CCNA Media And Qos Questions

Traffic shaping buffers and delays excess packets to conform to a configured average rate, which matches a provider policing at the CIR. Setting shape average to 4000000 bits per second on the outbound WAN interface smooths bursts so the policer sees a rate within contract, reducing drops and preserving voice quality across the link.

The service-policy is applied on the ingress interface, so show policy-map interface displays per-class matched and marked packet counters there. This confirms whether the class-map's marking (for example DSCP EF for voice) is actually being applied, revealing why one-way audio occurs.

The priority command in a policy-map creates a low-latency queue (LLQ) that provides strict priority scheduling for voice traffic while policing the priority class to a configured rate. This ensures voice gets absolute priority during congestion, and the policer prevents voice from starving other queues. Applying the policy-map to the WAN interface with service-policy output activates this behavior.

Even with correct markings, voice needs strict priority treatment during congestion to keep delay and jitter low. A low-latency queue on the WAN egress interface, typically matching DSCP EF with a bandwidth guarantee and policer, ensures voice packets are serviced ahead of data so call quality remains acceptable when the link is saturated.

This is correct because trust is broken at ingress (DSCP 0), so the router must reclassify. A class-map matching the RTP port range identifies voice bearer, the priority command gives it strict-priority LLQ treatment, and applying the policy-map outbound on the WAN interface shapes and queues the traffic in the correct direction toward the congestion point.

Exhausted transcoder resources leave no Media Termination Point capable of converting G.711 to G.729, so Cisco Unified Communications Manager cannot allocate the required resource for the new call and rejects it with a resource-unavailable cause code. The stem's transcoding constraint is therefore unsatisfiable, making rejection the expected outcome.

RSVP signals the requested bandwidth along the path; when a router finds insufficient unreserved capacity, it returns a PathErr and the admission decision rejects the flow, so the call setup is blocked rather than rerouted or downgraded.

CUCM registers MTP as a media resource for inserting or removing media streams, enabling supplementary services such as DTMF relay and RSVP. It satisfies the stem's requirement for a supported media resource type alongside transcoders, conference bridges and annunciators.

CS3 (Class Selector 3) is the recommended DSCP for call signaling traffic in Cisco collaboration networks. It provides a higher priority than best effort but lower than voice bearer, which is appropriate for signaling. This marking ensures that call setup and teardown messages are prioritized over data traffic but do not compete with voice media. Cisco documentation consistently recommends CS3 for call signaling.

LLQ assigns video traffic to a strict-priority queue with a configured bandwidth guarantee, so latency-sensitive packets are drained ahead of other classes. This directly minimises jitter and latency, satisfying the stem's video prioritisation requirement, while the policed priority limit prevents starvation of remaining traffic classes.

Re-marking at the network layer via an ACL classifies the DSCP 0 video flow, and the policy-map rewrites its DSCP value on ingress, so endpoints stay untouched. This satisfies the constraint of fixing marking without changing endpoint configuration.

Bandwidth reservation guarantees a minimum share of interface capacity to a class using the priority or bandwidth command within a policy-map. It satisfies the stem by being a core policy-map action, ensuring latency-sensitive or business-critical traffic is not starved during congestion.

Shaping smooths bursts by buffering excess packets and releasing them at the configured rate, so the interface queue is not overwhelmed by sudden spikes. This directly satisfies the stem's voice-traffic constraint: steady pacing keeps delay and jitter low, letting queuing mechanisms schedule voice reliably.

A class-map defines the traffic classification criteria, and match dscp ef identifies packets already marked with the Expedited Forwarding DSCP value. This is exactly the tool used to select voice bearer traffic that an upstream phone or switch has marked, so the policy-map can then apply the desired treatment to that class.

MTPs can act as a bridge for protocol-specific signaling needs.

Locations call admission control totals the bandwidth allocated to phones at a location and rejects new calls once that limit is exceeded, blocking them before they consume WAN capacity. This directly satisfies the stem's requirement to block calls when location bandwidth is exhausted.

G.711 is a narrowband waveform codec using pulse-code modulation at 64 kbps, delivering toll quality with minimal processing delay. It is the baseline codec in Cisco collaboration, satisfying the requirement for a common audio codec used across Cisco endpoints and gateways.

Locations define the maximum audio and video bandwidth allocated between sites or to a device pool. Applying the limit at the Location object enforces call admission control for video, which per-device or region settings cannot achieve.

Policing enforces a rate limit by discarding excess packets immediately, causing retransmissions and TCP throughput collapse. Shaping instead queues and delays outbound traffic to smooth bursts, avoiding drops. The stem's contrast hinges on this drop-versus-buffer mechanism, making the statement accurate.

