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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 5 Administering the Switch Managing the MAC Address Table
Building the Address Table
With multiple MAC addresses supported on all ports, you can connect any port on the switch to
individual workstations, repeaters, switches, routers, or other network devices. The switch provides
dynamic addressing by learning the source address of packets it receives on each port and adding the
address and its associated port number to the address table. As stations are added or removed from the
network, the switch updates the address table, adding new dynamic addresses and aging out those that
are not in use.
The aging interval is globally configured. However, the switch maintains an address table for each
VLAN, and STP can accelerate the aging interval on a per-VLAN basis.
The switch sends packets between any combination of ports, based on the destination address of the
received packet. Using the MAC address table, the switch forwards the packet only to the port associated
with the destination address. If the destination address is on the port that sent the packet, the packet is
filtered and not forwarded. The switch always uses the store-and-forward method: complete packets are
stored and checked for errors before transmission.
MAC Addresses and VLANs
All addresses are associated with a VLAN. An address can exist in more than one VLAN and have
different destinations in each. Unicast addresses, for example, could be forwarded to port 1 in VLAN 1
and ports 1, 9, and 10 in VLAN 5.
Each VLAN maintains its own logical address table. A known address in one VLAN is unknown in
another until it is learned or statically associated with a port in the other VLAN.
When private VLANs are configured, address learning depends on the type of MAC address:
• Dynamic MAC addresses learned in one VLAN of a private VLAN are replicated in the associated
VLANs. For example, a MAC address learned in a private-VLAN secondary VLAN is replicated in
the primary VLAN.
• Static MAC addresses configured in a primary or secondary VLAN are not replicated in the
associated VLANs. When you configure a static MAC address in a private VLAN primary or
secondary VLAN, you should also configure the same static MAC address in all associated VLANs.
For more information about private VLANs, see Chapter 12, “Configuring Private VLANs.”
If the switch is running the metro IP access or metro access image, you can disable MAC address
learning on a per-VLAN basis. Customers in a service provider network can tunnel a large number of
MAC addresses through the network and fill up the available MAC address table space. Y o u can control
MAC address learning on a VLAN and manage the MAC address table space that is available on the
switch by controlling which VLANs, and therefore which ports, can learn MAC addresses.
Before you disable MAC address learning, be sure that you are familiar with the network topology and
the switch system configuration. Disabling MAC address learning on a VLAN could cause flooding in
the network. See the “Disabling MAC Address Learning on a VLAN” section on page 5-27 for more
information.