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Appendix B      Concepts
  TACACS+
TACACS+
Cisco 1800 fixed-configuration 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 1800 fixed-configuration 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. 
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). 
						

							 
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Appendix B      Concepts
  Dial Backup
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 adaptation 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 (UBR) traffic. 
Cisco 1800 series routers also support AAL1 and 2 formats.
ATM encapsulation is the wrapping of data in a particular protocol header. The type of router to which 
you are connecting 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 ATM subsystem of the router 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.
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 a user to configure a backup 
modem line connection. The following can be used to bring up the dial backup feature in 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 a Basic 
Rate Interface (BRI), or an assigned backup dialer interface to be used in a dialer pool. While the primary  
						

							 
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Appendix B      Concepts
  NAT
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, asynchronous 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 alternative route. If this alternative 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.
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. 
						

							 
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Appendix B      Concepts
  Easy IP (Phase 1)
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 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, Cisco routers automatically negotiate a globally unique (registered) IP address for the 
dialer interface from the ISP router. 
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 
						

							 
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Appendix B      Concepts
  QoS
QoS
This section describes Quality of Service (QoS) parameters, including the following:
 IP Precedence
 PPP Fragmentation and Interleaving
 CBWFQ
 RSVP
 Low Latency Queuing
QoS refers to the capability of a network to provide better service to selected network traffic over various 
technologies, including ATM, Ethernet and IEEE 802.1 networks, and IP-routed networks that may use 
any or all of these underlying technologies. Primary goals of QoS include dedicated bandwidth, 
controlled jitter and latency (required by some real-time and interactive traffic), and improved loss 
characteristics. QoS technologies provide the elemental building blocks for future business applications 
in campus, WAN, and service provider networks. 
QoS must be configured throughout your network, not just on your router running VoIP, to improve voice 
network performance. Not all QoS techniques are appropriate for all network routers. Edge routers and 
backbone routers in your network do not necessarily perform the same operations; the QoS tasks they 
perform might differ as well. To configure your IP network for real-time voice traffic, you need to 
consider the functions of both edge and backbone routers in your network.
QoS software enables complex networks to control and predictably service a variety of networked 
applications and traffic types. Almost any network can take advantage of QoS for optimum efficiency, 
whether it is a small corporate network, an Internet service provider, or an enterprise network. 
IP Precedence
You can partition traffic in up to six classes of service using IP Precedence (two others are reserved for 
internal network use). The queuing technologies throughout the network can then use this signal to 
expedite handling.
Features such as policy-based routing and committed access rate (CAR) can be used to set precedence 
based on extended access-list classification. This allows considerable flexibility for precedence 
assignment, including assignment by application or user, by destination and source subnet, and so on. 
Typically this functionality is deployed as close to the edge of the network (or administrative domain) 
as possible, so that each subsequent network element can provide service based on the determined policy.
IP Precedence can also be set in the host or network client with the signaling used optionally. IP 
Precedence enables service classes to be established using existing network queuing mechanisms (such 
as class-based weighted fair queuing [CBWFQ]), with no changes to existing applications or 
complicated network requirements. 
PPP Fragmentation and Interleaving
With multiclass multilink PPP interleaving, large packets can be multilink-encapsulated and fragmented 
into smaller packets to satisfy the delay requirements of real-time voice traffic; small real-time packets, 
which are not multilink encapsulated, are transmitted between fragments of the large packets. The 
interleaving feature also provides a special transmit queue for the smaller, delay-sensitive packets, 
enabling them to be transmitted earlier than other flows. Interleaving provides the delay bounds for 
delay-sensitive voice packets on a slow link that is used for other best-effort traffic. 
						

