Link and Interface Types
All frames processed in a switch carry VLAN tags, but some devices connected to a switch cannot process tagged frames. To enable communication between the switch and these devices, the switch interfaces must be able to identify whether an Ethernet frame is tagged and remove VLAN tags from the frames.
Hosts in the same VLAN may be connected to different switches, and a VLAN can span multiple switches. To enable communication between hosts, interfaces between switches must be able to identify and send VLAN frames.
The following describes these interfaces and link types in detail. For more information about QinQ interfaces, see QinQ Configuration
Link Types
Ethernet links fall into the following types, depending on the number of allowed VLANs:
Access link
An access link can transmit data frames of only one VLAN. It connects a switch to a user terminal, such as a host or server. Generally, user terminals do not need to know the VLANs to which they belong and cannot identify tagged frames; therefore, only untagged frames are transmitted along an access link.
Trunk link
A trunk link can transmit data frames from multiple VLANs. It connects a switch to another switch or a router. Frames on a trunk link must be tagged so that other network devices can correctly identify VLAN information in the frames.
Interface Types
Ethernet interfaces are classified into the following types depending on the objects connected to them and the way they process frames:
Access interface
An access interface often connects to a user terminal such as a user host or server that cannot identify VLAN tags, or is used when VLANs do not need to be differentiated. In most cases, access interfaces can only receive and send untagged frames, and can add only a unique VLAN tag to untagged frames. However, if the VID and PVID are the same in tagged frames, access interfaces can receive and process the tagged frames.
Trunk interface
A trunk interface often connects to a switch, router, AP, or voice terminal that can receive and send tagged and untagged frames simultaneously. It allows tagged frames from multiple VLANs and untagged frames from only one VLAN.
Hybrid interface
A hybrid interface can connect to a user terminal (such as a user host or server) or network device (such as a hub or simplified Layer 2 switch) that cannot identify tags, but also a switch, router, voice terminal, or AP that can receive and send tagged and untagged frames. It allows tagged frames from multiple VLANs. Frames sent out from a hybrid interface are tagged or untagged depending on the VLAN configuration.
Hybrid and trunk interfaces can be interchanged in some scenarios, but hybrid interfaces must be used in specified scenarios, for example, selective QinQ scenario. Before packets from multiple VLANs provided by a service provider enter a user network, the outer VLAN tags must be removed. The trunk interface cannot be used here because the trunk interface allows only untagged packets from the default VLAN of the interface to pass through. For details about selective QinQ, see Configuring Selective QinQ in "QinQ Configuration".
QinQ interface
An 802.1Q-in-802.1Q (QinQ) interface often connects a private network to a public network. It can add an additional 802.1Q tag to a tagged frame. QinQ supports up to 4094 x 4094 VLANs, fulfilling network VLAN requirements. The outer tag is often called the public tag and identifies the VLAN ID of the public network, whereas the inner tag is often called the private tag and identifies the VLAN ID of the private network.
For details about the QinQ interface and QinQ frame format, see QinQ Fundamentals.