5-2
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 5 Configuring Ethernet VLAN Trunks
Understanding How VLAN Trunks Work
You can configure a trunk on a single Ethernet port or on an EtherChannel bundle. For more information
about EtherChannel, see
Chapter 6, “Configuring EtherChannel.”
Ethernet trunk ports support five different trunking modes (see
Table 5-1
). In addition, you can specify
whether the trunk will use ISL encapsulation, 802.1Q encapsulation, or whether the encapsulation type
will be autonegotiated.
For trunking to be autonegotiated, the ports must be in the same VLAN Trunking Protocol (VTP)
domain. However, you can use the
on
or
nonegotiate
mode to force a port to become a trunk, even if it
is in a different domain. For more information on VTP domains, see
Chapter 10, “Configuring VTP.”
Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP). DTP supports autonegotiation
of both ISL and 802.1Q trunks.
Trunking Modes and Encapsulation Type
Note
For a complete list of modules that do not support ISL encapsulation, refer to the
Catalyst 6500 Series
Release Notes
at this URL:
http://www.cisco.com/en/US/products/hw/switches/ps708/prod_release_notes_list.html
Table 5-1
lists the trunking modes that are used with the
set trunk
command and describes how they
function on Fast Ethernet, Gigabit Ethernet, and 10-Gigabit Ethernet ports.
Table 5-2
lists the encapsulation types that are used with the
set trunk
command and describes how they
function on Ethernet ports. You can enter the
show port capabilities
command to determine which
encapsulation types that a particular port supports.
Table 5-1
Ethernet Trunking Modes
Mode
Function
on
Puts the port into permanent trunking mode and negotiates to convert the link into a trunk link. The port
becomes a trunk port even if the neighboring port does not agree to the change.
off
Puts the port into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The
port becomes a nontrunk port even if the neighboring port does not agree to the change.
desirable
Makes the port actively attempt to convert the link to a trunk link. The port becomes a trunk port if the
neighboring port is set to
on
,
desirable
, or
auto
mode.
auto
Makes the port willing to convert the link to a trunk link. The port becomes a trunk port if the neighboring
port is set to
on
or
desirable
mode. This is the default mode for all Ethernet ports.
nonegotiate
Puts the port into permanent trunking mode but prevents the port from generating DTP frames. You must
configure the neighboring port manually as a trunk port to establish a trunk link.
