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Chapter 8      Advanced Router Configuration
Configuring TCP Maximum Segment Size for PPPoE
Configuring TCP Maximum Segment Size for PPPoE
The configuring TCP maximum segment size for PPP over Ethernet feature is 
supported on the following Cisco routers:
Cisco 806 and 831
Cisco 826 and 836
Cisco 827, 827H, 827-4V, and 837
Cisco SOHO 77, SOHO 77H, SOHO 78, SOHO 96, and SOHO 97
Cisco 828
If a Cisco router terminates the PPPoE traffic, a computer connected to the 
Ethernet interface may have problems accessing websites. The solution is to 
manually reduce the maximum transmission unit (MTU) configured on the 
computer by constraining the TCP maximum segment size (MSS). Enter the 
following command on the router’s Ethernet 0 interface:
ip tcp adjust-mss mss
where mss is 1452 or less.
Configuration Example
The following example shows a configuration of a PPPoE client.
vpdn enable
no vpdn logging
!
vpdn-group 1
 request-dialin
  protocol pppoe
!
interface Ethernet0
 ip address 192.168.100.1 255.255.255.0
 ip tcp adjust-mss 1452
 ip nat inside
!
interface ATM0
 no ip address
 no atm ilmi-keepalive
 pvc 8/35 
  pppoe-client dial-pool-number 1 
						

							 
Chapter 8      Advanced Router Configuration
Configuring Low Latency Queuing and Link Fragmentation and Interleaving
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dsl operating-mode auto
!
interface Dialer1
ip address negotiated
ip mtu 1492
ip nat outside
encapsulation ppp
dialer pool 1
dialer-group 1
ppp authentication pap callin
ppp pap sent-username sohodyn password 7 141B1309000528
!
ip nat inside source list 101 interface Dialer1 overload
ip route 0.0.0.0.0.0.0.0 Dialer1
access-list 101 permit ip 192.168.100.0.0.0.0.255 any
Configuring Low Latency Queuing and Link 
Fragmentation and Interleaving
Low latency queuing (LLQ) provides a low-latency, strict-priority transmit queue 
for voice over IP (VoIP) traffic. 
LLQ is supported on the following Cisco routers:
Cisco 806
Cisco 826 and 836
Cisco 827, 827H, 827-4V, 831, and 837
Cisco 828
Link fragmentation and interleaving (LFI) reduces voice traffic delay and jitter by 
fragmenting large data packets and interleaving voice packets within the data 
fragments.  
						

							 
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Chapter 8      Advanced Router Configuration
Configuring Low Latency Queuing and Link Fragmentation and Interleaving
Configuring Low Latency Queuing
Follow the steps below to configure the router for LLQ :
Step 1Ensure that the voice and data packets have different IP precedence values so 
that the router can differentiate between them. Normally, data packets should 
have an IP precedence of 0, and voice packets should have an IP precedence of 5. 
If the VoIP packets are generated from within the router, you may set the IP 
precedence to 5 for these packets by entering the ip precedence number 
command in dial-peer voice configuration mode as follows:
a.Enter the global configuration dial-peer voice 1 voip command.
b.Enter the ip precedence 5 command.
Step 2Create an access list and a class map for the voice packets.
a.Create an access list by entering the access-list 101 permit ip any any 
precedence 5 command.
b.Create a class map for the voice packets by entering class-map match-all 
voice command.
c.Link the class map to the access list by entering the match access-group 101 
command.
Step 3Create LLQ for voice traffic.
a.Create a policy map by entering the policy-map mypolicy command.
b.Define the class by entering the class voice command.
c.Assign the priority bandwidth to the voice traffic. The priority bandwidth 
assigned to the voice traffic depends on the codec used and the number of 
simultaneous calls that you allow. For example, a G.711 codec call consumes 
200 kbps; therefore, to support one G.711 voice call you would enter a 
priority 200 command.
Step 4Attach LLQ to the dialer interface.
a.Enter the global configuration interface dialer 1 command.
b.Create a service policy by entering the service-policy out mypolicy 
command. 
						

