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