Table of Contents
Advertisement
Chapter 11
Configuring VLANs
•
Table 11-4
Layer 2 Interface Modes
Mode
switchport mode access
switchport mode trunk
switchport mode dot1q-tunnel
switchport mode private-vlan
IEEE 802.1Q Configuration Considerations
The IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:
•
•
•
78-17058-01
To enable trunking, use the switchport mode trunk interface configuration command to change the
interface to a trunk.
Function
Puts the interface (access port) into permanent nontrunking mode and negotiates to
convert the link into a nontrunk link. The interface becomes a nontrunk interface
regardless of whether or not the neighboring interface is a trunk interface. This is the
default mode.
Puts the interface into permanent trunking mode and negotiates to convert the
neighboring link into a trunk link. The interface becomes a trunk interface even if the
neighboring interface is not a trunk interface.
Configures the interface as a tunnel (nontrunking) port to be connected in an asymmetric
link with an IEEE 802.1Q trunk port. The IEEE 802.1Q tunneling is used to maintain
customer VLAN integrity across a service provider network. See
"Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling,"
tunnel ports.
Configure the interface as a private VLAN host or promiscuous port (only NNIs can be
configured as promiscuous ports). For information about private VLANs, see
"Configuring Private VLANs."
In a network of Cisco switches connected through IEEE 802.1Q trunks, the switches maintain one
spanning-tree instance for each VLAN allowed on the trunks. Non-Cisco devices might support one
spanning-tree instance for all VLANs.
When you connect a Cisco switch to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco
switch combines the spanning-tree instance of the VLAN of the trunk with the spanning-tree
instance of the non-Cisco IEEE 802.1Q switch. However, spanning-tree information for each VLAN
is maintained by Cisco switches separated by a cloud of non-Cisco IEEE 802.1Q switches. The
non-Cisco IEEE 802.1Q cloud separating the Cisco switches is treated as a single trunk link between
the switches.
Make sure that the native VLAN for an IEEE 802.1Q trunk is the same on both ends of the trunk
link. If the native VLAN on one end of the trunk is different from the native VLAN on the other end,
spanning-tree loops might result.
Disabling spanning tree on the native VLAN of an IEEE 802.1Q trunk without disabling spanning
tree on every VLAN in the network can potentially cause spanning-tree loops. We recommend that
you leave spanning tree enabled on the native VLAN of an IEEE 802.1Q trunk or disable spanning
tree on every VLAN in the network. Make sure that your network is loop-free before disabling
spanning tree.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
Configuring VLAN Trunks
Chapter 13,
for more information on
Chapter 12,
11-15
Hide quick links:
Advertisement
Table of Contents
Troubleshooting
