198 C HAPTER 9: V IRTUAL LAN SEgress Rules These rules determine whether the outgoing frame is forwarded, filtered(dropped), or flooded; they also determine the frame’s tag status.Although the same standard bridging rules apply to both open andclosed VLANs, they result in different behavior depending on the allOpenmode (one address table for the system) versus allClosed mode (oneaddress table for each VLAN).Standard Bridging Rules for Outgoing FramesThe frame is handled according to these bridging rules:n If the frame’s destination address matches an address that waspreviously learned on the receive port, it is filtered (dropped).n If the frame’s destination address matches an address that was learnedon a port other than the receive port, it is forwarded to that port if thereceive port and transmit port are in the same VLAN or the system is inallOpen mode.n If a frame with an unknown, multicast, or broadcast destinationaddress is received, then it is flooded (that is, forwarded to all ports onthe VLAN that is associated with the frame, except the port on whichit was received). Those frames assigned to the null VLAN are notflooded to any ports because no ports are associated with the nullVLAN. See “Examples of Flooding and Forwarding Decisions” later inthis chapter.n If the frame’s destination address matches a MAC address of one ofthe bridge’s ports, it is further processed, not forwarded immediately.This type of frame is either a management/configuration frame (suchas a RIP update, SNMP get/set PDU, Administration Console Telnetpacket, or a Web Management Interface http packet), or it is a routedpacket. If it is a routed packet, the system performs the routingfunctions described in the appropriate routing chapter (for example, IP,OSPF, IPX, or AppleTalk).For example, if a frame is associated with VLAN A and has a destinationaddress associated with VLAN B, the frame is flooded over VLAN A inallClosed mode but forwarded untagged in allOpen mode.
