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Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
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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 Types
Table 5-1
lists the trunking modes used with the
set trunk
command and describes how they function
on Fast Ethernet and Gigabit Ethernet ports.
Table 5-2
lists the encapsulation types used with the
set trunk
command and describes how they
function on Ethernet ports. You can use the
show port capabilities
command to determine which
encapsulation types a particular port supports.
The trunking mode, the trunk encapsulation type, and the hardware capabilities of the two connected
ports determine whether a trunk link comes up and the type of trunk the link becomes.
Table 5-3
shows
the result of the possible trunking configurations.
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.
Table 5-2
Ethernet Trunk Encapsulation Types
Encapsulation
Function
isl
Specifies ISL encapsulation on the trunk link.
dot1q
Specifies 802.1Q encapsulation on the trunk link.
negotiate
Specifies that the port negotiate with the neighboring port to become an ISL (preferred) or 802.1Q trunk,
depending on the configuration and capabilities of the neighboring port.
