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Chapter 7      Router Feature Configuration
Configuring DHCP
For more information on the features not used in this configuration, refer to the 
Cisco IOS DHCP Server feature module. For more general information on DHCP 
servers, refer to Chapter 1, “Concepts.” 
Configuration Example
The following configuration shows a DHCP server configuration for the IP 
address 20.1.1.2.
!
ip dhcp pool CLIENT
   network 20.20.20.0 255.255.255.0
   domain-name cisco.com
   default-router 20.20.20.20
   netbios-name-server 1.1.1.1
   dns-server 1.1.1.2
   lease 0 1
!
Verifying Your Configuration
To verify that you have properly configured the DHCP server, enter the 
show dhcp server command and look for the assigned server IP. You should see 
a verification output like the example shown below.
router# sh dhcp server 
show ip dhcp binding
show ip dhcp conflict
show ip dhcp server statics
Configuring the DHCP Relay
This section describes how to configure the router to forward User Datagram 
Protocol (UDP) broadcasts, including IP address requests, from DHCP clients. 
Step 7lease days hours minutesSpecify the duration of the lease.
Step 8exitExit DHCP configuration mode. Command Task 
						

							 
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Configuring DHCP
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Follow the steps below to configure the DHCP relay, beginning in global 
configuration mode.
For complete information on the DHCP relay commands, refer to the 
Cisco IOS Release 12.0 documentation set. For more general information on 
DHCP relays, refer to Chapter 1, “Concepts.” 
Configuration Example
The following configuration contains commands relevant to DHCP relay only. 
You do not need 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.
!
int Ethernet0
ip address 192.168.100.1 255.255.255.0
ip helper-address 200.200.200.1
!
Command Task
Step 1interface Ethernet 0Enter configuration mode for the Ethernet 
interface.
Step 2ip helper-address addressForward default UDP broadcasts including IP 
configuration requests to the DHCP server.
Step 3no shutdownEnable the Ethernet interface and the 
configuration changes.
Step 4exitExit configuration mode for the Ethernet 
interface. 
						

							 
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Chapter 7      Router Feature Configuration
Configuring TACACS+
Verifying Your Configuration 
To verify that you have properly configured the DHCP relay, enter the 
show dhcp server command. You should see verification output like the example 
shown below. 
router#sh dhcp server 
   DHCP server:2.2.2.2
    Leases:  0
    Offers:  0      Requests:0     Acks:0     Naks:0
Declines:0      Releases:0     Bad: 0
Configuring TACACS+
The Cisco 806, 827, 831, 836, 837, 827H, and 827-4V routers and the 
Cisco SOHO 71, 91, 96, and 97 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.
To configure your router to support TACACS+, perform the following tasks: 
Command Task
Step 1aaa new-modelEnter the global configuration command to 
enable AAA. AAA must be configured to use 
TA C A C S + .
Step 2tacacs-server hostSpecify the IP address of one or more 
TACACS+ daemons.
Step 3tacacs-server keySpecify an encryption key that will be used to 
encrypt all exchanges between the network 
access server and the TACACS+ daemon. 
This same key must also be configured on the 
TACACS+ daemon.  
						

							 
Chapter 7      Router Feature Configuration
Configuring an Extended Access List
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You may need to perform other configuration steps to enable accounting for 
TACACS+ connections. For instructions on configuring TACACS+, refer to the 
Security Configuration Guide.
Configuring an Extended Access List
Follow the steps below to include one or more extended access lists in your router 
configuration, beginning in global configuration mode.
Step 4aaa authenticationDefine the method lists that use TACACS+ 
for authentication.
Step 5lineApply the defined method lists to various 
interfaces. Command Task
Command Task
Step 1access-list 100 permit tcp any ip ip 
address-mask establishedPermit any host on the network to access any 
Internet server.
Step 2access-list 100 deny ip ip adddress-mask anyDeny any Internet host from spoofing any 
host on the network.
Step 3access-list 100 permit tcp host ip 
address-maskPermit Internet DNS server to send TCP 
replies to any host on the network.
Step 4access-list 100 permit udp host ip 
address-maskPermit Internet DNS server to send UDP 
replies to any host on the network.
Step 5access-list 100 permit tcp any host ip 
addressPermit SMTP mail server to access any 
Internet server.
Step 6access-list 100 permit tcp any host ip 
addressPermit web server to access any Internet 
server.
Step 7access-list 100 permit tcp any host ip 
addressPermit FTP server to access any Internet 
server.
Step 8access-list 100 deny tcp any ip address-maskRestrict any Internet host from making a 
Telnet connection to any host on the network. 
						

