Specifications

4-5
System Management Software Configuration Guide for Cisco IE 2000U and Connected Grid Switches
Chapter 4 Administering the Switch
Prerequisites
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 the “Configuring Private VLANs” chapter in the Layer
2 Switching Software Configuration Guide for Cisco IE 2000U and Connected Grid Switches.
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. You 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. See the “Disabling MAC Address Learning on a VLAN” section on page 4-33 for more
information.
ARP Table
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 802 networks other than Ethernet is specified by the
Subnetwork Access Protocol (SNAP). By default, standard Ethernet-style ARP encapsulation
(represented by the arpa keyword) is enabled on the IP interface.
ARP entries added manually to the table do not age and must be manually removed.
For CLI procedures, see the IP Addressing: ARP Configuration Guide, Cisco IOS Release 15M&T.
Prerequisites
You should have at least a basic familiarity with the Cisco IOS environment and the command-line
interface.