including the PSTN.
■ NAT and PAT translate IP addresses within private internal networks into legal IP addresses for transport over public external networks such as the Internet without requiring a registered subnet address.
■ A router can get its interface address from a DHCP server.
■ Routing information takes the form of entries in a routing table, with one entry for each identified route. The routing table can be updated manually or automatically to accommodate network changes.
■ Distance vector routing algorithms enable each router to send all or some portion of its routing table to its neighbors.
■ Link-state routing algorithms maintain a complex database of topology information, which routers use to maintain full knowledge of distant routers.
■ Balanced hybrid routing algorithms combine aspects of both distance vector and linkstate routing.
■ RIP is used in small, homogeneous networks.
Комментариев нет:
Отправить комментарий