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Configuring Interfaces
Understanding Interface Types
Switch Ports
Switch ports are Layer 2 only interfaces associated with a physical port. Switch ports belong to one or more VLANs. A
switch port can be an access port, a trunk port, a private-VLAN port, or a tunnel port. You can configure a port as an
access port or trunk port. You configure a private VLAN port as a host or promiscuous port that belongs to a private-VLAN
primary or secondary VLAN. (Only NNIs can be configured as promiscuous ports.) You must manually configure tunnel
ports as part of an asymmetric link connected to an IEEE 802.1Q trunk port. Switch ports are used for managing the
physical interface and associated Layer 2 protocols and do not handle routing or bridging.
Configure switch ports by using the switchport interface configuration commands. Use the switchport command with
no keywords to put an interface that is in Layer 3 mode into Layer 2 mode.
Note:
When you put an interface that is in Layer 3 mode into Layer 2 mode, the previous configuration information related
to the affected interface might be lost, and the interface is returned to its default configuration.
Access Ports
An access port belongs to and carries the traffic of only one VLAN. Traffic is received and sent in native formats with no
VLAN tagging. Traffic arriving on an access port is assumed to belong to the VLAN assigned to the port. If an access port
receives an 802.1Q tagged packet, the packet is dropped, and the source address is not learned. 802.1x can also be
used for VLAN assignment.
Two types of access ports are supported:
Static access ports are manually assigned to a VLAN.
VLAN membership of dynamic access ports is learned through incoming packets. By default, a dynamic access port
is a member of no VLAN, and forwarding to and from the port is enabled only when the VLAN membership of the
port is discovered. UNIs begin forwarding packets as soon as they are enabled. Dynamic access ports on the switch
are assigned to a VLAN by a VLAN Membership Policy Server (VMPS). Dynamic access ports for VMPS are only
supported on UNIs and ENIs.
Trunk Ports
An 802.1Q trunk port carries the traffic of multiple VLANs and by default is a member of all VLANs in the VLAN database.
A trunk port supports simultaneous tagged and untagged traffic. An 802.1Q trunk port is assigned a default Port VLAN
ID (PVID), and all untagged traffic travels on the port default PVID. All untagged traffic and tagged traffic with a NULL
VLAN ID are assumed to belong to the port default PVID. A packet with a VLAN ID equal to the outgoing port default PVID
is sent untagged. All other traffic is sent with a VLAN tag.
Although by default a trunk port is a member of multiple VLANs, you can limit VLAN membership by configuring an
allowed list of VLANs for each trunk port. The list of allowed VLANs does not affect any other port but the associated
trunk port. By default, all possible VLANs (VLAN ID 1 to 4094) are in the allowed list. A trunk port can become a member
of a VLAN only if the VLAN is in the enabled state.
For more information about trunk ports, see
Tunnel Ports
Tunnel ports are used in 802.1Q tunneling to segregate the traffic of customers in a service-provider network from other
customers who are using the same VLAN number. You configure an asymmetric link from a tunnel port on a
service-provider edge switch to an 802.1Q trunk port on the customer switch. Packets entering the tunnel port on the
edge switch, already IEEE 802.1Q-tagged with the customer VLANs, are encapsulated with another layer of an 802.1Q
tag (called the metro tag), containing a VLAN ID unique in the service-provider network, for each customer. The
double-tagged packets go through the service-provider network keeping the original customer VLANs separate from
those of other customers. At the outbound interface, also a tunnel port, the metro tag is removed, and the original VLAN
numbers from the customer network are retrieved.
Configuring VLANs, page 271
19
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