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Chapter 4      Network Scenarios
Internet Access Scenarios
Configuring NAT
Follow the steps below to configure NAT, beginning in global configuration 
mode.
Configuration Example
In the following configuration example, you do not have to enter the commands 
marked “default.” These commands appear automatically in the configuration file 
that is generated when you use the show running-config command.
interface Ethernet0
ip address 192.168.1.1 255.255.255.0
no ip directed-broadcast (default)
ip nat inside
!
interface ATM0
no ip address
no ip directed-broadcast (default)
ip nat outside
no atm ilmi-keepalive (default)
pvc 8/35 
Command Task
Step 1ip nat inside source list 1 interface dialer 0 
overloadEnable dynamic translation of addresses 
permitted by the access list to one of 
addresses specified in the dialer interface.
Step 2ip route 0.0.0.0.0.0.0.0 dialerSet the ip route to point to the dialer interface 
as a default gateway.
Step 3access-list 1 permit 192.168.1 0 0.0.0.255Define a standard access list permitting 
addresses that need translation.
Step 4interface ethernet 0Enter configuration mode for the Ethernet 
interface.
Step 5ip nat insideEstablish the Ethernet interface as the inside 
interface.
Step 6no shutdownEnable interface and configuration changes 
just made to the Ethernet interface.
Step 7exitExit configuration mode for the Ethernet 
interface. 
						

							 
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Configuring Dial Backup over the Console Port
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dialer pool-member 1
!
bundle-enable
!
interface Dialer0
ip address negotiated
no ip directed-broadcast (default)
ip nat outside
encapsulation ppp 
dialer pool 1
!         
ip nat inside source list 1 interface Dialer0 overload
ip classless (default)
ip route 0.0.0.0 0.0.0.0 Dialer 0 (default gateway)
!
access-list 1 permit 192.168.1.0 0.0.0.255
!
end
Configuring Dial Backup over the Console Port
By allowing you to configure a backup modem line connection, dial backup 
provides protection against WAN downtime. Dial backup is inactive until it is 
configured. On Cisco 831, Cisco 837, Cisco SOHO 91, and Cisco SOHO 97 
routers, both the console port and the auxiliary port in the Cisco IOS software 
configuration are on the same physical RJ-45 port. Therefore, both ports cannot 
be activated simultaneously, and the command-line interface (CLI) must be used 
to enable or disable either one.
Configuring Dial Backup over the ISDN Interface
Like the Cisco 831 and 837 routers and the Cisco SOHO 91 and 97 routers, the 
Cisco 836 router supports dial-in (for remote management) and dial-out (for dial 
backup) capabilities across the ISDN interface. The Cisco SOHO 96 router 
supports only the dial-in feature. Unlike the Cisco 831 and 837 routers and the 
Cisco SOHO 91 and 97 routers, the dial backup and remote management 
functions are configured on the Cisco 836 and Cisco SOHO 96 routers through 
the router’s ISDN S/T port.  
						

							 
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Chapter 4      Network Scenarios
Configuring Dial Backup over the ISDN Interface
NoteThe remote management described next refers to backup remote management, 
which function allows external control of the router via the ISDN when the ATM 
link goes down.
Dial Backup Feature Limitations and Configuration
This section discusses the limitations and configuration of the dial backup feature 
on the Cisco 831, 836, and 837 routers and the Cisco SOHO 91, 96, and 97 
routers. 
Cisco 836 and 837 Routers and Cisco SOHO 96 and 97 Routers
The following can be used to bring up the dial backup feature in the Cisco IOS 
software for the Cisco 836 and 837 routers and the Cisco SOHO 96 and 97 routers:
Backup Interfaces
Floating Static Routes
Dialer Watch
For more information on the three features, see Chapter 1, “Concepts.”
Backup Interfaces
When the device receives an indication that the primary line is down, the backup 
interface is brought up. You can configure the backup interface to go down (after 
a specified time) when the primary connection is restored. 
The dial-on-demand routing (DDR) backup call is triggered by traffic of interest. 
Even if the backup interface comes out of standby mode, the router will not trigger 
the backup call unless it receives traffic of interest for that backup interface.
Floating Static Routes
Floating static routes depend on traffic of interest to trigger the DDR backup call. 
The router does not actually trigger the backup call unless it receives traffic of 
interest for that backup interface, even if the router installs the floating static route 
in the route table.  
						