							 
Chapter 8      Advanced Router Configuration
Configuring LFI
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Configuring LFI
Follow the steps below to configure the router for LFI.
NoteWhen you are configuring LFI, the data fragment size must be greater than the 
voice packet size; otherwise, the voice packets fragment, and voice quality 
deteriorates.
Step 1Configure the dialer bandwidth. The dialer interface has a default bandwidth of 
56 kbps, which may be less than the upstream bandwidth of your digital 
subscriber line (DSL) connection. You can find the upstream bandwidth of your 
DSL connection by entering the show dsl interface atm0 command in dialer 
interface configuration mode. If you have two or more PVCs sharing the same 
DSL connection, the bandwidth configured for the dialer interface must be the 
same as the bandwidth allocated to its assigned PVC.
Step 2Enable PPP multilink, and configure fragment delay and interleaving for the 
dialer interface. 
a.Enter the global configuration interface dialer 1 command.
b.Specify the dialer bandwidth by entering the bandwidth 640 command. The 
bandwidth is specified in kilobits per second (kbps).
c.Enter the ppp multilink command.
d.Specify PPP multilink interleaving by entering the ppp multilink interleave 
command.
e.Define the fragment delay by entering the ppp multilink fragment-delay 10 
command.
f.Calculate the fragment size using the following formula:
fragment size = (bandwidth in kbps/ 8) * fragment-delay in milliseconds (ms)
In this case, the fragment size = (640/8) * 10, resulting in a fragment size of 
800. The fragment size is greater than the maximum voice packet size of 200, 
which is G.711 20 ms. A low fragment delay corresponds to a fragment size 
that may be smaller than the voice packet size, resulting in reduced voice 
quality. 
						

							 
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Chapter 8      Advanced Router Configuration
Configuring Class-Based Traffic Shaping to Support Low Latency Queuing
Configuring Class-Based Traffic Shaping to Support 
Low Latency Queuing
Class-based traffic shaping (CBTS) is supported on the following Cisco routers:
Cisco 806 
Cisco 831 
CBTS can be used to control the WAN interface traffic transmission speed to 
match the speed of the attached broadband modem or of the remote target 
interface. CBTS ensures that the traffic conforms to the policies configured for it, 
thereby eliminating topology bottlenecks with data-rate mismatches. 
The shape average kbps and the shape peak kbps commands enable you to define 
traffic shaping for an interface.
NoteCBTS is supported on the Ethernet 1 interface.
Configuring CBTS for LLQ
Follow the steps below to configure CBTS, beginning in global configuration 
mode. This procedure shows how to create multiple traffic classes and associate 
them with policy maps, and then to associate the policy maps with a router 
interface. 
Step 1Define a traffic classification.
a.Enter the class-map map-name command to define a traffic classification. 
For example, the name voice could be used to specify that this is a class map 
for voice traffic.
b.Now in class configuration mode, enter the match ip precedence 5 command 
to match all IP voice traffic with a precedence of 5. Cisco Architecture for 
Voice, Video and Integrated Data (AVVID) documentation specifies a 
precedence value of 5 for voice-over-IP traffic.
c.Enter exit to leave class configuration mode. 
						

							 
Chapter 8      Advanced Router Configuration
Configuring Class-Based Traffic Shaping to Support Low Latency Queuing
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Step 2Define a policy map and associated classes for low-latency queuing.
a.Enter the policy-map map-name command in global configuration mode to 
construct policies and to allocate different network resources for the defined 
traffic classes. The name LLQ could be used to specify that this is the policy 
map for LLQ.
b.Now in policy-map mode, define a class to handle voice traffic by entering 
class QOS-class-name, using the class-map name you defined using the 
class-map command in Step 1. This command places the router in QOS-class 
configuration mode.
c.Enter priority number, where number is bandwidth in kilobits per second. A 
value of 300, as shown in the example configuration, provides enough 
bandwidth for two G.711 voice ports. Before setting a priority value, refer to 
the specification for the CODEC used for voice calls.
d.Enter exit to return to policy-map configuration mode.
e.Enter class class-default to use the default class for all traffic other than 
voice traffic. The name class-default is well known, and does not have to be 
predefined using the class-map command.
f.Apply WFQ to non-voice traffic by entering the fair-queue command.
g.Enter exit twice to return to global configuration mode.
Step 3Define a traffic-shaping policy map.
a.Enter policy-map map-name in global configuration mode. The name shape 
should be used to indicate this map defines overall traffic shaping that is 
compatible with the remote transmission rate bandwidth.
b.Enter class class-default to associate the default class with this policy map. 
c.Set the transmission speed to be used after traffic shaping to match the speed 
of the broadband modem or remote interface by entering the shape average 
kbps command, where kbps is a value in kilobits per second.
CautionThe transmission speed entered must be less than or equal to the TX 
bandwidth of the DSL or cable modem to which the router is attached. 
Specifying a value greater than the modem’s TX bandwidth will result 
in the modem’s becoming congested, and the benefits of applying QOS 
might be lost. 
						