							 
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Chapter 7      Router Feature Configuration
Configuring an Extended Access List
For more complete information on the extended access list commands, refer to the 
Cisco IOS Release 12.0 documentation set. For information on TCP and UDP 
port assignments, see Appendix C, “Common Port Assignments.”
Configuration Example
This configuration shows an access list being applied to IP address 192.168.1.0.
You do not need to enter the commands marked “default.” These commands 
appear automatically in the configuration file generated when you use the 
show running-config command.
!
access-list 101 permit tcp any host 192.168.1.0 0.0.0.255
! 
Step 9interface atm 0Enter configuration mode for the ATM 
interface.
Step 10dsl equipment-type co/cpeConfigure the DSL equipment type, if 
applicable.
Step 11dsl linerate number/autoSpecify the G.SHDSL line rate, if applicable. 
The range of valid numbers is between 72 and 
2312.
Step 12dsl operating-mode gshdsl symmetric 
annex annexSet the G.SHDSL operating mode, if 
applicable, and select the G.991.2 annex.
Step 13ip access-group 100 inActivate access list 100.
Step 14no shutdownEnable the interface and configuration 
changes made to the interface.
Step 15exitExit configuration mode for the ATM 
interface. Command Task 
						

							 
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Configuring Quality of Service Parameters
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Configuring Quality of Service Parameters
This section describes how to configure quality of service (QoS) parameters. The 
following are requirements for voice QoS:
Priority queuing for voice traffic
Fragmenting large data packets and interleaving voice packets
You can configure QoS in a single- or multiple-PVC environment. In a 
single-PVC environment, the traffic relies on IOS to provide priority queuing, 
using class-based weighted fair queuing (CBWFQ) to prioritize voice traffic and 
using MTU size reduction to perform Layer 3 fragmentation of data packets. In a 
multiple-PVC environment, the traffic relies on the ATM interface to provide 
priority queuing for voice and fragmentation and interleaving. 
NoteQoS parameters are supported only on routers with voice features.
For complete information on the QoS commands, refer to the Cisco IOS 
documentation set. For general information on QoS concepts, see Chapter 1, 
“Concepts.”
Configuring a Single-PVC Environment
In the single-PVC environment, the traffic relies on IOS to provide priority 
queuing (using CBWFQ). The tasks to configure a single-PVC environment are 
as follows:
Configuring IP Precedence 5 for voice packets
Configuring an access list and voice class
Configuring a policy map and specify priority queuing for voice class
Associating the policy map to the ATM PVC and decreasing the MTU of the 
ATM interface 
						

							 
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Chapter 7      Router Feature Configuration
Configuring Quality of Service Parameters
Configuring IP Precedence
IP Precedence gives voice packets a higher priority than other IP data traffic. The 
ip precedence command is used by the router to differentiate voice traffic from 
data traffic. Therefore, you need to ensure that the data IP packets do not have the 
same IP precedence as that of the voice packets.
Follow the steps below to configure real-time voice traffic precedence over other 
IP network traffic, beginning in global configuration mode.
NoteIn IP Precedence, the numbers 1 through 5 identify classes for IP flows; the 
numbers 6 through 7 are used for network and backbone routing and updates. It is 
recommended that IP Precedence 5 is used for voice packets. 
Configuring an Access List and Voice Class
Follow the steps below to create a policy map and to associate a priority queue 
with the voice class, beginning in global configuration mode. Command Task
Step 1dial-peer voice number voipEnter the dial peer configuration mode to 
configure a VoIP dial peer.
Step 2destination-pattern numberSet a destination pattern.
Step 3session target {ipv4:destination-address} Specify a destination IP address for the dial 
peer.
Step 4ip precedence numberSelect a precedence level for the voice traffic 
associated with that dial peer.
Step 5exitExit configuration mode for the dial peer 
interface.
Command Task
Step 1access-list 101 permit ip any any precedence 
5Configure an access list to match voice 
packets. 
						

