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Concepts
This chapter contains conceptual information that may be useful to Internet Service Providers or 
Network Administrators when configuring Cisco routers. To review some typical network scenarios, 
refer to “Network Scenarios” in Chapter 2. For information on specific configurations, refer to 
Chapter 3, “Basic Router Configuration” and Chapter 4, “Advanced Router Configuration.”
The following topics are included in this chapter:
Overview, page 1-1
ADSL, page 1-3
Network Protocols, page 1-3
Routing Protocol Options, page 1-4
PPP Authentication Protocols, page 1-5
TACACS+, page 1-6
Network Interfaces, page 1-6
Dial Backup, page 1-8
NAT, page 1-9
Easy IP (Phase 1), page 1-9
Easy IP (Phase 2), page 1-10
VoIP, page 1-10
QoS, page 1-11
Access Lists, page 1-13
Overview
The Cisco 826, 827, 828, 831, 836, and 837 and Cisco SOHO 76, 77, 78, 91, 96, and 97 routers are 
Cisco IOS-based members of the Cisco 800 router family with ATM/ADSL support. Depending on their 
feature set, the routers send data, voice, and video over high-speed ADSL lines to connect to the Internet 
or corporate intranets.
The data-only Cisco 826, 827, SOHO 76, and SOHO 77 have one 10Base-T Ethernet and one 
ADSL-over-ISDN or ADSL network port, respectively.  
						

							  
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Overview
The data and voice Cisco 827-4V has four FXS/POTS ports in addition to the 10Base-T Ethernet and 
one ADSL network port, and it supports Voice over IP (VoIP). The four FXS/POTS ports will support 
loop-start functions for connecting to POTS devices up to 500 ft. The Cisco 827-4V router includes a 
digital signal processor (DSP) chip to support VoIP over ATM adaptation layer (AAL5) protocol. 
AAL5 operates over the asymmetric digital subscriber line (ADSL) physical interface for both data and 
voice. The ADSL protocol supports EOC message sets defined in T1.413 DMT Issue 2 as limited by 
Digital Subscriber Line Access Multiplexers (DSLAMs). The ADSL controller and line interface unit 
are based on Alcatel chip sets. 
The Cisco 828 router is a Cisco IOS-based member of the Cisco 800 router family with ATM/SHDSL 
support. The SOHO 78 router also supports ATM/SHDSL. The routers send data, voice, and video over 
high-speed G.SHDSL lines to connect to the Internet or corporate intranets.
Both the Cisco 828 router and the SOHO 78 router provide a 4-port Ethernet hub, in addition to the 
G.SHDSL port.
Both the Cisco 831 router and the SOHO 91 Ethernet-to-Ethernet routers can connect a corporate 
telecommuter or small office to an ISP over a broadband or Ethernet connection to corporate LANs or 
the Internet. The routers are capable of bridging and multiprotocol routing between LAN and WAN 
ports. The Cisco 831 router is a hardware encryption–capable router offering business-class features to 
small offices and enterprise telecommuters. The SOHO 91 router offers software encryption capability 
without hardware encryption.
The Cisco 836 and Cisco SOHO 96 routers are ADSL routers with an integrated switch. These routers 
provide a 4-port Ethernet switch for the LAN and an ADSL physical interface for the WAN 
compatibility. The Cisco 836 router is a hardware encryption–capable, Ethernet
-to-ADSL router 
offering business-class features to small offices and enterprise telecommuters. The Cisco SOHO 96 
router offers software encryption capability without hardware encryption. Both these routers provide an 
ISDN basic rate interface (BRI) S/T interface as a backup for the ADSL interface.
The Cisco 837 router and SOHO 97 are ADSL routers with an integrated switch. These routers provide 
a 4-port Ethernet switch for LAN and an ADSL physical interface for WAN compatibility. The Cisco 837 
router is a hardware encryption–capable, Ethernet
-to-ADSL router offering business-class features to 
small offices and enterprise telecommuters. The SOHO 97 router offers software encryption capability 
without hardware encryption.