DSCP 46 maps to Expedited Forwarding, the per-hop behaviour the Cisco SRND specifies for voice RTP media. EF provides low latency, low jitter and guaranteed bandwidth through priority queuing, matching the marking requirement for audio media between endpoints.

A class map defines match criteria such as ACLs, DSCP values or protocol ports, identifying which packets belong to a traffic class. It performs classification only; the policy map then applies actions, so the class map's primary function is categorising and matching specific traffic flows.

Absent configured QoS policies, the phone applies its default markings: voice media as DSCP 46 (EF) for low-latency treatment and call signalling as CS3, allowing the network to prioritise voice traffic out of the box.

The SIP ALG inspects the Session Description Protocol inside SIP messages and rewrites embedded connection and media addresses so the public address is signaled instead of the private one. This lets the provider send RTP to the NAT-translated address, restoring two-way audio for the internal phone behind the router.

CS3 (24) is the standard DSCP marking for Call Signaling traffic in Cisco Unified Communications networks, matching the voice deployment model's per-hop behaviour classes. This satisfies the stem's constraint: the recommended DSCP value for Call Signaling.

EF (Expedited Forwarding), which corresponds to DSCP 46, is the recommended marking for voice bearer traffic. It indicates that packets require low latency, low jitter, and low loss, and network devices should provide priority queueing for EF-marked traffic. This ensures that voice packets are transmitted first during congestion, maintaining voice quality. Cisco's QoS baseline recommends EF for voice bearer.

EF (Expedited Forwarding) is the recommended DSCP for voice bearer traffic. It provides low loss, low latency, low jitter, and assured bandwidth. The service provider expects EF for voice, ensuring proper treatment across the network. Configuring IP phones to mark voice media as EF aligns with industry best practices.

Packet loss directly degrades video quality: dropped RTP packets cause macroblocking, freezing and audio artefacts, since real-time video cannot retransmit within its latency budget. It is one of the three factors, alongside latency and jitter, that determine call quality.

To apply different DSCP markings to voice and signaling, the router must first identify each traffic type. A class-map with match statements such as 'match ip dscp' is not sufficient if traffic is unmarked; instead, matching on UDP port ranges (e.g., 16384-32767 for RTP) or on protocols like SCCP/SIP is needed. This class-map then feeds into the policy-map where 'set dscp ef' and 'set dscp cs3' are applied.

Cisco IP Phones are designed to mark voice media as EF and signaling as CS3. When connected to a switch, the switch port can be configured to trust the DSCP markings from the phone. This simplifies QoS deployment and is a recommended practice in Cisco collaboration networks.

AF41 (Assured Forwarding Class 4, Drop Precedence 1) is the recommended DSCP for interactive video traffic in Cisco collaboration networks. It provides a higher priority than best effort but lower than voice, ensuring that video is prioritized over data while not competing with voice. For a Webex Room Kit sending 1080p video, marking with AF41 ensures appropriate queuing and bandwidth guarantees in the QoS policy.

LLQ combines strict priority queuing with a policer. It places voice traffic in a priority queue that is serviced before other queues, ensuring low latency and jitter. The policer limits the amount of traffic that can enter the priority queue, preventing starvation of other queues. Configuring LLQ with a bandwidth limit of 1 Mbps meets the requirement exactly.

A Location defines the total audio and video bandwidth available for calls to and from devices assigned to it. When the aggregate bandwidth of existing calls plus the new call would exceed the configured limit, CUCM rejects the call with a not-enough-bandwidth cause. This is the correct object for capping concurrent calls at a site with a constrained WAN link.

Class-based weighted fair queueing gives each branch class a guaranteed minimum bandwidth and shares excess proportionally, which prevents one branch from monopolizing the link. Adding low-latency queueing for voice within the same policy-map ensures voice bearer traffic gets strict-priority treatment, satisfying both the fairness and voice-priority requirements in this hub-and-spoke design.

Marking at the source lets the originating endpoint or access switch set DSCP values before any congestion or queuing occurs, so downstream devices can classify and prioritise on trusted markings rather than re-inspecting every flow. This satisfies the stem's best-practise requirement by preserving classification integrity end to end.

The show class-map command displays the match statements configured in each class-map. Verifying that the video class matches the correct DSCP value, such as AF41, or the correct access list confirms that the classification logic itself is correct before relying on interface statistics, which is essential when video is not being placed in the expected queue.

LLQ creates a strict priority queue that is always serviced first, ensuring minimal delay for voice. A built-in policer enforces the configured rate, dropping excess packets to prevent the priority queue from starving other queues. This combination provides low latency and jitter for voice while maintaining fairness for other traffic.