What is the purpose of a router's "subinterface"? What are its application scenarios?
Good evening, everyone!
Today, let's discuss a router-on-a-stick configuration:
"When configuring router-on-a-stick, when and where should the commands dot1q termination vid and arp broadcast enable be used?"
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The essence of router-on-a-stick is "logical segmentation and resource multiplexing"
The so-called "router-on-a-stick" refers to configuring multiple subinterfaces on a single physical interface of a router, using VLAN tagging to carry traffic for different VLANs, thereby enabling Layer 3 communication between VLANs.
The key point here is that the router's interface operates in a "subinterface + VLAN tag" mode, unlike traditional physical interfaces where "one interface = one subnet".
This brings us to the first command:
dot1q termination vid: Defines which VLAN a subinterface is bound to.
For example, if we configure the router like this:
This means: This subinterface processes VLAN 1 traffic - it will only handle incoming data frames tagged with VLAN 1.
Therefore, the prerequisites for using this command are:
1.The interface must be a subinterface
2.It must receive VLAN-tagged frames
Important note:
If a physical interface connects directly to end devices (like PCs) without VLAN tagging, this command is not needed!
arp broadcast enable: Allows broadcast transmission of ARP requests
This command determines whether the subinterface should forward broadcast ARP request frames when needed.
It applies in these scenarios:
◉ The interface is a subinterface
◉ Directly connected to end devices (e.g. PCs) that require ARP responses
◉ The network lacks switches to handle broadcast frame forwarding
Analysis of Two Typical Scenarios
✅ Scenario 1: Physical Interface Directly Connected to a PC
In this scenario:
◉ The router's two physical interfaces (GE 0/0/1 and GE 0/0/2) are directly connected to two PCs.
◉ Each interface carries an independent VLAN subnet (no shared physical link).
◉ The traffic is untagged (no 802.1Q VLAN headers).
◉ The interfaces operate as physical interfaces, not subinterfaces.
✔ No dot1q termination vid needed (physical interfaces don’t process VLAN tags).
✔ No arp broadcast enable required (ARP works natively on physical interfaces).
✔ Each interface acts as a standalone L3 gateway for its connected subnet.
✅ Scenario 2: Using Router-on-a-Stick with an Access Switch
In this more common real-world deployment scenario:
◉ A single physical interface on the router (e.g., GigabitEthernet 0/0/1) connects to a Layer 2 switch's Trunk port.
◉ All traffic from PC1 and PC2 carries VLAN tags (802.1Q).
◉ The router must create subinterfaces, such as GE0/0/1.1 and GE0/0/1.2, to process VLAN 10 and VLAN 20 traffic separately.
In this case, the following configurations are mandatory:
That's all for today's content! I hope this article helped clarify the usage scenarios for these two common subinterface configuration commands.
See you next time!
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