The Cisco 831, Cisco 836, Cisco 837, SOHO 91, SOHO 96, and SOHO 97 routers support switch 
functions which enable the routers to be connected as a 10/100 BASE-T device. These routers crossover 
functionality enable them to detect MDI/MDIX to any other PC or hub with a straight-through cable or 
crossover cable.
Ta b l e 1 - 1 summarizes what interface each Cisco model supports.
Table 1-1 Interface Supported in Each Cisco Router
Interface Supported Cisco Router Model
Ethernet to Ethernet 831, SOHO 91
Ethernet to ADSL over ISDN 826, SOHO 76, 836, SOHO 96
Ethernet  to  ADSL  over  POTS 827, 827H, 827-4V, 837, SOHO 77, SOHO 77H, 
SOHO 97 
						

							  
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ADSL
ADSL
ADSL is a technology that allows both data and voice to transmit over the same line. It is a packet-based 
network technology that allows high-speed transmission over twisted-pair copper wire on the local loop 
(“last mile”) between a network service provider (NSP) central office and the customer site, or on local 
loops created either within a building or campus. 
The benefit of ADSL over a serial or dial-up line is that it is always on and always connected, increasing 
bandwidth and lowering the costs compared with a dial-up or leased line. ADSL technology is 
asymmetric in that it allows more bandwidth from an NSP’s central office to the customer site than from 
the customer site to the central office. This asymmetry, combined with always-on access (which 
eliminates call setup), makes ADSL ideal for Internet and intranet surfing, video-on-demand, and remote 
LAN access. 
SHDSL
SHDSL is a technology based on the G.SHDSL (G.991.2) standard that allows both data and voice to be 
transmitted over the same line. SHDSL is a packet-based network technology that allows high-speed 
transmission over twisted-pair copper wire between a network service provider (NSP) central office and 
a customer site, or on local loops created within either a building or a campus. 
G.SHDSL devices can extend reach from central offices and remote terminals to approximately 
26,000 feet, at symmetrical data rates from 72 kbps up to 2.3 Mbps. In addition, it is repeatable at lower 
speeds, which means there is virtually no limit to its reach. 
SHDSL technology is symmetric in that it allows equal bandwidth between an NSP’s central office and 
a customer site. This symmetry, combined with always-on access (which eliminates call setup), makes 
SHDSL ideal for LAN access.
Network Protocols
Network protocols enable the network to pass data from its source to a specific destination over LAN or 
WAN links. Routing address tables are included in the network protocols to provide the best path for 
moving the data through the network.
IP
The best known Transmission Control Protocol/Internet Protocol (TCP/IP) at the internetwork layer is 
IP, which provides the basic packet delivery service for all TCP/IP networks. In addition to the physical 
node addresses, the IP protocol implements a system of logical host addresses called IP addresses. The 
IP addresses are used by the internetwork and higher layers to identify devices and to perform 
internetwork routing. The Address Resolution Protocol (ARP) enables IP to identify the physical address 
that matches a given IP address.
IP is used by all protocols in the layers above and below it to deliver data, which means that all TCP/IP 
data flows through IP when it is sent and received regardless of its final destination.
IP is a connectionless protocol, which means that IP does not exchange control information (called a 
handshake) to establish an end-to-end connection before transmitting data. In contrast, a 
connection-oriented protocol exchanges control information with the remote computer to verify that it  
						

							  
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Routing Protocol Options
is ready to receive data before sending it. When the handshaking is successful, the computers have 
established a connection. IP relies on protocols in other layers to establish the connection if 
connection-oriented services are required. 
IPX exchanges routing information using Routing Information Protocol (RIP), a dynamic 
distance-vector routing protocol. RIP is described in more detail in the following subsections.
Routing Protocol Options
Routing protocols include the following:
Routing Information Protocol (RIP) 
Enhanced Interior Gateway Routing Protocol (Enhanced IGRP)
RIP and Enhanced IGRP protocols differ in several ways, as shown in Ta b l e 1 - 2.
