5-3
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 5 Configuring Ethernet VLAN Trunks
Understanding How VLAN Trunks Work
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 5-2
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.
Table 5-3
Results of Possible Fast Gigabit Ethernet, and 10-Gigabit Ethernet Trunk Configurations
Neighbor Port
Trunk Mode
and Trunk
Encapsulation
Local Port Trunk Mode and Trunk Encapsulation
off
isl or
dot1q
on
isl
desirable
isl
auto
isl
on
dot1q
desirable
dot1q
auto
dot1q
desirable
negotiate
auto
negotiate
off
isl or dot1q
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
ISL trunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
1Q trunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
on
isl
Local:
Nontrunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
1Q trunk
1
Neighbor:
ISL trunk
1
Local:
Nontrunk
Neighbor:
ISL trunk
Local:
Nontrunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
desirable
isl
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
1Q trunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
auto
isl
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
1Q trunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
Nontrunk
Neighbor:
Nontrunk
on
dot1q
Local:
Nontrunk
Neighbor:
1Q trunk
Local:
ISL trunk
1
Neighbor:
1Q trunk
1
Local:
Nontrunk
Neighbor:
1Q trunk
Local:
Nontrunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk
desirable
dot1q
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
ISL trunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
1Q trunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk
Local:
1Q trunk
Neighbor:
1Q trunk