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Catalyst 3750 MetroSwitch Software Configuration Guide
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Chapter 9 Configuring Interface Characteristics
Understanding Interface Types
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. Dynamic access ports on the switch are
assigned to a VLAN by a VLAN Membership Policy Server (VMPS). The VMPS can be a Catalyst
6000 series switch; the Catalyst 3750 Metro switch cannot be a VMPS server.
You can also configure an access port with an attached Cisco IP Phone to use one VLAN for voice traffic
and another VLAN for data traffic from a device attached to the phone. For more information about voice
VLAN ports, see Chapter 12, “Configuring Voice VLAN.”
Trunk Ports
A trunk port carries the traffic of multiple VLANs and by default is a member of all VLANs in the VLAN
database. Two types of trunk ports are supported:
• In an ISL trunk port, all received packets are expected to be encapsulated with an ISL header, and
all transmitted packets are sent with an ISL header. Native (non-tagged) frames received from an
ISL trunk port are dropped.
• An IEEE 802.1Q 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 every VLAN known to the VTP, 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 only become a member of a VLAN if VTP knows of
the VLAN and the VLAN is in the enabled state. If VTP learns of a new, enabled VLAN and the VLAN
is in the allowed list for a trunk port, the trunk port automatically becomes a member of that VLAN and
traffic is forwarded to and from the trunk port for that VLAN. If VTP learns of a new, enabled VLAN
that is not in the allowed list for a trunk port, the port does not become a member of the VLAN, and no
traffic for the VLAN is forwarded to or from the port.
For more information about trunk ports, see Chapter 10, “Configuring VLANs.”
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 appear to be on the same VLAN. 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 802.1Q-tagged with the customer VLANs,
are encapsulated with another layer of 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.
Tunnel ports cannot be trunk ports or access ports and must belong to a VLAN unique for each customer.
For more information about tunnel ports, see Chapter 13, “Configuring IEEE 802.1Q and Layer 2
Protocol Tunneling.”
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