Cisco Catalyst 2960 Series, Configuration Manual

Page: 1612 / 2288

MAC Address Table Creation
With multiple MAC addresses supported on all ports, you can connect any port on the switch to 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 devices 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 9,
10, and 1 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.
Default MAC Address Table Settings
The following table shows the default settings for the MAC address table.
Table 141: Default Settings for the MAC Address
Default Setting Feature
300 seconds Aging time
Automatically learned Dynamic addresses
None configured Static addresses
ARP Table Management
To communicate with a device (over Ethernet, for example), the software first must learn the 48-bit MAC
address or the local data link address of that device. The process of learning the local data link address from
an IP address is called
address resolution .
The Address Resolution Protocol (ARP) associates a host IP address with the corresponding media or MAC
addresses and the VLAN ID. Using an IP address, ARP finds the associated MAC address. When a MAC
address is found, the IP-MAC address association is stored in an ARP cache for rapid retrieval. Then the IP
datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and
ARP requests and replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
1530
Information About Administering the Switch