Optical TAP vs. SPAN: Compare the Physical Visibility Scope

Start with the acquisition mechanism

A request for network visibility should identify where the observation is needed and which frames the receiving team needs to examine. Those questions determine whether a proposed acquisition method is suitable. The analyzer name alone leaves the physical purchase undefined.

Gigamon's G-TAP M Series data sheet describes passive optical splitting. Cisco's Nexus 9000 NX-OS 10.4(x) SPAN guide describes selected sources, traffic directions and destinations for copied packets. One branches the optical signal; the other creates copies through a configured switch feature.

Ask the proposal to identify that boundary explicitly: the optical relationship containing the branch, or the switch source selection and mirror destination. Then identify the physical path from the monitoring output to its appointed recipient. A drawing that ends at an unlabeled box called monitoring leaves both the endpoint and the receiving responsibility unresolved.

References: [G2][C1]

Treat the optical branch as a changed signal path

Gigamon's 2023 TAP white paper defines split ratio as the allocation of light between network and monitor paths. It is not a packet-sampling percentage. A passive splitter also needs no power of its own; that description does not extend to the complete receiving and analysis system.

The proposed branch therefore needs an optical design review. Gigamon's G-TAP M Series data sheet, copyright 2022-2025, lists fiber type, wavelength, connector, split ratio, network loss and monitor loss by module. Its loss footnote includes connector loss. These are product-specific inputs, not a universal allowance that can be applied to another TAP.

Have the appointed designer evaluate the actual module, media, endpoint optics, receiving equipment and complete proposed paths. The continuing network connection and each directional monitor path need their own applicability assessment. Retain the relevant product issue and the designer's disposition with the connection drawing. No recommended ratio or project loss budget follows from the category name passive TAP.

References: [G1][G2]

Keep directional monitor outputs distinct

The passive example in Gigamon's 2023 white paper has two TX-only monitor outputs, one for each direction. Preserve that distinction when translating an observation requirement into physical endpoints.

For the fictional A-to-B connection used in VIS-01 below, identify the A-to-B monitor output and its intended receiving input M-A separately from the B-to-A output and input M-B. These are conceptual direction labels, not a universal connector layout or cable quantity. The selected product and supported receiving arrangement determine the actual interfaces.

If the receiving proposal combines those outputs, ask its owner to identify the equipment and work that performs that function. A cabling line item ending at two monitor inputs does not, by itself, supply aggregation, capture storage or analysis.

References: [G1]

Ask what the named SPAN implementation can represent

Cisco's Nexus 9000 NX-OS 10.4(x) SPAN guide says replication uses hardware without involving the supervisor CPU. It also says packets containing frame check sequence (FCS) errors are not mirrored. Both statements belong to that named guide, not every switch or mirror feature.

Consequently, absence of FCS-error frames in that mirror output cannot establish their absence on the observed link. Source, direction and destination records describe the intended output; reception and analysis require separate evidence from the receiving team.

References: [C1]

Compare two proposals for one visibility question

Fictional comparison VIS-01 concerns one duplex connection between A and B. The fixed buyer requirement is to examine frames from both A-to-B and B-to-A, including the ability to capture and identify frames with FCS errors if present. An error count alone would not satisfy this stipulated frame-observation requirement. It concerns FCS-error frames, not physical-symbol-error analysis. No actual traffic, equipment combination, line rate or performed capture is assumed.

Proposal P uses the passive directional-split concept in Gigamon's 2023 example. Its fictional supplier returns a drawing for an optical branch, an A-to-B monitor path to M-A and a B-to-A monitor path to M-B, plus proposed placement, routing, labeling and connection records. The return identifies those physical deliverables but supplies neither a selected module and supported optical design nor the receiver's errored-frame capability.

Proposal S offers a mirror output under the cited Cisco Nexus 9000 NX-OS 10.4(x) SPAN guide. Its fictional network-owner return identifies destination D and maps the intended A-to-B and B-to-A source/direction scope to it; the supplier identifies the physical path from D to the receiver. Those documents make S's proposed endpoints clearer. The FCS exclusion in that guide still prevents its mirror output from establishing VIS-01's required error-frame population.

Return S as unsuitable for VIS-01. More complete direction or destination paperwork does not resolve that documented exclusion. Ask the network owner for an alternative acquisition scope that addresses the unchanged requirement. Removing error-frame observation would be a separate buyer requirement change and a fresh comparison; VIS-01 is not relaxed here.

Retain P only for a specifically defined applicability review. The design owner must return the actual TAP/module and document issue, media and connector arrangement, endpoint and receiving optics, complete network and monitor paths, and optical-power/loss assessment for each relevant receiving path. The disposition must state whether that exact arrangement is supported and identify unresolved conditions.

The receiver owner must separately identify the actual receiving hardware, interfaces and software/version, map M-A and M-B through any intermediate equipment, and provide applicable documentation explaining whether FCS-error frames are retained for capture and identified to the analyst or discarded or altered before that point. The answer must cover both directions through the proposed receiving chain. An analyzer name or a statement that it accepts the optical input does not answer that question.

If both owners support the exact proposed arrangement and the required error-frame handling, P can progress to a defined physical work package and the separately authorized receiving/capture scope. If either return is negative or incomplete, P remains unresolved or needs an alternative arrangement. An optical split and two output labels alone do not establish that the receiver captures or reports errored frames.

The present selection is therefore S rejected for this requirement and P awaiting those two technical returns. It is a decision about the next acquisition review, not an installed TAP or analyzer winner, an actual capture result or a performance ranking.

References: [G1][C1][G2]

Assign the physical and operating work separately

Define the physical installation around approved equipment references, locatable endpoints, routes, labels and required records. Identify the party supplying each selected component and the authority for any change to the existing connection. Passive describes the splitter mechanism; it is not permission to insert or remove equipment on an operating link.

Assign switch replication to the network owner. Assign receiving interfaces, any aggregation, capture and interpretation to the appointed monitoring specialists. The customer and its authorized network and security owners establish the permitted observation scope and data handling.

Make the evidence match each responsibility. A physical record identifies its documented path and agreed observations. A separately commissioned optical measurement answers its defined physical question. The network owner's configuration record describes the mirror scope, while the monitoring team's evidence describes what its receiving system actually observed. A packet copy cannot certify the passive cable plant, and a passive loss report cannot establish which frames were captured. Keep those outputs linked by connection identity without merging their conclusions.

Bring a defined physical package to the project inquiry

For Leviathan's physical-layer testing and installation-verification service, describe the proposed or installed connection package, site, endpoint drawing, accessible items and required evidence. State whether the request concerns visual inspection, cable identification, label checks, physical connectivity or a separately defined measurement. In VIS-01, include the P or alternative-scope disposition and identify any endpoints still awaiting review.

The service supports discussion of those physical checks, defect records and agreed documentation. Formal fiber measurements require confirmation of the actual instruments, qualified personnel, method, coverage and acceptance criteria for the engagement.

Use the project inquiry linked below to discuss that physical work and availability. Name the appointed design, component-supply, network and monitoring owners and the recipients of the physical records. Their outstanding technical decisions remain visible in the request; the inquiry does not purchase TAP selection, component supply, switch configuration or packet analysis.