Troubleshooting Cisco VoIP and Video Solutions Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

• Some common elements found in a typical Cisco IPT solution include the following:

1. One or more Call Agents
2. IP Phones
3. Application Servers
4. Voice Gateways
5. Voice Gatekeepers

• Call agents such as CUCM order and direct each step of call completion for the endpoints
• An IP phone is simply a telephony endpoint
• Application servers provide additional services such as voice mail and unified messaging
• Voice gateways connect the IP and TDM networks
• Gatekeepers perform address translation, CAC, bandwidth control and zone management

The Need for LAN and WAN Quality of Service

• Quality of Service (QoS) is a critical component of any IP Telephony or IP voice solution
• QoS is required to mitigate against or lessen the effects of the following:

1. Delay Issues
2. Bandwidth Issues
3. Jitter Issues
4. Packet Loss Issues

• There are several types of delay that may be experienced by packets or frames in networks
• Common causes of delay include serialization, queuing, processing, and forwarding delay
• The two categories of delay are fixed delay and variable delay
• Jitter is the variation in delay between consecutive packets
• Jitter is caused by variable queuing delays and is commonly referred to as variation delay
• Jitter can be mitigated using de-jitter buffers
• Packet loss occurs when one or more packets fail to reach their intended destination

LAN and WAN IPT QoS Implementation

• The three Quality of Service models are:

1. Best Effort Delivery (Default)
2. Integrated Services
3. Differentiated Services

• The Best-Effort (BE) model does not guarantee any level of service and is not recommended
• The BE model requires no QoS implementation within the internetwork
• Integrated Services (IntServ) performs admission control for each flow request
• IntServ provides a way to deliver end-to-end QoS for real-time applications
• RFC 1633 defines two components to provide guarantees per flow:

1. Resource reservation
2. Admission control

• IntServ uses Resource Reservation Protocol (RSVP) to signal the internetworking devices
• Queuing mechanisms can be used with RSVP to provide two kinds of services which are:

1. Guaranteed Rate Service
2. Controlled Load Service

• The DiffServ model requires no advance reservations and therefore scales very, very well
• DiffServ defines the concept of service classes
• DiffServ also allows each internetwork device to handle these packets on a per hop basis
• Differentiated Services are applicable to Layer 3
• Layer 2 frames use CoS bits that are contained within the 802.1Q or ISL-encapsulated frame
• DiffServ defines a new Differentiated Services Code Point (DSCP) field in the packet header
• The last 2 bits of the ToS byte (ECN bits) can be used to perform flow control
• DiffServ defines three sets of PHBs, which are the CS, AF and EF PHBs
• The Class Selectors are DSCP values that are compatible with IP Prec values
• The AF PHB set is used for two functions: queuing and congestion avoidance
• EF packets are given premium service (above all other classes)
• Catalyst switches support the following ingress QoS mechanisms:

1. Traffic Classification
2. Traffic Policing
3. Marking
4. Congestion Management and Avoidance

• Routers can be configured to use strict priority queuing or WRED when implementing QoS
• Strict priority queuing can be accomplished with CBWFQ
• WRED can be implemented in conjunction with Explicit Congestion Notification (ECN)
• Prior to configuring the Auto QoS Feature, the following should be taken into consideration

1. Cisco Express Forwarding (CEF) is required and must be globally enabled on the router
2. Remove any policy-maps currently applied to the interface
3. Ensure that the correct bandwidth has been configured for the interface
4. Ensure that the interface is configured with an IP address
5. The auto qos voip command is not supported on subinterfaces or gigabit interfaces
6. The auto qos voip command is supported with Frame Relay DLCIs
7. If the trust is not specified, a policy-map named AutoQoS-Policy-UnTrust is created
8. If the trust keyword is used, a policy-map named AutoQoS-Policy-Trust is created

Cisco IP Video Fundamentals

• While IP voice is delivered via Unicast, IP video traffic is typically delivered using Multicast
• Multicast uses the Class D address space ranging from 224.0.0.0 to 239.255.255.255
• This address range is exclusively reserved for IP Multicast traffic by IANA
• The following table shows the different Multicast address allocations by IANA:

