A dot1q VLAN subinterface is a virtual Cisco IOS interface that is associated with a VLAN ID on a routed physical interface.
A parent interface is a physical port. Subinterfaces can be created only on Layer 3 physical interfaces. A subinterface can
be associated with different functionalities such as IP addressing, forwarding policies, Quality of Service (QoS) policies,
and security policies.
Subinterfaces divide the parent interface into two or more virtual interfaces on which you can assign unique Layer 3 parameters
such as IP addresses and dynamic routing protocols. The IP address for each subinterface should be in a different subnet from
any other subinterface on the parent interface.
You can create a subinterface with a name that consists of the parent interface name (for example, HundredGigabitEthernet 1/0/33) followed by a period and then by a number that is unique for that subinterface. For example, you can create a subinterface
for HundredGigabitEthernet interface 1/0/33 named HundredGigabitEthernet 1/0/33.1, where .1 indicates the subinterface.
One of the uses of subinterfaces is to provide unique Layer 3 interfaces to each VLAN that is supported by the parent interface.
In this scenario, the parent interface connects to a Layer 2 trunking port on another device. You can configure a subinterface
and associate the subinterface to a VLAN ID using 802.1Q trunking.
You can configure subinterfaces with any normal range or extended range VLAN ID in VLAN Trunking Protocol (VTP) transparent
mode. Because VLAN IDs 1 to 1005 are global in the VTP domain and can be defined on other network devices in the VTP domain,
you can use only extended range VLANs with subinterfaces in VTP client or server mode. In VTP client or server mode, normal-range
VLANs are excluded from subinterfaces.
Use bridge groups on VLAN interfaces (also called fall-back bridging) to bridge nonrouted protocols. Bridge groups on VLAN
interfaces are supported on the route processor (RP) software.
You can configure the same VLAN ID on a Layer 2 VLAN or Layer 3 VLAN interface and on a Layer 3 subinterface.
The following features and protocols are supported on Layer 3 subinterfaces:
Addressing and routing—IPv4 and IPv6.
Unicast routing—Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), Routing Information Protocol (RIP), Border Gateway Protocol (BGP), and static routing.
Multicast routing—Internet Group Management Protocol (IGMP), Protocol-Independent Multicast Sparse Mode (PIM-SM), and Source
Specific Multicast (SSM).
First-Hop Redundancy Protocol (FHRP) protocols—Hot Standby Router Protocol (HSRP), Virtual Router Redundancy Protocol (VRRP),
and Gateway Load Balancing Protocol (GLBP).
Bidirectional Forwarding Detection (BFD), Unicast Reverse Path Forwarding (uRPF), and Equal-Cost Multipath (ECMP).
Maximum transmission unit (MTU) and IPv4 fragmentation.
Virtual routing and forwarding (VRF) lite.
Router access control list and policy-based routing (PBR).
Quality of Service (QoS)—Marking and policing.
Services—Network Address Translation (NAT) IPv4, Security Group Access Control List (SGACL) enforcement, DHCP Server/Relay, SGT Exchange Protocol (SXP), and NetFlow.