Cisco Router 800 Series Software Configuration Guide
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8-5 Cisco 800 Series Software Configuration Guide 78-5372-06 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 8-6 Cisco 800 Series Software Configuration Guide 78-5372-06 ! 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.
8-7 Cisco 800 Series Software Configuration Guide 78-5372-06 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 8-8 Cisco 800 Series Software Configuration Guide 78-5372-06 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.
8-9 Cisco 800 Series Software Configuration Guide 78-5372-06 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 8-10 Cisco 800 Series Software Configuration Guide 78-5372-06 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.
8-11 Cisco 800 Series Software Configuration Guide 78-5372-06 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
8-12
Cisco 800 Series Software Configuration Guide
78-5372-06 policy-map LLQ
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
8-13 Cisco 800 Series Software Configuration Guide 78-5372-06 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 8-14 Cisco 800 Series Software Configuration Guide 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