Netgear XCM8806 Chassis User Manual - Page 280
Managing MAC Address Learning, Creating Blackhole FDB Entries, How FDB Entries Get Added
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NETGEAR 8800 User Manual • Creating Blackhole FDB Entries on page 283 Managing MAC Address Learning By default, MAC address learning is enabled on all ports. MAC addresses are added to the FDB as described in How FDB Entries Get Added on page 272. When MAC address learning is disabled on a port, the switch no longer stores the source address information in the FDB. However, the switch can still examine the source MAC address for incoming packets and either forward or drop the packets based on this address. The source address examination serves as a preprocessor for packets. Forwarded packets are forwarded to other processes, not to other ports. For example, if the switch forwards a packet based on the source address, the packet can still be dropped based on the destination address or the egress flooding configuration. When MAC address learning is disabled, the two supported behaviors are labeled as follows in the software: • forward-packets • drop-packets The forward-packets behavior forwards all received packets for further processing. No action is taken based on the source address. When the drop-packets option is chosen and all unicast, multicast, and broadcast packets from a source address not in the FDB are dropped. No further processing occurs for dropped packets. The disable learning forward-packets option saves switch resources (FDB space), however, it can consume network resources when egress flooding is enabled. When egress flooding is disabled or the drop-packet option is specified, disabling learning adds security by limiting access to only those devices listed in the FDB. To disable learning on specified ports, use the following command: disable learning {drop-packets | forward-packets} port [ | all] Note: If neither option is specified, the drop-packets behavior is selected. To enable learning on specified ports, use the following command: enable learning {drop-packets | forward-packets} ports [all | ] MAC address learning is disabled on a service VLAN (SVLAN) or backbone VLAN (BVLAN) to create a Provider Backbone Bridge (PBB). To enable or disable learning on an entire SVLAN or BVLAN, use the following commands with the appropriate options: enable learning vlan 280 | Chapter 10. FDB
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