• All Multicast frames begin with the 24-bit prefix of 0x0100.5E. This is assigned by IANA
• Each Layer 2 Multicast MAC can represent 32 different IP Multicast addresses
• IGMP allows receivers to request a Multicast data stream from a particular group address
• The IGMP messages are encapsulated in IP datagrams, with an IP protocol number of 2
• Every IGMP message carries an IP Router Alert option in its IP header
• The IP Router Alert option notifies routers to closely examine the contents of the packet
• There are three versions of IGMP
• IGMP version 1 is defined in RFC 1112
• IGMPv1 provides the most basic services to a Multicast host
• IGMPv1 uses only two message types: Host Membership Queries and Membership Reports
• IGMP Queries are sent by IGMPv1-enabled routers to the group address 224.0.0.1
• IGMP Reports are sent unilaterally by hosts or in response to an IGMP Query
• IGMP version 2 is defined in RFC 2236
• IGMPv2 is backward compatible with version 1
• There are several changes made to IGMPv2. These changes include the following:

1. The Querier Election Mechanism
2. The ‘Leave Group’ Message
3. The Group-Specific Query

• IGMPv3 is backward-compatible with versions 1 and 2
• IGMPv3 changes include General Query and the Group-and-Source-Specific Query
• The default IGMP version enabled in Cisco IOS software is IGMPv2
• Multicast forwarding is the manner in which Multicast traffic is forwarded
• There are three Multicast forwarding mechanisms, which are:

1. Reverse Path Forwarding
2. Dense Mode Forwarding
3. Sparse Mode Forwarding

• Reverse Path Forwarding is used to prevent loops within the Multicast network or domain
• Dense mode is referred to as a flood-and-prune mechanism to forward Multicast packets
• Dense mode flooding builds what is referred to as a source-based tree
•  The source-based tree uses a (S, G) notation
• Sparse mode flooding builds what is referred to as a shared tree
• The shared tree uses a (*, G) notation
• Sparse mode forwarding requires the use of a Rendezvous Point (RP)
• There are three Multicast routing protocols,  which are:

1. Distance Vector Multicast Routing Protocol (DVMRP)
2. Multicast Open Shortest Path First (MOSPF)
3. Protocol Independent Multicast (PIM)

• PIM is a Multicast routing protocol that was developed by Cisco
• There are two versions of PIM, version 1 and version 2
• PIMv1 packets are encapsulated in IGMP Type 14 packets
• PIMv1 has been replaced by PIMv2
• PIMv2 is not encapsulated in IGMP and uses IP protocol number 103
• PIMv2 packets are sent to the Multicast group address 224.0.0.13
• In Cisco IOS Software, there are three ways in which the location of the RP can be specified:

1. By statically configuring the router or switch with the RP IP address
2. Automatically, using the Cisco Auto-RP mechanism
3. Automatically, using the Bootstrap Router mechanism with PIMv2

LAN and WAN Video QoS Implementation

• IP video traffic should be given the same QoS considerations as IP voice traffic
• Like IP voice traffic, IP video traffic should be correctly classified and marked or remarked
• Classification and marking should happen at the Access layer
• Classification can be based on ACLs or by using the switch port trust states
• Distribution switches should be configured to trust the settings specified at the Access layer
• There should be no marking or re-marking performed at the Distribution layer
• Traffic destined into the Distribution / Core should be given priority based on QoS markings

Troubleshooting Converged Networks

• IP Telephony or VoIP troubleshooting targets include the following:

1. Power over Ethernet
2. Switch Port Configuration
3. IP Services
4. Packet Loss and Congestion
5. Queuing

• If you notice packet loss, look for the following problem indicators and locations:

1. Interface drops
2. Buffer drops
3. Policy-map drops
4. Interface congestion
5. Link congestion

• IP voice troubleshooting targets include the following:

1. Packet Loss and Congestion
2. Queuing
3. Bandwidth Utilization
4. IP Multicast