Cisco QoS FAQ: LAN QoS

Cisco QoS FAQ: LAN QoS

Q1. What is instantaneous buffer overrun?

Answer: Instantaneous buffer overrun occurs when a switch port TX queue fills for an instant causing packet loss, which can adversely affect real-time applications. In a VoIP conversation, for example, 40 ms of congestion causes an audible clip in the conversation.

Q2. What is meant by the term “1p1q4t?”

Answer: This term denotes an interface that has a priority queue, a standard queue, and four drop thresholds.

Q3. What packet or frame marking(s) can a Layer 2 switch use to prioritize traffic?

Answer: CoS only.

Q4. What packet or frame marking(s) can a Layer 3 switch use to prioritize traffic?

Answer: CoS, IP precedence, or IP DSCP.

Q5. What must be done for a Layer 2 switch to properly classify a packet received from a host across the WAN?

Answer: The router must map the IP precedence or IP DCSP value on the packet into a CoS value for the Layer 2 switch to properly classify the packet.

Q6. For a Layer 2 switch to receive a CoS value from a router, what must exist?

Answer: An 802.1Q trunk must exist between the router and switch for the CoS value to be present.

Q7. What type of router interface supports an 802.1Q trunk to a Layer 3 switch?

Answer: An Ethernet subinterface must exist on the router to form an 802.1Q trunk with a Layer 2 switch.

Q8. What is a drop threshold?

Answer: Drop thresholds define the amount of the total Layer 2 buffer use that must be reached before a specified class of traffic is dropped.

Q9. What is a trust boundary, and where should it be set?

Figure: Trust Boundaries

cisco-qos-faq-lan-qos

Answer: A trust boundary is the first point in your network where you trust the CoS or ToS markings. Typically this point is on the IP Phones or access layer switches. Catalyst 6500 Series of Switches

Q10. Name three of the five fields that the Policy Feature Card (PFC) rewrites to perform Layer 3 switching.

Answer:
Layer 2 (MAC) Destination Address
Layer 2 (MAC) Source Address
Layer 3 IP Time-to-Live (TTL)
Layer 3 Checksum
Layer 2 (MAC) Checksum (also called the frame checksum or FCS)

Q11. Explain why the first packet in a flow may not retain the proper CoS values when a Catalyst 6500 is configured with a PFC.

Answer: If this flow is not in the flow cache of the PFC or the CEF FIB of the PFC2, the first packet of the flow is forwarded to the MSFC for a routing decision. After a routing decision has been made, the fist packet in the flow is routed to the correct interface. This rewrites the CoS value to 0. The routing decision populates the flow cache or the CEF FIB, causing subsequent packets to be switched and thereby retain the original CoS value.

Q12. What is the difference between Hybrid mode and Native mode?

Answer: Hybrid mode refers to the Catalyst operating system, whereas Native mode refers to an IOS operating system on the Catalyst 6500 series switches.

Q13. What command is used on a Catalyst 6500 running in Hybrid mode to place ports 2/1 through 2/10 in VLAN 10?

Answer: set port auxiliaryvlan 2/1-10 10

Q14. What command enables QoS on a 6500 running in Native mode?

Answer: mls qos

Q15. What queue and threshold is recommended for call control traffic on a Catalyst 6500?

Answer: 2q1t

Q16. Which series of Ethernet line cards are preferred in the Catalyst 6500 series for QoS? Why?

Answer: The 65xx series of Ethernet line cards are preferred due to the increase in buffer size and the addition of a priority queue. Catalyst 4500/4000 Series of Switches

Q17. Which supervisor engine is preferred on the Catalyst 4500/4000 series? Why?

Answer: The Supervisor Engine III or the Supervisor Engine IV are preferred because of the addition of a priority queue, three standard queues, and the ability to use QoS access lists.

Q18. In which queue number does the priority queue reside on a Catalyst 4500 or 4000 with a Supervisor II Engine?

Answer: There is no priority queue on a Catalyst 4500 or 4000 with a Supervisor II Engine.

Q19. In which queue number does the priority queue reside on a Catalyst 4500 or 4000 with a Supervisor III Engine?

Answer: The priority queue in a Catalyst 4500 or 4000 with a Supervisor III Engine resides in Queue 3.

Q20. What keyword(s) is used to enable the priority queue on a Catalyst 4500 or 4000 with a Supervisor III module?

Answer:
tx-queue 3
priority high

Q21. Name the four methods a Catalyst 4500 with a Supervisor III uses to classify traffic.

Answer:
CoS values
IP precedence values
IP DSCP values
IP ACLs

Q22. What command enables you to rewrite a the CoS value of a PC attached to an IP Phone with Catalyst IOS switches?

Answer: switchport priority extend cos

Q23. What trust state are the ports if a 4500 with a Supervisor III in when QoS is enabled?

Answer: Untrusted.

Q24. What is dynamic buffer limiting?

Answer: DBL is an automated algorithm that manages misbehaving traffic flows by tracking the buffering for each traffic flow and limiting the flow if excessive demands are placed on a buffer. Catalyst 3550/3524 Series of Switches

Q25. In which queue number does the priority queue reside on a Catalyst 3550?

Answer: The priority queue in a Catalyst 3550 resides in Queue 4.

26 What command enables QoS on the Catalyst 3550?

Answer: mls qos

Q27. What trust state are the ports of a 3550 in when QoS is enabled?

Answer: Untrusted.

Q28. Name the four methods a Catalyst 3550 can use to classify traffic.

Answer:
CoS values
IP precedence values
IP DSCP values
IP ACLs

Q29. What command enables QoS on the Catalyst 3524?

Answer: QoS is enabled by default in a Catalyst 3524. No command is needed.

Q30. When is a GigaStack GBIC module an acceptable QoS choice for cascading Catalyst 3524 switches together?

Answer:
The GigaStack modules are acceptable for QoS under either of the two following conditions:
— The Catalyst 3524 switches are cascaded using both ports of the GigaStack modules causing half-duplex connections; however, no real-time applications are present on the network.
— The Catalyst 3524 switches are cascaded using only a single port of the GigaStack modules, resulting in full-duplex connections. Real-time applications can be present on the network.

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