RIP
RIP is an associated protocol for IP, and is widely used for routing Internet protocol traffic. RIP is a 
distance-vector routing protocol, which means that it uses distance (hop count) as its metric for route 
selection. Hop count is the number of routers that a packet must traverse to reach its destination. For 
example, if a particular route has a hop count of 2, then a packet must traverse two routers to reach its 
destination.
By default, RIP routing updates are broadcast every 30 seconds. You can reconfigure the interval at 
which the routing updates are broadcast. You can also configure triggered extensions to RIP so that 
routing updates are sent only when the routing database is updated. For more information on triggered 
extensions to RIP, refer to the Cisco IOS 12.0(1)T documentation set. For information on accessing the 
documentation, see the “Obtaining Documentation” in “About This Guide.”
Table 1-2 RIP and Enhanced IGRP Comparison
Protocol Ideal Topology Metric Routing Updates
RIP Suited for topologies with 
15 or fewer hops.Hop count. Maximum hop 
count is 15. Best route is one 
with lowest hop count.By default, every 30 seconds. 
You can reconfigure this value 
and also use triggered 
extensions to RIP.
Enhanced 
IGRPSuited for large topologies 
with 16 or more hops to 
reach a destination.Distance information. Based 
on a successor, which is a 
neighboring router that has a 
least-cost path to a 
destination that is 
guaranteed to not be part of 
a routing loop.Hello packets sent every 
5 seconds plus incremental 
updates sent when the state of 
a destination changes. 
						

							  
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PPP Authentication Protocols
Enhanced IGRP
Enhanced IGRP is an advanced Cisco proprietary distance-vector and link state routing protocol, which 
means it uses a metric more sophisticated than distance (hop count) for route selection. Enhanced IGRP 
uses a metric based on a successor, which is a neighboring router that has a least-cost path to a 
destination that is guaranteed not to be part of a routing loop. If a successor for a particular destination 
does not exist but neighbors advertise the destination, the router must recompute a route.
Each router running Enhanced IGRP sends hello packets every 5 seconds to inform neighboring routers 
that it is functioning. If a particular router does not send a hello packet within a prescribed period, 
Enhanced IGRP assumes that the state of a destination has changed and sends an incremental update.
Because Enhanced IGRP supports IP, you can use one routing protocol for multi-protocol network 
environments, minimizing the size of the routing tables and the amount of routing information.
PPP Authentication Protocols
The Point-to-Point Protocol (PPP) encapsulates network layer protocol information over point-to-point 
links. 
PPP originally emerged as an encapsulation protocol for transporting IP traffic over point-to-point links. 
PPP also established a standard for the assignment and management of IP addresses, asynchronous 
(start/stop) 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 data-compression negotiation. PPP supports these functions by 
providing an extensible Link Control Protocol (LCP) and a family of Network Control Protocols (NCPs) 
to negotiate optional configuration parameters and facilities.
The current implementation of PPP supports two security authentication protocols to authenticate a PPP 
session:
Password Authentication Protocol (PAP)
Challenge Handshake Authentication Protocol (CHAP)
PPP with PAP or CHAP authentication is often used to inform the central site which remote routers are 
connected to it. 
PAP
PAP uses a two-way handshake to verify the passwords between routers. To illustrate how PAP works, 
imagine a network topology where a remote office Cisco 827 router is connected to a corporate office 
Cisco 3600 router. After the PPP link is established, the remote office router repeatedly sends a 
configured username and password until the corporate office router accepts the authentication. 
PAP has the following characteristics:
The password portion of the authentication is sent across the link in clear text (not scrambled or 
encrypted). 
PAP provides no protection from playback or repeated trial-and-error attacks. 
The remote office router controls the frequency and timing of the authentication attempts. 
						

							  
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TACACS+
CHAP
CHAP uses a three-way handshake to verify passwords. To illustrate how CHAP works, imagine a 
network topology where a remote office Cisco 827 router is connected to a corporate office Cisco 3600 
router. 
