A Subinterface is a virtual interface that is created by dividing one physical interface into many logical interfaces, the subinterface uses the parent physical interface in order to send and receive messages like Fast Ethernet 0/0.1 is the subinterface Fast Ethernet Port 0/0. To know more about What is a Subinterface in a Cisco Router, keep reading this article till the end.

The commonly use of Cisco router subinterface is done when we want to enable communication between two different VLANs. If we have a router that has only one interface and there is required to configured two networks, then we can create sub-interfaces assign each sub-interface with the appropriate IP address and route the data between subnets.

A Sub-interface is configured the same as physical interface and if you look number range that could be configured at any physical Interface is too huge.

Sub-Interface Range on Physical Interface

Before configuring the sub-interfaces, firstly make the interface up by using the following command.

Making the Interface up

Now create the subinterface with the encapsulation of 802.1Q for inter VLAN routing of VLAN 100.

Inter VLAN  Communication

Also, read…

How to configure Layer 3 sub-interface?

The following steps should be followed in order to configure the layer-3 sub interface.

  • Enable
  • Configure the terminal
  • Encapsulation dot1q VLAN-number
  • End
Step Command Purpose
01 Enable


Router > enable

Enable privilege exex-mode
02 Configure terminal


Router# Configure terminal

Enable global configuration mode.
03 Interface slot/port.sub interface


Interface S0/0.1

 Select an interface then sub-interface
04 Encapsulation dot1q vlan-id


Router(config-subif)#encapsulation dot1q 99

Define encapsulation type
05 IP address


Router(config-subif)# IP address

Configure IP address

Sub-interface configuration for Inter-vlan communication

Remember the layer 3 interface forwards packets from one subnet to another, you can use layer-3 interface for IP routing and inter VLAN routing of Layer-2 Traffic.

The following features are supported at Layer-3 sub interfaces

  • IPV4 addressing and routing.
  • Unicast routing
  • Multicast routing
  • FHRP (First Hop redundancy protocol), HSRP (hot standby routing protocol), VRRP (Virtual router Redundancy protocol) and Gateway Load Balancing Protocol (GLBP).
  • VRF (Virtual routing and forwarding)
  • QoS (Quality of service)

In frame relay network there could be two types of sub-interfaces Multi-point and point to point-sub interfaces. In point to pint frame relay, the sub-interfaces act as leased line and each point to point connection require its own subnet, while in multi-point frame-relay network sub-interfaces acts as NBMA network and they can save the network as they use the single subnet.

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