							 
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Configuring Quality of Service Parameters
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Configuring a Policy Map and Specifing Voice Queuing
Follow the steps below to configure a policy map and to specify voice queuing, 
beginning in global configuration mode.
Configuring a Policy Map and Specifying Priority Queuing for Voice Class
Follow the steps below to associate the policy map to the ATM PVC and decrease 
the MTU of the ATM interface so that large data packets are fragmented, 
beginning in global configuration mode.
Step 2class-map voiceConfigure a voice class.
Step 3match access-group 101Associate the voice class with the access list. Command Task
Command Task
Step 1policy map nameConfigure a policy map.1
1. Total bandwidth for the policy map may not exceed 75 percent of the total PVC bandwidth.
Step 2class voiceSpecify the class for queuing.
Step 3priority numberSpecify the priority for queuing.
Command Task
Step 1policy map nameConfigure a policy map.1
1. Total bandwidth for the policy map may not exceed 75 percent of the total PVC bandwidth.
Step 2class voiceSpecify the class for queuing.
Step 3priority bandwidthSpecify the priority for queuing.
Step 4exitExit configuration mode for the policy map. 
						

							 
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Chapter 7      Router Feature Configuration
Configuring Quality of Service Parameters
Associating the Policy Map to the ATM PVC and Decreasing the ATM Interface 
MTU
Use the following table to associate the policy map to the ATM PVC and decrease 
the MTU, beginning in global configuration mode. It is recommended that 300 is 
used for the MTU size because it is larger than the size of the voice packets 
generated by the different codecs. 
NoteThe default service class for configuring the ATM interface is unspecified bit rate 
(ubr). In order to attach the policy map to the ATM PVC, you must use a service 
class of vbr-nrt or vbr-rt.
Command Task
Step 1interface ATM 0Enter configuration mode for the ATM 
interface.
Step 2ip address ip-address maskSet the IP address and subnet mask for the 
ATM interface.
Step 3pvc vpi/vciCreate an ATM PVC for each end node with 
which the router communicates.
Step 4encapsulation protocolSpecify the encapsulation type for the PVC. 
Encapsulations can be specified as 
AAL5SNAP or AAL5MUX PPP.
Step 5service policy out nameAssociate the service policy name.
Step 6vbr-rt pcr scr bs Specify the service class.
Step 7exitExit configuration mode for the ATM PVC.
Step 8mtu numberSpecify the MTU for the ATM interface.
Step 9no shutdownEnable the ATM interface.
Step 10exitExit configuration mode for the ATM 
interface. 
						

							 
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Configuring a Single-PVC Environment Using RFC 1483 Encapsulation
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Configuration Example
The following example shows a voice QoS configuration in a single-PVC 
environment using AAL5SNAP encapsulation.
!
dial-peer voice 105 voip  
destination-pattern 3.. 
session target ipv4:10.1.2.3 
ip precedence 5
access-list 101 permit ip any any precedence critical
class-map voice  
match access-group 101
policy-map mypolicy 
class voice   
priority 480
int atm0
mtu 300
pvc 8/35  
encapsulation aal5snap   
service-policy out mypolicy 
vbr-rt 640 640 10
!
Configuring a Single-PVC Environment Using RFC 
1483 Encapsulation
This section describes configuring of a single-PVC environment using RFC 1483.
In a single-PVC environment using RFC 1483 encapsulation, the traffic relies on 
Cisco IOS to provide priority queuing using low latency queuing (LLQ). The 
following tasks are needed to configure a single-PVC environment:
Differentiating Between Data and Voice Packets
Configuring an Access List and Voice Class
Configuring a Policy Map and Specifying Voice Queuing