Cisco UCS vNIC Placement vs RSS for CPU Offload

300-610 By islever Editorial Last updated

Evaluate options for network device virtualization in a data center (Cisco VIC adapters)

Short answer

C Enable Receive Side Scaling (RSS) in the Ethernet adapter policy to distribute network receive processing across multiple CPUs.

Question

An engineer is experiencing performance issues on a Cisco UCS blade server. The B-Series blade server contains four CPUs, most of which are idle. The engineer notices that the CPU is suffering from too many requests sent by the NIC. Additionally, the number of queues appears to be insufficient and only a single CPU is processing the network traffic. Which policy must be used to alleviate these issues?
  1. LAN connectivity
  2. dynamic vNIC connection
  3. Ethernet adapter Correct Answer
  4. vNIC placement

Key Takeaways

The core issue is single-CPU processing of network traffic, which is resolved by enabling Receive Side Scaling (RSS) within the Ethernet adapter policy.

This question addresses how to distribute network traffic across multiple CPUs on a Cisco UCS B-Series blade server using the Ethernet adapter policy.

Common mistake: Many candidates incorrectly select 'vNIC placement' (D), confusing physical pinning with software-based load balancing across available cores.

Expert Analysis

Why the Answer Is Correct

The correct answer is C (Ethernet adapter). The scenario describes a classic case where network interrupts are not being distributed across the available CPU cores, causing a bottleneck on a single processor despite idle resources. This is solved by enabling Receive Side Scaling (RSS) in the Ethernet adapter policy. As noted by user Fcpoultry, "RSS distributes network receive processing across multiple CPUs... enabled—Network receive processing is shared across processors whenever possible." By configuring the Ethernet adapter policy, the engineer enables RSS, allowing the NIC and OS to hash incoming packets to different queues mapped to different CPUs.

Why the Other Options Are Wrong

Option D (vNIC placement) is a common distractor. While vNIC placement controls which physical PCIe slots or NUMA nodes a vNIC is pinned to, it does not inherently solve the problem of a single CPU handling all interrupts if RSS is not enabled. Option A (LAN connectivity) is too generic and refers to the general path rather than a specific configuration policy. Option B (dynamic vNIC connection) relates to how vNICs are assigned to service profiles dynamically, which has no bearing on CPU interrupt distribution.

Community Comment Notes

The community consensus strongly favors C. User zeppie confirms that RSS is enabled on the ethernet adapter policy. User CoAsT_x emphasizes that this is a non-negotiable setting for such issues. One dissenting vote suggests D, but the technical reality of interrupt affinity points to the adapter's internal scaling features.

Official Reference

https://www.cisco.com/en/US/docs/unified_computing/ucs/sw/gui/config/guide/141/UCSM_GUI_Configuration_Guide_141_chapter19.html#d4900e910_navtitle

This link opens the official vendor documentation in a new tab.

Exam Strategy

When seeing "single CPU processing" or "interrupt storm" on multi-core systems, look for RSS (Receive Side Scaling) or IOMMU settings first. In UCS specifically, these are configured in the Adapter Policy, not just the vNIC definition.

Frequently Asked Questions

Why isn't vNIC placement the solution?

VNIC placement pins vNICs to specific hardware paths but doesn't split interrupt handling across cores unless RSS is also enabled.

Does RSS work on all Cisco UCS adapters?

Most modern VIC adapters support RSS, but it must be explicitly enabled in the Ethernet adapter policy within UCS Manager.

More 300-610 FAQ →

Community Votes

C
100%

100% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 4 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 4

  1. Fcpoultry 👍 4 Answered C
    "RSS distributes network receive processing across multiple CPUs in multiprocessor systems. This can be: disabled—Network receive processing is always handled by a single processor even if additional processors are available. enabled—Network receive processing is shared across processors whenever possible." https://www.cisco.com/en/US/docs/unified_computing/ucs/sw/gui/config/guide/141/UCSM_GUI_Configuration_Guide_141_chapter19.html#d4900e910_navtitle
  2. CoAsT_x 👍 1 Answered C
    It is 1000% Ethernet adapter policies. C. non-negotiable. https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/unified-computing-system-adapters/victuning-wp.html#Transmitandreceivequeues
  3. zeppie 👍 1 Answered C
    You enable RSS on the ethernet adapter policy https://www.cisco.com/en/US/docs/unified_computing/ucs/sw/gui/config/guide/141/UCSM_GUI_Configuration_Guide_141_chapter19.html#d4900e910_navtitle
  4. lurker8000 👍 1 Answered D
    D vNIC placement should be the answer here...

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