provide complete redundancy. But if voice is layered onto the network, these requirements need to be revisited. With Cisco Architecture for Voice, Video and Integrated Data (AVVID) technology, Cisco CallManager clusters provide a way to design redundant hardware. When using gatekeepers, you can configure backup devices as secondary gatekeepers in case the primary gatekeeper fails. Redundant devices and Cisco IOS services, like Hot Standby Router Protocol (HSRP), also provide high availability. For proactive network monitoring and trouble reporting, a network management platform such as CiscoWorks2000 provides a high degree of responsiveness to network issues.

Content 7.1 Planning for Implementation of Voice in a Campus 7.1.8 Power Requirements in Support of VoIP Accurate calculations of power requirements are critical for an effective IP telephony solution. IP phones are best implemented with PoE. Power can be supplied to the IP phones directly from Cisco Catalyst switches with inline power capabilities or by inserting a Cisco Catalyst Inline Power Patch Panel. In addition to IP phones, failover power and total load must be considered for all devices in the IP telephony availability definition, including Building Distribution and Campus Backbone submodules, gateways, Cisco CallManager, and other servers and devices. Power calculations must be network-based rather than device-based. Also, as with wireless access points, VoIP phones are best implemented with Power over Ethernet (PoE). To provide highly available power protection, you need either a UPS with a minimum battery life of 1 hour for power system failures, or a generator. This solution must include UPS or generator backup for all devices associated with the IP telephony network. In addition, consider UPS systems that have auto-restart capability and a service contract for 4-hour support response. Recommendations for IP telephony high-availability power and environment include the following:

UPS and generator backup
UPS systems with auto-restart capability
UPS system monitoring
4-hour service response contract for UPS system problems
Recommended equipment operating temperatures maintained at all times

Content 7.2 Accommodating Voice Traffic on Campus Switches 7.2.1 QoS and Voice Traffic in the Campus Module Regardless of the speed of individual switches or links, speed mismatches, many-to-one switching fabrics, and aggregation can cause congestion and latency. If congestion management features are not in place, some packets will be dropped, causing retransmissions that inevitably increase network load even more. QoS can mitigate latency caused by congestion on campus devices. QoS classifies and marks traffic at one device. Other devices can then prioritize or queue the traffic according to the marks applied to individual frames or packets. Figure describes how QoS is applied in the campus network.

Content 7.2 Accommodating Voice Traffic on Campus Switches 7.2.2 LAN-Based Classification and Marking Classification and marking identifies traffic for proper prioritization as the traffic traverses the network. Traffic is classified by examining information at different layers of the Open Systems Interconnection (OSI) model. The classified traffic receives a mark or QoS value. IP traffic can be classified according to any values configurable in an access control list (ACL) or any of the following criteria :

Layer 2 parameters: MAC address, Multiprotocol Label Switching (MPLS), ATM cell loss priority (CLP) bit, Frame Relay discard eligible (DE) bit, or ingress interface
Layer 3 parameters: IP precedence, differentiated services code point (DSCP), QoS group, IP address, or ingress interface
Layer 4 parameters: TCP or UDP ports, or ingress interface
Layer 7 parameters: Application signatures or ingress interface

All traffic classified or grouped according to these criteria will be marked according to that classification. QoS marks establish priority levels or priority classes of service for network traffic as it is processed by each switch. Once traffic is marked with a QoS value, QoS policies on switches and interfaces handle traffic according to the values contained in the individual frames and packets. As a result of classification and marking, traffic is prioritized accordingly at each switch to ensure that delay-sensitive traffic receives priority processing as the switch manages congestion, delay, and bandwidth allocation. QoS Layer 2 classification examines information in the Ethernet or 802.1Q header, such as the destination MAC address or VLAN ID. QoS Layer 2 marking occurs in the Priority field of the 802.1Q header. LAN Layer 2 headers have no means of carrying a QoS value, so 802.1Q encapsulation is required if Layer 2 QoS marking is to occur. The Priority field is 3 bits long and is also known as the 802.1p User Priority or Class of Service (CoS) value. This 3-bit field supports CoS values from 1 to 7, with 1 being associated with delay tolerant traffic such as TCP/IP. Voice traffic, which by nature is not delay tolerant, receives higher default CoS values. A CoS value of 5 is given to Voice Bearer traffic, which is the phone conversation itself, so voice quality is impaired if packets are dropped or delayed. Call signaling to create, maintain, and tear down a voice call receives a CoS of 3. As a result of Layer 2 classification and marking, the following QoS operations can occur:

Input queue scheduling: When a frame enters a port, it can be assigned to a port-based queue prior to being scheduled for switching to an egress port. Typically, multiple queues are used where traffic requires different service levels.
Policing: Frames are inspected to see if a predefined rate of traffic within a certain timeframe has been exceeded. The timeframe is typically a fixed number internal to the switch. If a frame has exceeded the rate limit, it can either be dropped or the CoS value can be marked down.
Output queue scheduling: The switch places the frame into an appropriate outbound (egress) queue for switching. The switch ensures that the buffer does not overflow on the queue.

QoS Layer 3 classification examines header values, such as the destination IP address or protocol. QoS Layer 3 marking occurs in the Type of Service (ToS) byte in the IP header. The first three bits of the ToS byte are occupied by IP Precedence, which correlates to the three CoS bits carried in the Layer 2 header. The ToS byte can also be used for DSCP marking. DSCP allows prioritization hop by hop as packets are processed on each switch and interface. Figure shows how DSCP uses ToS bits. The first three DSCP bits, correlating to Precedence and CoS, identify the DSCP CoS for the packet. The next three DSCP bits establish a drop precedence for the packet. Packets with a high DSCP drop precedence value are dropped before those with a low value if a device or queue becomes overloaded. Voice traffic is marked with a low value to minimize voice packet drop. Each 6-bit DSCP value is also given a DSCP name. DSCP classes 1-4 are Assured Forwarding (AF) classes. If the DSCP class value is 3 and the drop precedence is 1, the DSCP would be AF31.

Content 7.2 Accommodating Voice Traffic on Campus Switches 7.2.3 Describing QoS Trust Boundaries Trust boundaries establish a border for traffic entering the campus network. As traffic traverses the switches of the campus network, it is handled and prioritized according to the marks received or trusted when the traffic originally entered the network at the trust boundary. At the trust boundary device, QoS values are trusted if they accurately represent the type of traffic and precedence processing the traffic should receive as it enters the campus network. If untrusted, the traffic is marked with a new QoS value appropriate for the policy in place