							 
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  QoS
In general, multilink PPP with interleaving is used in conjunction with CBWFQ and RSVP or IP 
Precedence to ensure voice packet delivery. Use multilink PPP with interleaving and CBWFQ to define 
how data is managed; use Resource Reservation Protocol (RSVP) or IP Precedence to give priority to 
voice packets.
CBWFQ
In general, class-based weighted fair queuing (CBWFQ) is used in conjunction with multilink PPP and 
interleaving and RSVP or IP Precedence to ensure voice packet delivery. CBWFQ is used with multilink 
PPP to define how data is managed; RSVP or IP Precedence is used to give priority to voice packets. 
There are two levels of queueing; ATM queues and Cisco IOS queues. CBWFQ is applied to Cisco IOS 
queues. A first-in-first-out (FIFO) Cisco IOS queue is automatically created when a PVC is created. If 
you use CBWFQ to create classes and attach them to a PVC, a queue is created for each class.
CBWFQ ensures that queues have sufficient bandwidth and that traffic gets predictable service. 
Low-volume traffic streams are preferred; high-volume traffic streams share the remaining capacity, 
obtaining equal or proportional bandwidth.
RSVP
RSVP enables routers to reserve enough bandwidth on an interface to ensure reliability and quality 
performance. RSVP allows end systems to request a particular QoS from the network. Real-time voice 
traffic requires network consistency. Without consistent QoS, real-time traffic can experience jitter, 
insufficient bandwidth, delay variations, or information loss. RSVP works in conjunction with current 
queuing mechanisms. It is up to the interface queuing mechanism (such as CBWFQ) to implement the 
reservation. 
RSVP works well on PPP, HDLC, and similar serial-line interfaces. It does not work well on 
multi-access LANs. RSVP can be equated to a dynamic access list for packet flows.
You should configure RSVP to ensure QoS if the following conditions describe your network:
 Small-scale voice network implementation
 Links slower than 2 Mbps
 Links with high utilization
 Need for the best possible voice quality
Low Latency Queuing
Low latency queuing (LLQ) provides a low-latency strict priority transmit queue for real-time traffic. 
Strict priority queuing allows delay-sensitive data to be dequeued and sent first (before packets in other 
queues are dequeued), giving delay-sensitive data preferential treatment over other traffic. 
						

							 
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Appendix B      Concepts
  Access Lists
Access Lists
With basic standard and static extended access lists, you can approximate session filtering by using the 
established keyword with the permit command. The established keyword filters TCP packets based on 
whether the ACK or RST bits are set. (Set ACK or RST bits indicate that the packet is not the first in the 
session and the packet therefore belongs to an established session.) This filter criterion would be part of 
an access list applied permanently to an interface. 
						

							 
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  Access Lists 
						

							 
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APPENDIXC
ROM Monitor
The ROM monitor firmware runs when the router is powered up or reset. The firmware helps to initialize 
the processor hardware and boot the operating system software. You can use the ROM monitor to 
perform certain configuration tasks, such as recovering a lost password or downloading software over 
the console port. If there is no Cisco
 IOS software image loaded on the router, the ROM monitor runs 
the router.
This appendix contains the following sections:
 Entering the ROM Monitor
 ROM Monitor Commands
 Command Descriptions
 Disaster Recovery with TFTP Download
 Configuration Register
 Console Download
 Debug Commands
 Exiting the ROM Monitor
Entering the ROM Monitor
To use the ROM monitor, you must be using a terminal or PC that is connected to the router over the 
console port.
Perform these steps to configure the router to boot up in ROM monitor mode the next time it is rebooted.
CommandPurpose
Step 1enableEnters privileged EXEC mode. If an enable password is configured, you 
must enter the enable command and the enable password to enter privileged 
EXEC mode.
Step 2configure terminalEnters global configuration mode.
Step 3config-reg 0x0Resets the configuration register. 
						

							 
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Appendix C      ROM Monitor
  ROM Monitor Commands
TimesaverBreak (system interrupt) is always enabled for 60 seconds after the router reboots, regardless of whether 
it is set to on or off in the configuration register. During this 60-second window, you can break to the 
ROM monitor prompt by pressing the Break key. 
ROM Monitor Commands
Enter ? or help at the ROM monitor prompt to display a list of available commands and options, as 
follows:
rommon 1 > ?alias set and display aliases command
boot boot up an external process
break set/show/clear the breakpointconfreg configuration register utility
cont continue executing a downloaded image
context display the context of a loaded image
cookie display contents of cookie PROM in hexdir List files in directories-dir 
dis display instruction stream
format Format a filesystem-format frame print out a selected stack frame
fsck Check filesystem consistency-fsck 
help monitor builtin command helphistory monitor command history
meminfo main memory information
more Concatenate (type) file(s)-cat repeat repeat a monitor command
reset system reset
set display the monitor variablesstack produce a stack trace
sync write monitor environment to NVRAM
sysret print out info from last system returntftpdnld tftp image download
unalias unset an alias
unset unset a monitor variablexmodem x/ymodem image download
Commands are case sensitive. You can halt any command by pressing the Break key on a terminal. If 
you are using a PC, most terminal emulation programs halt a command when you press the Ctrl and the 
Break keys at the same time. If you are using another type of terminal emulator or terminal emulation 
software, see the documentation for that product for information on how to send a Break command. 
Step 4exitExits global configuration mode.
Step 5reloadReboots the router with the new configuration register value. The router 
remains in ROM monitor and does not boot the Cisco
 IOS software.
As long as the configuration value is 0x0, you must manually boot the 
operating system from the console. See the boot command in the “
Command 
Descriptions” section in this appendix. 
After the router reboots, it is in ROM monitor mode. The number in the 
prompt increments with each new line.
Command Purpose