							 
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Chapter 8      Advanced Router Configuration
Configuring Class-Based Traffic Shaping to Support Low Latency Queuing
d.Enter service-policy name to associate the LLQ policy map with the 
traffic-shaping policy map. If the map name for the low-latency queue were 
LLQ, then name would be LLQ.
e.Enter exit twice to return to global configuration mode.
Step 4Apply these policies to the Ethernet 1 interface.
a.Enter the interface Ethernet 1 command.
b.Apply the service policy to the Ethernet 1 interface by entering 
service-policy output
 name, where name matches the policy defined in the 
traffic-shaping policy map. If the traffic-shaping policy map name were 
shape, the service-policy name would also be shape. 
Step 5Enter end to leave router configuration mode.
Configuration Example
The following example shows how a Cisco 806 router can be configured to 
connect to a broadband modem with limited bandwidth, while ensuring voice line 
quality. Two policy maps are configured: 
Policy map LLQ
Policy map shape
Policy map LLQ ensures that voice traffic has a strict priority queue with 
bandwidth of up to 300 kbps. The policy map shape limits the total throughput to 
2.2 MBps.
!
version 12.2
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password encryption
!
hostname 806-uut
!
ip subnet-zero
!
class-map match-all voice
 match ip precedence 5
!
! 
						

							 
Chapter 8      Advanced Router Configuration
Configuring the Length of the PVC Transmit Ring
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  class voice
    priority 300
  class class-default
   fair-queue
policy-map shape
  class class-default
   shape average 2250000
   service-policy LLQ
!
interface Ethernet0
 ip address 1.7.65.11 255.255.0.0
!
interface Ethernet1
 ip address 192.168.1.101 255.255.255.0
service-policy output shape
!
ip classless
ip http server
ip pim bidir-enable
!
line con 0
 stopbits 1
line vty 0 4
 login
!
!
scheduler max-task-time 5000
end
!
Configuring the Length of the PVC Transmit Ring
The length of the PVC transmit ring can be configured on the following Cisco 
routers:
Cisco 826 and 836
Cisco 827, 827H, 827-4V, and 837
Cisco SOHO 77, SOHO 77H, SOHO 78, SOHO 96, and SOHO 97
Cisco 828  
						

							 
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Chapter 8      Advanced Router Configuration
Configuring the Length of the PVC Transmit Ring
If both voice and data packets share the same PVC, it is important to reduce the 
PVC transmit (TX) ring size. This reduces the maximum number of data packets 
and fragments that can be in front of a voice packet in the hardware queue, thus 
reducing latency.
Follow these steps to reduce the PVC TX ring size:
Step 1Enter the global configuration int atm 0 command.
Step 2Specify the PVC number by entering the pvc 1/100 command.
Step 3Reduce the PVC TX ring size to 3 by entering the tx-ring-limit 3 command.
Configuration Example
The following example combines LFI, LLQ, and the PVC TX ring configurations.
class-map match-all voice
match access-group 101
!
policy-map mypolicy
 class voice
  priority 200 
 class class-default
  fair-queue
!
interface Ethernet0
ip address 70.0.0.1 255.255.255.0
no ip mroute-cache
!
interface ATM0
 no ip address
 bundle-enable
 dsl operating-mode auto
!
interface ATM0.1 point-to-point
 no ip mroute-cache
 pvc 1/40 
 encapsulation aal5mux ppp dialer
 dialer pool-member 1
 tx-ring-limit 3
!
interface Dialer1
 bandwidth 640 
						

							 
Chapter 8      Advanced Router Configuration
Configuring DHCP Server Import
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78-5372-06  ip address 60.0.0.1 255.255.255.0
 encapsulation ppp
 dialer pool 1
 service-policy output mypolicy
 ppp multilink
 ppp multilink fragment-delay 10
 ppp multilink interleave
!
ip classless
no ip http server
!
access-list 101 permit ip any any precedence 5
!
voice-port 1
!
voice-port 2
!
voice-port 3
!
voice-port 4
dial-peer voice 110 pots
 destination-pattern 1105555
 port 1
!
dial-peer voice 210 voip
 destination-pattern 2105555
 session target ipv4:60.0.0.2
 codec g711ulaw
 ip precedence 5
Configuring DHCP Server Import
The Cisco IOS DHCP server has been enhanced to allow configuration 
information to be updated automatically by PPP. You can enable PPP to 
automatically configure the Domain Name System (DNS), the Windows 
Information Name Server (WINS), or the NetBIOS Name Service (NBNS), and 
the server IP address information within a Cisco IOS DHCP server pool.
This feature is supported on the following Cisco routers:
Cisco 806 and 831
Cisco 826 and 836
Cisco 827, 827H, 827-4V, and 837