184Figure 63 Single-tagged Ethernet frame header and double-tagged Ethernet frame headerThe default maximum transmission unit (MTU) of an interface is 1500 bytes. The size of an outer VLANtag is 4 bytes. H3C recommends that you increase the MTU of each interface on the service providernetwork to at least 1504 bytes. For more information about interface MTU configuration, see"Configuring Ethernet interfaces."Implementations of QinQH3C provides the following QinQ implementations: basic QinQ and selective QinQ.• Basic QinQBasic QinQ enables a port to tag any incoming frames with its port VLAN ID (PVID) tag, regardlessof whether they have been tagged or not. If an incoming frame has been tagged, it becomes adouble-tagged frame. If not, it becomes a frame tagged with the PVID tag.• Selective QinQSelective QinQ is more flexible than basic QinQ. In addition to all the functions of basic QinQ,selective QinQ enables a port to perform the following per-CVLAN actions for incoming frames:{ Tag frames from different CVLANs with different SVLAN tags.{ Mark the outer VLAN 802.1p priority based on the existing inner VLAN 802.1p priority.{ Modify the inner VLAN ID.Besides being able to separate the service provider network from the customer networks, selectiveQinQ provides abundant service features and enables more flexible networking.Protocols and standardsIEEE 802.1Q: IEEE standard for local and metropolitan area networks: Virtual Bridged Local AreaNetworksQinQ configuration task listWhen you configure QinQ, follow these guidelines:• QinQ requires configurations only on the service provider network.• QinQ configurations made in Ethernet interface view take effect on the interface only. Those madein Layer 2 aggregate interface view take effect on the aggregate interface and all member ports inthe aggregation group. Those made in port group view take effect on all member ports in the portgroup.
