Case Study: QoS in the Acme, Inc. Network

Acme, Inc. currently uses five different classes of service on the Acme network. For simplicity's sake, and because of the 3-bit resolution of some QoS technologies, such as 802.1p LAN CoS, MPLS EXP, and IP precedence, Acme IT limits its QoS to classes that can be identified in 3 bits. Traffic is classified at the WAN edge by matching IP precedence valuesonly the first 3 bits of the ToS byte, which cover 7 DSCP values each.

The policy template shown in Table 5-5 deals with policy marking of packets in Acme. It shows QoS policy for voice, video, and data classifications and their breakout assignments based on circuit speed, allocation per class as a percentage, and the classifications assigned.

Table 5-5. Sample Policy Template Breakdown

Policy Number	30	25	20	15	10	5
	Bandwidth at Interface Line Rate (in kbps)
Class	622000	155000	45000 to 34000	34000 to 2048	2048 to 1024	1024 to 256
Management	2%	2%	2%	2%	2%	6%
Voice EF	10%	33%	33%	33%	33%	33%
Video AF4	5%	5%	8%	7%	10%	5%
Signaling AF3	5%	5%	5%	5%	5%	10%
Default BE	58%	45%	50%	43%	40%	36%
Scavenger CS1	20%	10%	10%	10%	10%	10%
Total	100%	100%	100%	100%	100%	100%

IP precedence values 6 and 7 are also used for network control traffic (routing protocols). Most of this traffic is link-local (CE-PE only), allowing an individual class of traffic to be set up for this traffic on a WAN port with minimum bandwidth. On the CE side of the CE-to-PE links, it is recommended that a separate class be used for management traffic. On the PE side of the CE-to-PE link, this tends to vary with each provider. This traffic must, at a minimum, be mapped to a high-priority data class of service in the service provider cloud.

QoS for Low-Speed Links: 64 kbps to 1024 kbps

LFI allows large packets on a serial link to be divided into using MLP or Frame Relay encapsulation with FRF.12 fragmentation.

To determine the LFI fragment size, you must consider the packet flow through the router. Following the link fragmentation process and LLQ/CBWFQ's fragment ordering, fragments are placed on a transmit ring buffer or TX ring. The TX ring then queues packets onto the physical interface in a FIFO fashion.

Example 5-5 shows examples of applications using MLP and FRF.12.

Example 5-5. LFI on MLP

interface Multilink1                   Multilink bundle interface bandwidtd XXX                          Enter Link bandwidth in kbps  ip address 10.52.255.1 255.255.255.252  no ip redirects  no ip proxy-arp  ip authentication mode eigrp 109 md5  ip authentication key-chain eigrp 100 apple_key  max-reserved-bandwidth 100  service-policy output WAN-EDGE        Apply service policy outbound  ppp multilink                         Configure Multilink  ppp multilink fragment-delay 10       Set the max packet delay in ms                                        (determines fragment size)  ppp multilink interleave              Enable LFI  multilink-group 1                     Apply template to multilink group #1 ! ! interface Serial S:P  description Multilink PPP group member  bandwidth XXX                         Configure bandwidth equal to full line rate  no ip address  no ip redirects  no ip proxy-arp  encapsulation ppp  fair-queue  ppp multilink                         enable multilink on interface  multilink-group 1                     Assign interface to multilink group 1 ! interface Serial S:P                   Next I/F when MLP Bundling is required  description Multilink PPP group member  bandwidth XXX

Slow-Speed (768-kbps) Leased-Line Recommendation: Use MLP LFI and cRTP

For slow-speed leased lines, LFI is required to minimize serialization delay. Therefore, MLP is the only encapsulation option on slow-speed leased lines because MLP LFI is the only mechanism available for fragmentation and interleaving on such links. Optionally, cRTP can be enabled either as part of the modular QoS command-line interface (MQC) policy map or under the multilink interface (using the ip rtp header-compression command). Ensure that MLP LFI and cRTP, if enabled, are configured on both ends of the point-to-point link, as shown in Example 5-6.

Example 5-6. Slow-Speed (768-kbps) Leased-Line QoS Design Example

! policy-map WAN-EDGE class Voice priority percent 33 ! Maximum recommended LLQ value compress header ip rtp ! Enables Class-Based cRTP class Call Signaling bandwidth percent 5 ! BW guarantee for Call-Signaling ! interface Multilink1 description 768 kbps Leased-Line to RBR-3745-Left ip address 10.1.112.1 255.255.255.252 service-policy output WAN-EDGE ! Attaches the MQC policy to Mu1 ppp multilink ppp multilink fragment delay 10 ! Limits serialization delay to 10 ms ppp multilink interleave ! Enables interleaving of Voice with Data ppp multilink group 1 ! ... ! interface Serial1/0 bandwidth 786 no ip address encapsulation ppp ppp multilink ppp multilink group 1 ! Includes interface Ser1/0 into Mu1 group !

These examples cover the application of the WAN-EDGE service policy discussed in Example 5-4. For more examples of configuring the WAN edge, refer to the Cisco QoS Solution Reference Network Design, http://www.cisco.com/application/pdf/en/us/guest/netsol/ns432/c649/ccmigration_09186a008049b062.pdf.