GSLC · Question #572
A router has a single Ethernet connection to a switch. In the router configuration, the Ethernet interface has three sub-interfaces, each configured with ACLs applied to them and 802.1q trunks…
The correct answer is B. The switch has several VLANs configured on it. A single physical router interface with multiple 802.1q sub-interfaces is the router-on-a-stick design, used specifically to route between multiple VLANs on the connected switch.
Question
A router has a single Ethernet connection to a switch. In the router configuration, the Ethernet interface has three sub-interfaces, each configured with ACLs applied to them and 802.1q trunks. Which of the following is MOST likely the reason for the sub-interfaces?
Options
- AThe network uses the subnet of 255.255.255.128.
- BThe switch has several VLANs configured on it.
- CThe sub-interfaces are configured for VoIP traffic.
- DThe sub-interfaces each implement quality of service.
How the community answered
(47 responses)- A4% (2)
- B72% (34)
- C15% (7)
- D9% (4)
Why each option
A single physical router interface with multiple 802.1q sub-interfaces is the router-on-a-stick design, used specifically to route between multiple VLANs on the connected switch.
The subnet mask 255.255.255.128 defines IP address block sizing and does not drive the need for sub-interfaces; sub-interface count is determined by the number of VLANs, not subnet masks.
When a switch hosts multiple VLANs, a router can inter-route between them over a single trunk link by creating one sub-interface per VLAN, each tagged with the corresponding 802.1q VLAN ID and assigned its own IP gateway address. This router-on-a-stick topology eliminates the need for a dedicated physical router port per VLAN. The presence of 802.1q trunking and multiple sub-interfaces is the definitive indicator that the switch has multiple VLANs requiring inter-VLAN routing.
While a sub-interface could carry VoIP traffic within one VLAN, multiple sub-interfaces with 802.1q trunking specifically indicate a multi-VLAN routing design, not a VoIP-only configuration.
QoS policies can be applied to sub-interfaces as a secondary configuration, but QoS alone does not explain why multiple 802.1q sub-interfaces are required; the switch VLAN topology is the root cause.
Concept tested: Router-on-a-stick sub-interfaces with 802.1q inter-VLAN routing
Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/inter-vlan-routing/41860-howto-L3-intervlanrouting.html
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