							 
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Configuring Dial Backup over the ISDN Interface
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Floating static routes are independent of line protocol status. This is an important 
consideration on Frame Relay circuits wherein line protocol may not go down if 
the data-link connection identifier (DLCI) is inactive. Floating static routes are 
also encapsulation independent. 
NoteWhen static routes are configured, the primary interface protocol must go down 
in order to activate the floating static route.
Dialer Watch
Only the Extended Interior Gateway Routing Protocol (EIGRP) link-state 
dynamic routing protocols are supported.
There is a bottleneck in supporting bridging over console backup interfaces 
because bridging is not supported over slower interfaces such as console ports or 
auxiliary ports. 
In the Cisco 836 and 837 routers, the dial backup feature is supported for the 
encapsulations identified in Ta b l e 4 - 1.  
						

							 
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Chapter 4      Network Scenarios
Configuring Dial Backup over the ISDN Interface
Cisco 831 and Cisco SOHO 91 Routers
Support for the dial backup feature on the Cisco 831 router is limited because the 
Ethernet WAN interface is always up, even when ISP connectivity is down across 
the modem connected to the Cisco 831 router. Support for dial backup is possible 
only for the PPPoE environment. The only way to bring up the backup interface 
is to simultaneously use the dialer watch feature. You also need to add the IP 
addresses of the peer in the dialer watch command and in the static route 
command to enable the dial backup when primary line goes down. 
Table 4-1 Encapsulation Types Supported by Dial Backup Feature—Cisco 836 and 837 Routers
Encapsulation 
Ty p e  ( W A N )Dial Backup 
PossibleType of Dial Backup 
Method Limitations
PPP over ATM 
PPP over 
EthernetYe s
Backup interface 
method
Floating static 
routes
Dialer watchFloating static route and dialer watch 
need a routing protocol to run in the 
router. The dialer watch method 
brings up the backup interface as 
soon as the primary link goes down. 
The backup interface is brought 
down as soon as the dialer timeout is 
reached and the primary interface is 
up. Router checks the primary 
interface only when the dialer 
timeout expires. The backup 
interface remains up until the dialer 
timeout is reached, even though the 
primary interface is up.
For the dialer watch method, a 
routing protocol does not need to be 
running in the router, if the IP address 
of the peer is known.
RFC 1483 
(AAL5, SNAP, 
and MUX)Ye s
Backup interface 
method
Floating static 
routes
Dialer watchIf bridging is done through the WAN 
interface, it is not supported across 
the auxiliary port. 
						

							 
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Configuring Dial Backup over the ISDN Interface
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For the Cisco SOHO 91 router, only dial-in capability is supported. 
Ta b l e 4 - 2 shows the encapsulation types supported by the Cisco 831 router dial 
backup. 
Configuring Dial Backup and Remote Management for the 
Cisco 837 and Cisco SOHO 97 Routers
Figure 4-8 and Ta b l e 4 - 4 show how dial backup and remote management work in 
a network system when the primary line goes down. 
Table 4-2 Encapsulation Types Supported by Dial Backup—Cisco 831 Router
Encapsulation TypeDial Backup 
PossibleType of Dial Backup 
Method Limitations
PPPoE Yes Dialer watch Bridging is not supported across a 
slow interface, for example, an 
auxiliary port. The peer IP address 
of the ISP provider is needed to 
configure the dialer watch 
command and the IP static route.
Normal IP in cable 
modem scenarioNo Dialer watch The IP addresses of the peers are 
needed for dialer watch to work 
properly. If a lease time obtained 
by DHCP is not set short enough 
(one or two minutes), dial backup 
will not be supported. 
						