After the PPP link is established, the corporate office router sends a challenge message to the remote 
office router. The remote office router responds with a variable value. The corporate office router checks 
the response against its own calculation of the value. If the values match, the corporate office router 
accepts the authentication. The authentication process can be repeated any time after the link is 
established.
CHAP has the following characteristics:
The authentication process uses a variable challenge value rather than a password.
CHAP protects against playback attack through the use of the variable challenge value, which is 
unique and unpredictable. Repeated challenges limit the time of exposure to any single attack.
The corporate office router controls the frequency and timing of the authentication attempts.
NoteCisco recommends using CHAP because it is the more secure of the two protocols. 
TACACS+
Cisco 800-series routers support the Terminal Access Controller Access Control System Plus 
(TACACS+) protocol through Telnet. TACACS+ is a Cisco proprietary authentication protocol that 
provides remote access authentication and related network security services, such as event logging. User 
passwords are administered in a central database rather than in individual routers. TACACS+ also 
provides support for separate modular authentication, authorization, and accounting (AAA) facilities 
that are configured at individual routers.
Network Interfaces 
This section describes the network interface protocols that Cisco 800-series routers support. The 
following network interface protocols are supported:
Ethernet 
AT M
Ethernet 
Ethernet is a baseband LAN protocol that transports data and voice packets to the WAN interface using 
carrier sense multiple access collision detect (CSMA/CD). The term is now often used to refer to all 
CSMA/CD LANs. Ethernet was designed to serve in networks with sporadic, occasionally heavy traffic 
requirements, and the IEEE 802.3 specification was developed in 1980 based on the original Ethernet 
technology.  
						

							  
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Network Interfaces
Under the Ethernet CSMA/CD media-access process, any host on a CSMA/CD LAN can access the 
network at any time. Before sending data, CSMA/CD hosts listen for traffic on the network. A host 
wanting to send data waits until it detects no traffic before it transmits. Ethernet allows any host on the 
network to transmit whenever the network is quiet. A collision occurs when two hosts listen for traffic, 
hear none, and then transmit simultaneously. In this situation, both transmissions are damaged, and the 
hosts must retransmit at some later time. Algorithms determine when the colliding hosts should 
retransmit.
ATM
Asynchronous Transfer Mode (ATM) is a high-speed, multiplexing and switching protocol that supports 
multiple traffic types including voice, data, video, and imaging.
ATM is composed of fixed-length cells that switch and multiplex all information for the network. An 
ATM connection is simply used to transfer bits of information to a destination router or host. The ATM 
network is considered a LAN with high bandwidth availability. Unlike a LAN, which is connectionless, 
ATM requires certain features to provide a LAN environment to the users. 
Each ATM node must establish a separate connection to every node in the ATM network that it needs to 
communicate with. All such connections are established through a permanent virtual circuit (PVC).
PVC
A PVC is a connection between remote hosts and routers. A PVC is established for each ATM end node 
with which the router communicates. The characteristics of the PVC that are established when it is 
created are set by the ATM adaption layer (AAL) and the encapsulation type. An AAL defines the 
conversion of user information into cells. An AAL segments upper-layer information into cells at the 
transmitter and reassembles the cells at the receiver. 
Cisco routers support the AAL5 format, which provides a streamlined data transport service that 
functions with less overhead and affords better error detection and correction capabilities than AAL3/4. 
AAL5 is typically associated with variable bit rate (VBR) traffic and unspecified bit rate traffic (UBR). 
Cisco 800-series routers also support AAL1 and 2 formats.
ATM encapsulation is the wrapping of data in a particular protocol header. The type of router you are 
connecting to the router determines the type of ATM PVC encapsulation types. 
The routers support the following encapsulation types for ATM PVCs:
LLC/SNAP (RFC 1483)
VC-MUX (RFC 1483)
PPP (RFC 2364)
Each PVC is considered a complete and separate link to a destination node. Users can encapsulate data 
as needed across the connection. The ATM network disregards the contents of the data. The only 
requirement is that data be sent to the routers ATM subsystem in a manner that follows the specific AAL 
format.
