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Access Layer Policy

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(NIC) to make forwarding decisions. The switches create a MAC table that
includes dynamic, permanent, and static addresses. This filter table is created
when hosts send a frame and the switch learns the source MAC address and
from which segment and port it was received.

The switch keeps adding into the MAC filter table new MAC addresses

that are sent on the network. As hosts are added or removed, the switch
dynamically updates the MAC filter table. If a device is removed, or if it is
not connected to the switch for a period of time, the switch will age out the
entry.

You can see the switch's filter table by using the command show mac-

address-table

. The following output shows the information received

when using the show mac-address-table command:

Todd1900EN#sh mac-address-table

Number of permanent addresses : 0

Number of restricted static addresses : 0

Number of dynamic addresses : 4

Address Dest Interface Type Source Interface List

-----------------------------------------------------------------

00A0.246E.0FA8 Ethernet 0/2 Dynamic All

0000.8147.4E11 Ethernet 0/5 Dynamic All

0000.8610.C16F Ethernet 0/1 Dynamic All

00A0.2448.60A5 Ethernet 0/4 Dynamic All

The addresses in the table above are from the four hosts connected to a

1900 switch. They are all dynamic entries, which means the switch looked at
the source address of a frame as it entered the switch interface and placed
that address in the filter table. Notice that we have hosts in interfaces 1, 2,
4, and 5.

Administrators can specifically assign permanent addresses to a switch

port. These addresses are never aged out. You can do this to provide security
to a port, which means that unless you specifically configure a hardware
address to a switch port, the hardware address won't work. Administrators
can also create static entries in the switch; these entries actually create a path
for a source hardware address. This can be really restrictive, and you need to
be careful when setting static entries because you can basically shut your
switch down if you do not plan the configuration carefully.

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