							 
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Chapter 4      Network Scenarios
Configuring Dial Backup over the ISDN Interface
Figure 4-8 Cisco 837 Router Dial Backup and Remote Management
Configuring Dial Backup and Remote Management for the 
Cisco 836 and Cisco SOHO 96 Routers
Figure 4-9, Figure 4-10, and Ta b l e 4 - 1 1 and Ta b l e 4 - 1 2 show how dial backup 
and remote management work in a network system when the primary line goes 
down. Two scenarios are typical applications of the Cisco 836 and the 
Cisco SOHO 96 routers. In Figure 4-9, the dial backup link goes through CPE 
splitter, DSLAM, and CO splitter before connecting to the ISDN switch. In Figure 
4-10, the dial backup link goes directly from the Cisco 836 router to the ISDN 
switch. Callout 
Number Description
1Main WAN link; primary connection to Internet service provider
2Dial backup; serves as a failover link when primary line goes down
3Remote management; serves as dial-in access to allow changes or updates to Cisco IOS 
configurations 
82269
1
3
2
23
AT M
837
Modem Modem
Modem
PC3620
PSTN 
						

							 
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Configuring Dial Backup over the ISDN Interface
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Figure 4-9 Cisco 836 Router Dial Backup and Remote Management—Dial Backup Through CPE 
Splitter, DSLAM, and CO Splitter
Callout 
Number Description
1Primary ADSL interface
2Dial backup and remote management via ISDN interface; serves as a failover link 
when primary line goes down
3Administrator remote management via ISDN interface when the primary ADSL link 
is down; serves as dial-in access to allow changes or updates to Cisco IOS 
configuration
82892
Cisco 836CPE
splitterAT M
networkATM Aggregator
Internet
Web
server Administrator ISDN switch
ISDNISDN Peer
Router CO
splitter DSLAM
2
3
1 
						

							 
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Chapter 4      Network Scenarios
Configuring Dial Backup over the ISDN Interface
Figure 4-10 Cisco 836 Router Dial Backup and Remote Management—Dial Backup Directly from 
Router to ISDN Switch
PPP over ATM with Centrally Managed Addressing and with Dial 
Backup
When customer premises equipment such as a Cisco 837 router is connected to an 
ISP, an IP address is dynamically assigned to the router, or the IP address may be 
assigned by its peer through the centrally managed function. The dial backup 
feature can be added to provide a failover route in case the primary line fails.  Callout 
Number Description
1Primary ADSL interface
2Dial backup and remote management via ISDN interface; serves as a failover link 
when primary line goes down
3Administrator remote management via ISDN interface when the primary ADSL 
link is down; serves as dial-in access to allow changes or updates to Cisco IOS 
configuration
88208
PC836CPE
splitterAT M
networkAggregator
Internet
Web
server
Administrator ISDN
switch DSLAM
2
3
1 
						

							 
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Configuring Dial Backup and Remote Management for the Cisco 837 Router
Follow the steps below to configure dial backup and remote management for the 
Cisco 837 router.
Command Task
Step 1ip name-server 206.13.28.12Enter your ISP DNS IP address.
Step 2ip dhcp pool 1Configure CPE as a local DHCP server.
Step 3vpdn enableEnable VPDN.
Step 4vpdn-group 1Specify VPDN group for protocol PPPoE.
Step 5chat-script Dialout ABORT ERROR 
ABORT BUSY ““ “AT” OK “ATDT 
5555102 T” TIMEOUT 45 CONNECT \c Configure a chat script for a modem.
Step 6interface Async1Enter configuration mode for the async 
interface.
Step 7interface Dialer3Enter configuration mode for the dialer 
interface.
Step 8dialer watch-group 1Specify the group number for watch-list.
Step 9ip nat inside source list 101 interface 
Dialer3 overloadEstablish the Ethernet interface as the inside 
interface.
Step 10ip route 0.0.0.0 0.0.0.0 ! (dial backup peer 
address @ISP)Set the IP route to point to the dialer interface 
as a default gateway.
Step 11access-list 101 permit ip 192.168.0.0 
0.0.255.255 anyDefine an extended access list permitting 
addresses that need translation.
Step 12dialer watch-list 1 ip ! (ATM peer address 
@ISP) 255.255.255.255Evaluate the status of the primary link, based 
on the existence of routes to the peer. 
Step 13line con 0Enter configuration mode for the console 
interface.
Step 14modem enableChange the console port to auxiliary port 
function.
Step 15line aux 0Enter configuration mode for the auxiliary 
interface.
Step 16flow control hardwareEnable hardware signal flow control