Operation Manual – VLANH3C S5100-SI/EI Series Ethernet Switches Chapter 1 VLAN Overview1-4II. MAC address learning mechanism of VLANsSwitches make forwarding decisions based on destination MAC addresses. For thispurpose, each switch maintains a MAC address table, of which each entry records theMAC address of a terminal connected to the switch and to which port this terminal isconnected, assuming that no VLAN is involved. For the ease of management, a MAClearning mechanism is adopted on switches. With this mechanism, a switch canpopulate its MAC address table automatically by learning the source MAC address ofincoming traffic and on which port the traffic is received. When forwarding trafficdestined for the learned MAC address, the switch looks up the table and forwards thetraffic according to the entry.After VLANs are configured, a switch adopts one of the following MAC address learningmechanisms:z Shared VLAN learning (SVL), where the switch records all learned MAC addressentries in one MAC address table, regardless of in which VLAN they are learned.This table is called the shared MAC address forwarding table. Packets received inany VLAN on a port are forwarded according to this table.z Independent VLAN learning (IVL), where the switch maintains an independentMAC address forwarding table for each VLAN. The source MAC address of apacket received in a VLAN on a port is recorded to the MAC address forwardingtable of this VLAN only, and packets received in a VLAN are forwarded accordingto the MAC address forwarding table for the VLAN.Currently, the H3C S5100-SI/EI series Ethernet switches adopt the IVL mode only. Formore information about the MAC address forwarding table, refer to the “MAC AddressForwarding Table Management” part of the manual.1.1.4 VLAN InterfaceHosts in different VLANs cannot communicate with each other directly unless routers orLayer 3 switches are used to do Layer 3 forwarding. The S5100-SI/EI series Ethernetswitches support VLAN interfaces configuration to forward packets in Layer 3.VLAN interface is a virtual interface in Layer 3 mode, used to realize the layer 3communication between different VLANs, and does not exist on a switch as a physicalentity. Each VLAN has a VLAN interface, which can forward packets of the local VLANto the destination IP addresses at the network layer. Normally, since VLANs can isolatebroadcast domains, each VLAN corresponds to an IP network segment. And a VLANinterface serves as the gateway of the segment to forward packets in Layer 3 based onIP addresses.