Dialer Interface
A dialer interface assigns PPP features (such as authentication and IP address assignment method) to a 
PVC. Dialer interfaces are used when configuring PPP over ATM. 
						

							  
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Dial Backup
Dialer interfaces can be configured independently of any physical interface and applied dynamically as 
needed. 
Dial Backup
Dial backup provides protection against WAN downtime by allowing user to configure a backup modem 
line connection. The following can be used to bring up the dial backup feature in the Cisco IOS software: 
Backup Interface
Floating Static Routes
Dialer Watch
Backup Interface
A backup interface is an interface that stays idle until certain circumstances occur, such as WAN 
downtime, at which point it is activated. The backup interface can be a physical interface such as Basic 
Rate Interface (BRI), or an assigned backup dialer interface to be used in a dialer pool. While the primary 
line is up, the backup interface is placed in standby mode. In standby mode, the backup interface is 
effectively shut down until it is enabled. Any route associated with the backup interface does not appear 
in the routing table.
Because the backup interface command is dependent on the router’s identifying that an interface is 
physically down, it is commonly used to back up ISDN BRI connections and async lines and leased lines. 
The interfaces to such connections go down when the primary line fails, and the backup interface quickly 
identifies such failures.
Floating Static Routes
Floating static routes are static routes that have an administrative distance greater than the administrative 
distance of dynamic routes. Administrative distances can be configured on a static route so that the static 
route is less desirable than a dynamic route. In this manner, the static route is not used when the dynamic 
route is available. However, if the dynamic route is lost, the static route can take over, and the traffic can 
be sent through this alternate route. If this alternate route uses a Dial-on-Demand Routing (DDR) 
interface, then that interface can be used as a backup feature.
Dialer Watch
Dialer watch is a backup feature that integrates dial backup with routing capabilities. Dialer watch 
provides reliable connectivity without having to define traffic of interest to trigger outgoing calls at the 
central router. Hence, dialer watch can be considered regular DDR with no requirement for traffic of 
interest. By configuring a set of watched routes that define the primary interface, you are able to monitor 
and track the status of the primary interface as watched routes are added and deleted.
When a watched route is deleted, dialer watch checks for at least one valid route for any of the IP 
addresses or networks being watched. If there is no valid route, the primary line is considered down and 
unusable. If there is a valid route for at least one of the watched IP networks defined and the route is 
pointing to an interface other than the backup interface configured for dialer watch, the primary link is 
considered up and dialer watch does not initiate the backup link. 
						

							  
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NAT
NAT
Network address translation (NAT) provides a mechanism for a privately addressed network to access 
registered networks, such as the Internet, without requiring a registered subnet address. This mechanism 
eliminates the need for host renumbering and allows the same IP address range to be used in multiple 
intranets.
NAT is configured on the router at the border of an inside network (a network that uses nonregistered IP 
addresses) and an outside network (a network that uses a globally unique IP address; in this case, the 
Internet). NAT translates the inside local addresses (the nonregistered IP addresses assigned to hosts on 
the inside network) into globally unique IP addresses before sending packets to the outside network.
With NAT, the inside network continues to use its existing private or obsolete addresses. These addresses 
are converted into legal addresses before packets are forwarded onto the outside network. The translation 
function is compatible with standard routing; the feature is required only on the router connecting the 
inside network to the outside domain.
Translations can be static or dynamic. A static address translation establishes a one-to-one mapping 
between the inside network and the outside domain. Dynamic address translations are defined by 
describing the local addresses to be translated and the pool of addresses from which to allocate outside 
addresses. Allocation occurs in numeric order and multiple pools of contiguous address blocks can be 
defined.
NAT eliminates the need to readdress all hosts that require external access, saving time and money. It 
also conserves addresses through application port-level multiplexing. With NAT, internal hosts can share 
a single registered IP address for all external communications. In this type of configuration, relatively 
few external addresses are required to support many internal hosts, thus conserving IP addresses.
Because the addressing scheme on the inside network may conflict with registered addresses already 
assigned within the Internet, NAT can support a separate address pool for overlapping networks and 
translate as appropriate. 
Easy IP (Phase 1)
The Easy IP (Phase 1) feature combines Network Address Translation (NAT) and PPP/Internet Protocol 
Control Protocol (IPCP). This feature enables a Cisco router to automatically negotiate its own 
registered WAN interface IP address from a central server and to enable all remote hosts to access the 
Internet using this single registered IP address. Because Easy IP (Phase 1) uses existing port-level 
multiplexed NAT functionality within the Cisco IOS software, IP addresses on the remote LAN are 
invisible to the Internet.
The Easy IP (Phase 1) feature combines NAT and PPP/IPCP. With NAT, the router translates the 
nonregistered IP addresses used by the LAN devices into the globally unique IP address used by the 
dialer interface. The ability of multiple LAN devices to use the same globally unique IP address is known 
as overloading. NAT is configured on the router at the border of an inside network (a network that uses 
nonregistered IP addresses) and an outside network (a network that uses a globally unique IP address; in 
this case, the Internet).
With PPP/IPCP, the Cisco routers automatically negotiate a globally unique (registered) IP address for 
the dialer interface from the ISP router.  
						

							  
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Easy IP (Phase 2)
Easy IP (Phase 2)
The Easy IP (Phase 2) feature combines Dynamic Host Configuration Protocol (DHCP) server and relay. 
DHCP is a client-server protocol that enables devices on an IP network (the DHCP clients) to request 
configuration information from a DHCP server. DHCP allocates network addresses from a central pool 
on an as-needed basis. DHCP is useful for assigning IP addresses to hosts connected to the network 
temporarily or for sharing a limited pool of IP addresses among a group of hosts that do not need 
permanent IP addresses.
DHCP frees you from having to assign an IP address to each client manually.
DHCP configures the router to forward UDP broadcasts, including IP address requests, from DHCP 
clients. DHCP allows for increased automation and fewer network administration problems by:
Eliminating the need for the manual configuration of individual computers, printers, and shared file 
systems
Preventing the simultaneous use of the same IP address by two clients
Allowing configuration from a central site
NoteWhen using NAT, DHCP relay cannot be used on the Cisco 800-series routers. The built-in DHCP server 
should be used instead.
VoIP
The Cisco 827-4V router is a voice-and-data-capable router that provides Voice-over-IP (VoIP) 
functionality and can carry voice traffic (such as telephone calls and faxes) over an IP network.
Cisco voice support is implemented using voice packet technology. There are two primary applications 
for VoIP: 
It provides a central-site telephony termination facility for VoIP traffic from multiple 
voice-equipped remote office facilities. 
It provides a PSTN gateway for Internet telephone traffic. VoIP used as a PSTN gateway leverages 
the standardized use of H.323-based Internet telephone client applications. 
In VoIP, the digital signal processor (DSP) segments the voice signal into frames and stores them in voice 
packets. These voice packets are transported by using IP in compliance with H.323 signaling standards.
H.323
H.323 is an International Telecommunication Union (ITU-T) standard that describes packet-based video, 
audio, and data conferencing. H.323 is an umbrella standard that describes the architecture of the 
conferencing system and refers to a set of other standards (H.245, H.225.0, and Q.931) to describe its 
actual protocol.
Cisco H.323 Version 2 support upgrades Cisco IOS software to comply with the mandatory requirements 
and several of the optional features of the version 2 specification. This upgrade enhances the existing 
VoIP gateway and the Multimedia Conference Manager (gatekeeper and proxy). A gateway allows H.323 
terminals to communicate with non-H.323 terminals by converting protocols, and it is an endpoint on 
the LAN that provides real-time, two-way communications between H.323 terminals on the LAN and 
other ITU-T terminals in the WAN or to another H.323 gateway.